1
Effects of harvest intensity and organic matter
removal on nitrogen availability and leaching
at a high productivity coastal Douglas-fir site
Barry Lynn Flaming
2001
2
Effects of harvest intensity and organic matter removal
on nitrogen availability and leaching at a high
productivity coastal Douglas-fir site
Barry Lynn Flaming
A thesis submitted in partial fulfillment of the
requirements for the degree of
Master of Science
University of Washington
2001
Program Authorized to Offer Degree: College of Forest Resources
University of Washington
Graduate School
3
This is to certify that I have examined this copy of a master’s thesis by
BARRY LYNN FLAMING
and have found that it is complete and satisfactory in all respects,
and that any and all revisions required by the final
examining committee have been made.
Committee members:
Robert B. Harrison
Charles L. Henry
Robert L. Edmonds
Thomas A. Terry
Date:
July 26, 2001
4
Master’s Thesis
In presenting this thesis in partial fulfillment of the requirements for a Master’s
degree at the University of Washington, I agree that the Library shall make its copies
freely available for inspection. I further agree that extensive copying of this thesis is
allowable only for scholarly purposes, consistent with “fair use” as prescribed in the
U.S. Copyright Law. Any other reproduction for any purposes or by any means shall
not be allowed without my written permission.
Signature
Date
5
Abstract: The effects of harvesting-related nutrient losses on sustained forest
productivity are not clearly understood. The removal of logging residues from a site
following intensive biomass utilization may impact the rate at which nutrients are
made available to seedlings and alter leaching losses of dissolved nutrients. The
objectives of this study were to examine the impact of intensive harvesting on
nitrogen availability and leaching at a high productivity (Site Index 42 m in 50 years)
coastal Douglas-fir site in southwestern Washington. Nitrogen concentrations in soil
solution were monitored at 0.2- and 1.0-m depths during the 8- to 12-month period
following four experimental harvesting treatments: bole-only harvest; total-tree
harvest; total-tree plus harvest; and nonharvested forest. During the study period
(March to June 2000), intensive harvesting was found to significantly increase N
availability (nitrification) in the 0.2-m depth (p=0.06). Total N concentrations at 0.2m depth in March 2000 were 1.7, 2.6, and 3.0 mg L-1 in bole-only, total-tree, and
total-tree plus harvesting treatments, respectively. These concentrations increased to
5.6, 8.2, and 6.8 mg L-1 in June 2000. Lower N concentrations in the bole-only
treatment were due to the presence of logging slash, which acted as a net sink for N.
Total soil solution N concentrations decreased on the order of 1 to 4 times as solution
passed from 0.2- to 1.0-m depths, indicating the high buffering capacity of these
volcanic soils. There were no treatment differences in total N leaching at 1.0-m depth
(p=0.24), and total N leaching was estimated at 3 to 4.5 kg ha-1 6 mo-1. Nitrogen
losses were primarily in the form of nitrate in the harvested treatments but dissolved
organic N in the nonharvested forest. While harvest intensity was found to affect
short-term soil N availability, it is not yet clear if these differences will affect
seedling growth rates.
6
Chapter 1. Introduction
The sustained productivity of intensively managed forests has been the subject
of considerable concern (Ballard and Gessel 1983; Dyck and Mees 1990).
Management efforts aimed at increasing the amount of woody biomass produced and
utilized from a forested site may potentially lead to a reduction in the productivity of
subsequent stands as a result of nutrient depletion (Weetman and Webber 1972).
Practices such as whole-tree harvesting increase the biomass yields capable from a
given area but remove a larger quantity of nutrients compared to conventional
bolewood harvesting, as foliage, branches and bark generally have higher nutrient
concentrations. In addition, site preparation practices intended to enhance seedling
establishment, such as windrowing and the piling and burning of logging residues
(branches and foliage) may also displace organic matter and nutrients. Nutrient
cycling studies have indicated that typical bolewood harvesting generally causes
nutrient losses from the forest ecosystem at rates that are comparable to rates of
natural nutrient inputs (Keenan and Kimmins 1993). However, harvesting on short
rotations and biomass harvesting may cause removals that exceed rates of natural
replacement (Wells and Jorgensen 1979). While evidence of forest productivity
declines following intensive management have been reported (Stone and Will 1965;
Keeves 1966), they are often associated more with the removal of topsoil or from
repeated removals of forest floor organic matter than from increasing harvest
intensity itself (Fox 2000).
7
Retaining logging residues on site has been suggested as a means to sustain
long-term forest productivity (Balneaves and Dyck 1992). The potential of this
organic matter in supplying nutrients to the regenerating forest will depend on the
quantity and rate at which this material decomposes and releases nutrients following
harvesting. In addition to its role as a source of nutrients, logging residues may also
have many indirect effects on the physical environment. Residues may act as a
mulch, retaining soil moisture over longer periods in the spring and early summer,
and as an insulator buffering soil temperature fluctuations. In covering the mineral
soil, logging residues may reduce erosion and influence the establishment of
competing vegetation, which can impact seedling growth and survival.
These
influences on the physical environment will in turn affect the activity of soil
microorganisms and the rate at which organic matter is decomposed and nutrients are
made available to the regenerating forest.
Nitrogen (N) is particularly important since it is the availability of this
element that commonly limits forest productivity in the Pacific Northwest (Gessel et
al. 1973). While soils in this region generally contain large amounts of N, it typically
exists in an organic form that is unavailable to plants.
The release of mineral
(available) N from decomposing organic matter is a complex and dynamic process
that is controlled primarily by the activity of microbial decomposer organisms, which
in turn is governed by the chemical constituents of the organic matter and by
environmental factors such as temperature and moisture. The contribution of logging
residues to forest productivity therefore depends on their effects on soil temperature
8
and moisture as well as the N dynamics of the decomposing organic matter. In
addition, the timing of N mineralization and availability to coincide with the nutrient
demand of the subsequent stand is of considerable concern. Nitrogen that is available
in excess of vegetative uptake may be leached to the groundwater as nitrate,
contributing to soil acidification and downstream eutrophication, while an insufficient
supply of available nutrients may limit tree growth or cause a nutrient deficiency in
the regenerating stand. While these processes have been investigated in low and
medium productivity Douglas-fir stands (Bigger 1988; Compton and Cole 1990),
little information exists concerning the response of high productivity sites to intensive
management.
9
Chapter 2. Study Objectives and Hypotheses
OBJECTIVES
The goal of this study was to examine how the intensity of harvesting and organic
matter removal affected soil nitrogen dynamics on a highly productive coastal
Douglas-fir site. Soil nitrogen availability, leaching, and rates of decomposition and
N release from forest floor organic matter were estimated during a four-month period
(March to June, 2000) 8 to 12 months after the installation of several treatments. The
treatments investigated included: (1) a conventional, bole-only (BO) harvest where
only merchantable tree stemwood was removed, with branches and foliage (logging
slash) being retained on the site; (2) a total-tree (TT) harvest where the entire
aboveground tree biomass was removed; (3) a total-tree plus (TP) harvest where
aboveground tree biomass was removed in addition to coarse woody debris and some
residual organic matter; and (4) an adjacent 47-year-old nonharvested reference stand
(FT).
10
HYPOTHESES
Hypothesis 1:
Null hypothesis: Increasing the intensity of harvesting and organic matter removal
will have no significant effect on the availability of nitrogen during the 8- to 12month period (March to June, 2000) following harvest.
Alternative Hypothesis: Increasing harvest intensity may decrease (or increase) the
availability of nitrogen during the 8- to 12-month period after harvest.
The mineralization of N from decomposing logging residues (branches and
foliage) depends on the chemical nature of the organic matter, as well as soil
temperature and moisture conditions. Generally, it is believed that organic substrates
with a carbon:nitrogen ratio (C:N ratio) between 20 and 30:1 will mineralize N, while
ratios higher than this (>30:1) will immobilize N in microbial biomass (Edmonds
1991). However, some forest residues may exhibit mineralization at C:N ratios as
high as 50 to 100:1 (Paul and Clark 1996). Logging residues contain relatively high
concentrations of N relative to other woody tree components and can act as a source
of nutrients as they decompose.
The retention of logging residues on site also
conserves moisture and its physical presence may stimulate N mineralization
(Emmett et al. 1991a). Harvesting of the entire aboveground tree biomass removes
this organic matter from the site, therefore eliminating its potential as a nutrient
source. Drier surface conditions following slash removal may also limit rates of
decomposition and N mineralization. On the other hand, microbes in decomposing
logging residues may immobilize N making it temporarily unavailable for plant
growth, thereby temporarily reducing N availability.
11
Hypothesis 2:
Null hypothesis: Increasing the intensity of harvesting and organic matter removal
will have no significant effect on the leaching of nitrogen during the 8- to 12-month
period (March to June, 2000) following harvest.
Alternative Hypothesis: Increasing harvest intensity may decrease (or increase) the
leaching of nitrogen during the 8- to 12-month period after harvest.
The quantity of N that is leached deeper in the soil profile and below the
seedling rooting zone is expected to be proportional to the quantity present (available)
in the surface soil horizons. Therefore, if the removal of logging residues during
intensive harvesting leads to reduced N availability (as proposed in Hypothesis 1),
then reduced N leaching will also be expected. Alternatively, if intensive harvesting
leads to an increase in N availability, then increased leaching may be observed.
Hypothesis 3:
Null hypothesis: Increasing the intensity of harvesting and organic matter removal
will have no significant effect on decomposition rates of forest floor organic matter
during the 8- to 12-month period (March to June, 2000) following harvest.
Alternative Hypothesis: Increasing harvest intensity may decrease decomposition
rates of forest floor organic matter during the 8- to 12-month period after harvest.
Organic matter with high N concentrations and low C:N ratios (<30:1)
generally decompose faster than materials with low N and high C:N ratios (Edmonds
1991).
The residual forest floor organic matter left on site following intensive
harvesting consists primarily of nutrient-poor, high C:N ratio (>50:1) woody material,
12
which decomposes slowly, while logging residues, with greater N concentrations and
lower C:N ratios, are expected to decompose more rapidly. In addition, removing
logging residues may result in drier moisture conditions in the residual organic
matter, thus limiting the activity of decomposer organisms.
Hypothesis 4:
Null hypothesis: Increasing the intensity of harvesting and organic matter removal
will have no significant effect on net N mineralization rates of forest floor organic
matter during the 8- to 12-month period (March to June, 2000) following harvest.
Alternative Hypothesis: Increasing harvest intensity may decrease (or increase) net
N mineralization rates of forest floor organic matter during the 8- to 12-month period
after harvest.
Logging residues act as a source of nutrients and will release (mineralize) N
as they decompose. Intensive harvesting removes this potential N source from the
site and the nutrient-poor (high C:N ratio) organic matter that remains will tend to
immobilize N or to release it more slowly. On the other hand, microbial populations
may temporarily immobilize N in their biomass as they decompose the logging slash,
thereby reducing rates of net N mineralization.
13
Chapter 3. Literature Review
3.1 Organic Matter and Forest Productivity
Soil organic matter has a major influence on plant growth because of its
beneficial effects on the physical, chemical, and biological properties and processes
occurring in soil (Chen and Aviad 1990). Its presence greatly modifies soil structure
and increases water infiltration and the water holding capacity of soil. Organic matter
also has a high cation exchange capacity and is a source of nutrients and energy for
microbial and plant growth. In the cool, moist climate of the Pacific Northwest where
decomposition rates are slow, large amounts of organic matter may accumulate on the
soil surface.
Gessel et al. (1973) estimated that the forest floor of Douglas-fir
(Pseudotsuga menziesii (Mirb.) Franco) stands in western Washington contain
between 15 and 30 Mg ha-1 of organic matter, emphasizing the important role of the
forest floor in regulating nutrient supply in these ecosystems. Soil organic matter and
its influence on the productivity of Pacific Northwest forests has also been the focus
of numerous symposia (Grier et al. 1989; Perry et al. 1989; Gessel et al. 1990).
The relationships between organic matter and forest productivity are still not
well understood (Edmonds and Chappell 1994), but there is evidence to suggest that
the removal and depletion of soil organic matter and its associated nutrients may have
negative impacts on sustained forest productivity. Early observations of productivity
declines came from intensively managed pine forests in Europe in the late 19th
century. Ebermeyer (1876) noticed that in areas where continued litter raking and
14
removal had occurred over long periods of time, tree growth was greatly reduced
compared to areas where this practice did not occur. He associated this decline in
forest productivity with nutrient depletion caused by litter removal and stressed the
importance of retaining the litter layer as a source of nutrients for tree growth.
More recently, productivity declines have been observed in second rotation
Radiata pine (Pinus radiata D. Don) plantations grown on sandy, infertile soils in
Australia and New Zealand. These declines have been attributed to a deficiency in
nitrogen (N) caused by the loss of organic matter during harvesting and site
preparation (Stone and Will 1965; Keeves 1966). Studies of successive rotations of
Radiata pine have led to practices that conserve organic matter in logging residues
and the litter layer on site in order to improve N and moisture availability to seedlings
(Squire et al. 1979; Squire 1983; Balneaves 1990).
The impact of intensive harvesting and organic matter removal on the
sustained productivity of Douglas-fir stands has recently been the focus of
investigation (Bigger 1988; Compton and Cole 1990). In western Washington, three
experimental levels of harvesting intensity were studied at both a high productivity
(Site Class II; site index 36 m in 50 years; King 1966) and low productivity (Site
Class IV; site index 25 m in 50 years; King 1966) Douglas-fir site. Harvesting
treatments involved (i) a conventional, bole-only harvest where logging slash
(branches and foliage) was left on the site; (ii) a whole-tree harvest where the entire
aboveground tree biomass was removed; and (iii) a complete removal treatment in
which all aboveground tree biomass and forest floor material was removed. Ten
15
years after harvesting and replanting, researchers observed that seedling growth
strongly reflected the intensity of organic matter removal, with a 40% height
reduction in the complete removal treatment relative to the bole-only harvest. On a
parallel set of plots, the addition of 200 kg N ha-1 of urea fertilizer at year 5
completely eliminated these growth differences, indicating that amendments of
limiting nutrients may be able to ameliorate the negative impacts of intensive
harvesting (Cole 1995).
Results from numerous studies have shown that the harvest and utilization of
the entire aboveground tree biomass (whole-tree harvesting) may greatly improve the
biomass yields from a given site (Weetman and Webber 1972; Keays and Hatton
1975). Whole-tree harvesting has recently become more common in the south and
southeast regions of North America (Kimmins 1977) and whole-tree yarding with
cable systems is routinely practiced in the Pacific Northwest (Terry 2001). In wholetree yarding, harvested trees are processed at a central landing where branches and
tops are cut. Logging slash is therefore removed from the site, but this practice is not
entirely efficient as limbs may break when the trees hit the ground and some of the
slash remains on the site rather than being transported to the landing. Site preparation
practices such as windrowing or slash piling, which are often utilized to aid in
seedling establishment, also resemble whole-tree harvesting in terms of removing
logging residues from the site but in addition may inadvertently displace topsoil.
The importance of slash and litter to the total N reserves of a site may be
substantial. On a high productivity Douglas-fir site in Washington (site index 36 m at
16
50 years), the N content of slash and litter was estimated at 464 kg N ha-1 and was
equivalent to the quantity of N removed during conventional bolewood harvesting
(Bigger and Cole 1983). The impacts of intensive harvesting may also depend on the
quantity of total nutrient reserves present at a site. Whole-tree harvesting of a low
productivity site removes a proportionately greater amount of total ecosystem
nutrients than on a high productivity site since more of the total ecosystem nutrients
are tied up in aboveground biomass. For example, bole-only harvesting of a 53-yearold Douglas-fir site removed 16% of total ecosystem N from both high- (Site Index
II) and low-productivity (Site Index IV) sites.
However, whole-tree harvesting
increased these removals to 32% of total N on the low site compared to 24% on the
high site (Edmonds and Bigger 1983).
While the retention of logging residues following harvesting has been
promoted as an operational practice to maintain forest productivity (Weetman and
Webber 1972; Balneaves and Dyck 1992), conflicting results reveal that the
importance of slash retention may be specific to the local environmental conditions
present at a given site. Some studies have supported the findings of Compton and
Cole (1990) with observations that seedlings grown in logging slash had better
growth rates than where this material had been removed, whether by harvesting or
site preparation. Ballard (1978) reported that Radiata pine trees in New Zealand
growing on harvested areas where slash was retained had 40% greater volume than
trees growing between windrowed areas, but 19% less volume than trees growing
within the windrows. The biomass of Sitka spruce (Picea sitchensis) seedlings after
17
15 months of growing in slash was twice that of seedlings growing where slash was
removed and also had greater foliar N concentrations (Emmett et al. 1991b). Titus
and Malcolm (1987) also observed that height growth of Sitka spruce seedlings in
Scotland was greater when planted in slash and attributed this to improved
microclimatic conditions, as growth was higher than in whole-tree harvested areas
that were amended with NPK fertilizer. On the other hand, some studies have
observed increased growth rates following slash removal. Valentine (1975) found
that Douglas-fir seedling growth in scarified plots where organic matter had been
removed from the surface were actually better than where slash was retained.
Logging residues act as a barrier to energy and water exchange between the soil and
atmosphere and their removal led to warmer soil temperatures and subsequently
improved root growth of seedlings grown in the scarified plots. Clearly the effect of
retaining logging residues on tree growth involves complex interactions between soil
processes that influence nutrient availability, temperature regimes, and soil moisture
dynamics.
3.2 Organic Matter Decomposition
Decomposition is the physical and chemical breakdown of organic materials
by soil organisms, primarily fungi and bacteria. Temperature, water availability, and
the quantity and quality of organic substrates have a substantial influence on the rate
at which organic matter is decomposed and nutrients are released. In general, organic
substrates with high N concentrations and low C:N ratios decompose more rapidly
than substrates with low N concentrations and high C:N ratios (Edmonds 1991).
18
Temperature has a profound effect on rates of chemical and physiological reactions.
Rates of microbial activity exponentially increase with rising temperature,
approximately doubling for every 10oC increase. This pattern holds until the upper
limit of thermotolerance is reached, typically about 35-400C. When temperatures
reach levels below 50C, soil microbial activity and organic matter decomposition
virtually cease (Brady and Weil 1999). At a given temperature, microbial processes
attain maximum rates when soil is at or near field capacity (i.e., at a matric potential
of –0.01 MPa) and decline linearly as soil matric potential becomes more negative
(Barnes et al. 1998). In addition, interactions between temperature and moisture also
affect microbial processes. For example, Zak et al. (1999) reported that high rates of
microbial activity at warm soil temperatures (e.g., 250C) are limited by the diffusion
of substrate to active cells, but that this limitation lessens as substrate demand and
physiological activity decline at cooler temperatures (e.g., 50C).
The retention or removal of logging residues will have important implications
in terms of the temperature and moisture conditions at a site and therefore will affect
the activity of soil organisms. Logging residues may act to retain soil moisture and
buffer against soil temperature fluctuations, and its ability to do this is largely a
function of the thickness of the organic layer on the soil surface. In one study,
clearcutting a Radiata pine plantation in Australia increased evaporation by 295% and
dried the litter layer at most times of the year where slash had been removed. Where
slash was retained, the organic matter had a mulching effect causing increased water
contents in litter and soil during the summer months (Smethurst and Nambiar 1990).
19
Drier surface conditions resulting from direct exposure to wind and sunlight also
contributed to the reduced decay rates of pine needles in whole-tree harvested plots in
the Rocky Mountains (Hendrickson et al. 1985). Entry et al. (1986) reported that
whole-tree harvesting reduced microbial biomass pools in summer and winter and
they were positively correlated with moisture and negatively correlated with soil
temperature.
Soil temperatures and diurnal temperature fluctuation are typically
higher where slash is removed due to the direct exposure of mineral soil to solar
radiation. Whole-tree harvesting of a Douglas-fir site led to mean maximum soil
temperatures at 5-cm depth that were about 4-50C greater in mid-spring compared to
bole-only harvesting and 9-100C higher in mid-summer (Valentine 1975). At another
site in Oregon, the diurnal temperature range 5-cm below the soil surface was two
times greater (17 versus 80C) under bare mineral soil compared to soil under slash
(Hermann 1963). Soil temperatures at this site reached a maximum of 430C in
exposed mineral soil but only 320C where litter was present. Despite high
temperatures, summer drought conditions (i.e., low soil moisture) common in the
Pacific Northwest may be more important in influencing litter decomposition rates
than temperature (Edmonds 1979).
It is often assumed that decomposition rates of organic matter on the soil
surface will increase following harvesting compared to nonharvested stands due to
increased surface temperatures following removal of the forest canopy and increased
moisture conditions associated with reduced evapotranspiration. However, results
from many studies have found that rates are highly variable and have been found to
20
either increase (Gadgil and Gadgil 1978; Edmonds 1991; Prescott et al. 1993),
decrease (Whitford et al. 1981; Binkley 1984; Prescott 1997; Prescott et al. 2000), or
remain unchanged (Will et al. 1983; Wallace and Freedman 1986) compared to
adjacent forested stands. Analyses by Yin et al. (1989) suggested that the effects of
harvesting on decomposition rates depended primarily on the regional and local
temperature and moisture regimes and are therefore site specific. In cold climates
(high latitudes) decomposition may increase in response to increased temperatures
following harvest, but temperature increases may be so extreme as to inhibit
decomposer activity in warm climates (low latitudes) (Seastedt and Crossley 1981).
Even if the decomposition potential of a site increases due to improved
temperature and moisture conditions following harvesting, decomposition rates of
organic matter may still be primarily limited by the chemical nature of the material
(Prescott 1997). The elimination of litterfall inputs following harvesting causes a
reduction in the supply of a readily available carbon (C) source to decomposers, and
organic matter decomposition rates may be slower than in the standing forest where C
is continuously added in fresh litter and canopy throughfall (Yin et al. 1989).
The effects of clearcutting on environmental conditions may also influence
decomposition rates differently depending on the depth in the forest floor at which
they are measured. Binkley (1984) found no difference in cellulose weight loss
between clearcut and nonharvested sites on Vancouver Island, British Columbia near
the forest floor surface at the interface of litter and fragmented organic layers.
However, decomposition rates were 3 to 5 times greater deeper down between the
21
fragmented and humified layers and between the humified layer and mineral soil.
This was attributed to the more favorable moisture conditions found there.
3.3 Nitrogen Mineralization
The dynamics of organic matter decomposition influence the rate at which
nutrients are released and made available to plants. Forest growth in the Pacific
Northwest is typically limited by the availability of nitrogen (Gessel et al. 1973),
even though large quantities (from 2000 to >9000 kg N ha-1) and up to 85% of the
total amount of N in a forested ecosystem are stored in soil (Cole et al. 1967;
Edmonds and Chappell 1993). Soil N exists primarily in an organic form in a wide
range of chemical compounds, such as plant and animal detritus, living biomass of
soil microorganisms, and stable organic matter and humus. At any given time, only a
very small fraction (about 1-2%) of this soil N exists in an inorganic, mineral form
that is available for plant uptake (Cole 1995).
Soil microorganisms have a major effect on soil N availability because their
growth and maintenance requirements affect the amount of inorganic N that is
released from soil organic matter. Nitrogen is made available to plants through a
process called mineralization or ammonification.
This process involves the
enzymatic digestion of simple organic compounds (proteins, amino sugars, and
nucleic acids) by microbes and the subsequent release of the ammonium (NH4+-N)
ion into the soil solution (Paul and Clark 1996).
In N limited systems such as forests in the Pacific Northwest, there is fierce
competition between plant roots, heterotrophic decomposers and autotrophic
22
microbes for available forms of N.
Once ammonium is released through
mineralization, there are many processes that control its fate. It may be rapidly
incorporated by plant roots or microbes (immobilized), chemically fixed into humus
or clays, or nitrified.
While heterotrophic decomposer organisms are generally
thought to be the most efficient competitors for NH4+, their growth and maintenance
requirements are primarily governed by a continual supply of energy in the form of
labile C (Johnson 1992; Hart et al. 1994).
3.4 Nitrogen Immobilization and C:N Ratio
Whether soil microorganisms release nutrients into or assimilate them from
the soil solution depends on the chemical constituents of the plant litter and their
suitability as an energy substrate. In addition to a labile source of C as an energy
supply, heterotrophic decomposer organisms also require external sources of N for
their growth and metabolism. The uptake and assimilation of NH4-N by microbes
into organic compounds is referred to as immobilization since ammonium is removed
from the pool of N that is potentially available for plant uptake.
The degree to which mineralized N will be available for plant uptake or
immobilized by microbes can generally be predicted by the C:N ratio of the substrate
being decomposed. Soil fungi have a C:N ratio ranging from 15:1 to 5:1 whereas
those of soil bacteria range from 5:1 to 3:1 (Paul and Clark 1996). Since leaf litter
has a C:N ratio much wider (e.g., 50:1) than that of microbial cells, its decomposition
requires the presence of an additional source of N, and immobilization of available N
typically characterizes the initial stages of litter decomposition (Edmonds 1991). In
23
general, a high C:N ratio indicates that microbes will immobilize nearly all
mineralized N. It is for this reason that the retention of carbon-rich coarse woody
debris has been proposed as a means of conserving N on site (Entry et al. 1986). A
low C:N ratio indicates that available N exceeds microbial requirements and
mineralized N will be available for plant uptake. In general, net mineralization of
needle and leaf litter is thought to occur at C:N ratios between 20 and 30:1, while net
immobilization occurs at C:N ratios above 30:1 (Paul and Clark 1996). However, this
boundary is dependent to a large extent upon the quality of the substrate (Edmonds
1991). Since forest residues often have a high lignin content that degrades slowly and
is not substantially incorporated into microbial biomass, they can often show net
mineralization at C:N ratios as high as 50:1 and even 100:1 (Paul and Clark 1996).
For woody substrates, this range may be much higher [i.e. >100:1 for twigs and
branches and >300:1 for large logs (Edmonds 1987; Sollins et al. 1987)].
In addition to N immobilization by microbial uptake and assimilation,
Johnson (1992) argued that N retention in soils might also be affected by nonbiological immobilization of N into soil humus. Humic substances may react with N
to form recalcitrant N compounds that are very slow to decompose. The dissolution
and release of organic N compounds from soil humic fractions may therefore be an
important process of the N cycle in temperate forest soils (Hedin et al. 1995). Both of
these processes are poorly understood.
24
3.5 Nitrification
When ammonium is present in excess of plant and microbial requirements,
chemautotrophic bacteria may utilize it as a source of energy in a process known as
nitrification. The oxidation of ammonium to nitrate (NO3-) is a two-step process
whereby ammonium is used as an energy source to fix CO2 from the soil atmosphere,
according to the following biochemical reactions:
NH4+ + 1.5O2  NO2- + 2H+ + H2O
NO2- + H2O  NO3- + 2H
2H + 0.5O2  H20
The nitrifying bacteria Nitrosomonas and Nitrobacter that perform this
transformation are sensitive to pH, water potential, and aeration. While a soil pH of
6.0 is the ideal range for nitrifying bacteria, nitrification in acid forest soils may occur
at pH values of less than 4.0. Nitrification rates are most rapid at matric potentials of
–0.01 to –1.0 MPa, and require the presence of O2 (Paul and Clark 1996).
Nitrification occurs most readily at temperature near 300C and is negligible at low
soil temperatures, below about 8 to 100C (Brady and Weil 1999). Compared to
microbial heterotrophs, nitrifying bacteria are generally believed to be poor
competitors for NH4+ and consequently their populations are typically low in N
limited systems (Fisk and Fahey 1990).
However, nitrifiers may be successful
competitors under conditions of stationary or declining populations of heterotrophs
(Hart et al. 1994), as may occur following clearcutting and the removal of tree
nutrient uptake (Entry et al. 1986; Emmett et al. 1991b). Net nitrification in many
25
acid forest soils is not frequently observed but an active nitrifier population may still
be present and high rates of gross nitrification may be taking place with rapid
assimilation of nitrate by plants and microbes (Stark and Hart 1997).
Like ammonium, nitrate may also be utilized as a source of available N by
plants and microbes for growth and metabolism. Nitrate may also be denitrified and
lost as a gas or leached with groundwater. Nitrification results in the production of
H+ ions and therefore contributes to soil acidification (Johnson and Lindberg 1991).
The release of hydrogen ions in the formation of nitrate and the high mobility of the
nitrate anion itself also promotes the mobilization and loss of base cations from the
soil exchange complex. The leaching of nitrate with groundwater represents a loss of
available nutrients (both N and cations) and is also a cause for concern over the
degradation of water quality (Marco et al. 1999).
3.6 Nitrogen Availability
The net availability of nitrogen in the soil solution is a complex interaction
between many processes that either add or remove nutrient ions to solution (Fig. 1).
Nutrients are added through atmospheric deposition, throughfall, weathering, and
mineralization, and removed via plant and microbial uptake (immobilization),
leaching, or gaseous losses (denitrification). The microbially mediated processes of
N mineralization and immobilization control the transfer of N from soil organic
matter to the soil solution where it is available for uptake up by plant roots. Plants
absorb mineral elements mostly in the form of ions from the soil solution and directly
26
Figure 1. Generalized fluxes of nutrient elements to and from soil solution.
Source: Swift et al. 1979.
from soil colloids (Cole 1995).
Douglas-fir trees are capable of absorbing and
assimilating both ammonium and nitrate ions (Radwan and Brix 1986), and recent
evidence also suggests that some plant species may be able to directly utilize some
simple forms of dissolved organic N (DON, e.g., amino acids) through symbiotic
associations with mycorrhizal fungi (Nasholm et al. 1998).
Increases in the availability of N and other nutrients are commonly reported
after forests are clearcut (Bormann and Likens 1979; Vitousek et al. 1979; Smethurst
and Nambiar 1990) and this has been a well-known phenomenon supplying shortterm increases in soil nutrient availability to traditional slash and burn cultivation.
After clearcutting there are often changes to populations of soil invertebrate fauna and
of fungal and bacterial decomposers due to changes in the soil environment (Seastedt
27
and Crossley 1981; Maybury 1993). Moisture stress is often diminished due to
reduced transpiration rates and increased temperatures near the soil surface stimulate
the activity of soil organisms. Large quantities of organic matter in logging residues
and dead roots supply a source of energy and nutrients to these organisms. In the
absence of plant uptake, a flush of available nutrients typically begins during the year
or two following harvest and continues until the readily decomposed organic matter
has been immobilized by microbes, mineralized, or converted into stable humus
(Kimmins 1997). At a site on Vancouver Island, clearcutting of mixed conifer forest
led to the accumulation of 7 to 20 times more inorganic N on ion exchange resins
than in nonharvested sites, reflecting a combination of reduced uptake, increased
mineralization rates, and/or the increased mobility of ions associated with moister soil
(Binkley 1984). Elevated concentrations of mineral N may persist in clearcuts for
several years after harvesting even despite leaching losses (Smethurst and Nambiar
1990).
Decomposition and mineralization of the residual organic matter following
harvesting are often observed and may contribute to improved N availability, but this
is not always the case (Covington 1981; Kimmins 1997). Reduced plant uptake of
mineral N following clearcutting may by itself explain observations of increased
nitrogen availability, even if mineralization rates remained unaltered by cutting.
Where increased mineralization rates have been reported, they were attributed to
physical changes following harvesting, such as warmer and wetter soil conditions
(Marks and Bormann 1972; Stone 1973) and increased frequency and intensity of
28
wetting and drying cycles in the forest floor (Campbell et al. 1971). Inputs of readily
decomposable organic matter in branches, foliage, and roots combine to produce a
more beneficial environment for microbial activity (Edmonds and McColl 1989).
Increased rates of N mineralization after clearcutting have also been attributed to
decreased competition between saprophytic microorganisms and mycorrhizae (Gadgil
and Gadgil 1978).
Two contrasting theories have been described to predict the implications of
retaining logging residues on the availability of nutrients and their subsequent losses.
Wells and Jorgensen (1979) speculated that removing logging residues that would be
subject to decomposition and nutrient release would lead to decreased N
mineralization rates relative to bole-only harvesting, decreased pools of available N,
and therefore decreased leaching. On the other hand, Vitousek (1981) proposed that
slash removal would decrease the potential for microbial immobilization, thereby
increasing net N mineralization, available N pool sizes, and leaching losses.
Many studies have supported the theory of Wells and Jorgensen (1979) with
observations of greater nutrient availability where slash has been left to decompose
(Vitousek and Matson 1985a, 1985b; Hendrickson et al. 1985, 1989; Rosen and
Lundmark-Thelin 1987; Smethurst and Nambiar 1990). Soluble nutrients may be
leached from slash and the physical presence of organic matter may also stimulate
mineralization (Emmett et al. 1991a). Increased mineralization was observed under
slash for three consecutive years (17%, 17%, and 32% greater than slash removal)
following clearcutting of Radiata pine stands and was attributed to higher rates of
29
mineralization in the litter layer which was wetter under slash (Smethurst and
Nambiar 1990).
Vitousek and Matson (1985a) also observed higher rates of
mineralization in soil under loblolly pine (Pinus taeda) slash (65 kg N ha-1 yr-1 in
bole-only harvest compared to 55 kg N ha-1 yr-1 in whole-tree harvest), but changes in
soil temperature and moisture after clearcutting could not account for these
differences.
Slash retention also increased overall microbial biomass pools,
stimulated microbial activity and the decomposition of organic matter, as well as the
nitrification rate and subsequent release of nitrate.
Removing slash reduced
nitrification rates and nitrate losses (Emmett et al. 1991b).
However, other studies have supported the theory of Vitousek (1981). Higher
N availability in clearcuts may be related more to the reduced assimilatory capacity of
microbes in the absence of labile C sources (litterfall and root exudates), rather than
faster N mineralization rates per se (Prescott 1997). A reduced input of fresh carbon
in litter after clearcutting may reduce microbial biomass and its capacity for
immobilizing N.
Reductions in soil microorganisms have been reported after
clearcutting of forests (Vlug and Borden 1973; Sundman et al. 1978; Seastedt and
Crossley 1981), but retaining slash has been shown to increase microbial biomass
pools relative to whole-tree harvesting (Hendrickson et al. 1985; Entry et al. 1986;
Emmett et al. 1991b). Vitousek and Matson (1985a) reported that immobilization of
N in microbial biomass was the major process retaining N after clearcutting loblolly
pine forests and that retaining residues as substrates for microbes increased N
retention. A diminished capacity for microbial immobilization was found where
30
intensive site preparation had occurred (Vitousek and Matson 1985b) and has been
implicated as a cause of elevated N losses (Stark and Hart 1997). The importance of
N immobilization was further demonstrated in a tracer study in a loblolly pine
ecosystem, where 60-85% of added
15
N-ammonium sulfate was held in microbial
biomass after one growing season (Vitousek and Matson 1984).
3.7 Nitrogen Leaching
The high demand for growth-limiting nutrients such as N by overstory trees
typically maintains low concentrations of these elements in soil solution. In nondisturbed forests, nitrogen losses are small and occur primarily as dissolved organic
matter or soil suspended in streamflow (Fredriksen 1971). Nitrate concentrations in
streams draining mature Douglas-fir stands may average only 0.002 mg L-1 and
represent annual losses of 0.04 kg N ha-1 (Fredriksen 1972).
Following harvesting, nutrient uptake by vegetation is reduced or nearly
eliminated (particularly when combined
with
herbicide treatments) while
decomposition and mineralization may continue or increase, thus increasing the
amount of dissolved inorganic nutrients in solution. Decreased evapotranspiration
causes greater soil water movement, and along with increased ion concentrations
leads to increased ion transport to streams.
The high mobility of nitrate in
combination with the low anion exchange capacity of most soils makes nitrate the
form of N most often leached from terrestrial ecosystems.
The most well-known and controversial evidence of increased nitrate leaching
following clearcutting came from the Hubbard Brook Experimental Forest in New
31
Hampshire (Likens et al. 1970; Bormann and Likens 1979).
Clearcutting of a
northern hardwoods watershed followed by complete herbicide control of regrowing
vegetation for three years led to a dramatic loss of up to 105 kg NO3-N ha-1 yr-1 (a 65
fold increase over an uncut reference watershed) and created much concern for the
effects of clearcutting and herbicide application on water quality. When vegetative
regrowth in this hardwood ecosystem was not controlled, NO3- concentrations in
streamwater still increased substantially after clearcutting, from 1 mg L-1 to a high of
23 mg L-1 (Pierce et al. 1972). Subsequent studies have rarely reported leaching
losses of the magnitude seen at Hubbard Brook, but some have demonstrated that the
leaching of inorganic N may be slightly elevated in recently clearcut forests compared
to mature reference stands (Fredriksen 1971; McColl and Grigal 1979; Sollins and
McCorison 1981; Stevens and Horung 1990).
Other studies have reported that
clearcutting had no significant effect on N leaching losses (Cole and Gessel 1965;
Aubertin and Patric 1974; Silkworth and Grigal 1982).
In a series of watershed studies in Oregon, Fredriksen et al. (1975) reported
very small increases in streamwater nitrate concentrations after clearcutting, with a
typical delay of one to two years. Feller and Kimmins (1984) observed that losses of
dissolved nitrogen in streamwater in British Columbia were approximately 4 times
greater than pre-harvest levels the first year after clearcutting and 14 times greater
during the second year.
This lag period has been attributed to several factors,
including low summer rainfall, decomposition products are not flushed out of the soil
32
until midwinter, and because it usually takes at least one summer before appreciable
mineralization can occur.
In addition to the leaching of inorganic forms of nutrients, dissolved organic
N (DON) also represents an important form of N and generally comprises most of the
total N leaching from the forest floor of intact forests (Sollins et al. 1980; Emmett et
al. 1991a; Johnson and Lindberg 1991; Qualls et al. 1991). DON has been reported
to be the most important form of N (over 95%) in streamwater draining from many
mature undisturbed or aggrading forested watersheds (Sollins and McCorison. 1981;
Hedin et al. 1995). Dissolved organics may originate from many sources, including
the leaching of tannins from dead and fragmented bark, the leaching of soluble
organics from increasingly porous and fragmented decaying wood, the dissolution of
lignin and other constituents by microbial enzymes, and the leaching of microbial
biomass such as that of shelf fungi (Qualls et al. 2000). The slow gradual release of
potentially soluble organics from detritus may also be an important mechanism
controlling N losses (Vitousek et al. 1979).
DON concentrations may increase
following disturbance in some forests (Sollins and McCorison 1981) but usually not
to the very high levels of nitrate observed (Vitousek and Melillo 1979). While many
studies have focused on the leaching of inorganic nutrients, the importance of DON in
solution transport in intact forests and the sudden inputs of potentially soluble
nutrients in logging slash suggest that organic forms may also be important in the
retention or loss of nutrients after clearcutting. Bormann and Likens (1979) estimated
that a large amount of N lost from the forest floor after clearcutting at Hubbard Brook
33
could not be accounted for in either streamwater, inorganic N, or in vegetation
regrowth, and they hypothesized that it may have been translocated to deeper soil
horizons and stored.
The relative importance of logging residues in contributing to nutrient losses
or retaining nutrients on site remains uncertain. It has been suggested that forest floor
disturbance be minimized and organic matter (including woody debris) retained
during logging to provide for nutrient retention and N fixation (Harvey et al. 1980;
Entry et al. 1987; Jurgensen et al. 1992).
Nutrient availability has often been
reported to be greater under patches of slash (Vitousek and Matson 1985a;
Hendrickson et al. 1985, 1989; Smethurst and Nambiar 1990; Emmett et al. 1991a),
leading to greater hydrologic losses of nutrients with conventional harvesting
compared to whole-tree harvesting (Rosen and Lundmark-Thelin 1987). Despite
larger microbial N pools where slash is retained, increased rates of microbial turnover
and the stimulation of nitrification may lead to increased nitrate leaching losses
(Emmett et al. 1991b). Other studies have observed no differences in nitrate leaching
between bole-only and whole-tree harvesting (Johnson et al. 1982; Bigger 1988).
Data from 11 different forest types across the U.S. indicated that the differences in
nitrate losses between these two harvesting types were small (Mann et al. 1988).
Several well-known retention mechanisms explain the buffering capacity of
soils and why leaching losses rarely approach the potential losses represented by
increases in N availability.
Mechanisms controlling nitrate loss are primarily
biological and hydrologic, while the loss of DON is controlled by geochemical and
34
hydrologic mechanisms. Nutrient uptake by vegetation and microbial decomposers
and denitrification remove ammonium, the energy supply for nitrifiers, from the soil
solution.
Competition between nitrifiers, decomposers, and plant roots prior to
disturbance may cause low initial populations of nitrifying bacteria (Vitousek et al.
1979; Fisk and Fahey 1990), and in combination with allelochemical inhibition of
nitrifying bacteria (Vitousek et al. 1982), may keep nitrification rates low or delayed.
Nitrate and DON adsorption on anion exchange sites and by iron and aluminum
oxides may also remove these constituents from soil solution as it moves through the
soil profile. A lack of percolating water, particularly in dry summer months, also
reduces leaching losses.
In the Pacific Northwest, large amounts of carbon-rich organic matter on the
forest floor and less active nitrifier populations in these N-limited systems ensure that
annual leaching losses are typically low (Vitousek et al. 1982). Leaching losses
following harvesting are reported to be quite small, on the order of <5 kg N ha-1 yr-1
(Bigger 1988; Compton and Cole 1990; Johnson and Lindberg 1991) and probably do
not have a long-term detrimental effect on site productivity (Cole 1981). Leaching
losses are often short-lived and negligible when compared to the total losses
associated with nutrient removals that occur during harvesting (Sollins and
McCorison 1981; Mann et al. 1988; Hendrickson et al. 1989). Natural inputs in
atmospheric deposition and free-living N2 fixation over the course of a year (1 to 6 kg
N ha-1 yr-1) may generally balance the loss of N from leaching (Vitousek and Melillo
1979; Hornbeck et al. 1986; Tiedemann et al. 1988).
35
Chapter 4. Materials and methods
4.1 Site
The site investigated in this study was a high productivity (Site Index II+)
Douglas-fir stand located in Pacific County, southwestern Washington (46o43’N,
123o36’W) on Weyerhaeuser Company’s McDonald Tree Farm. The Boistfort soil
series found in this area is a very deep, well-drained, medium- to moderately-fine
textured silt loam developed from Eocene basalt with 5–20 % volcanic ash in the
surface horizon (Steinbrenner and Gehrke 1973). It is classified as a mesic Typic
Fulvudand and has a dark brown (7.5 YR 3/2) A horizon about 15-20 cm deep with a
mean bulk density of 0.50 g cm-3 and pH of 4.2. The site is gently sloping with an
elevation of 300 m and a west aspect. This region experiences a maritime climate
with cool dry summers and moderate wet winters. During 1999, the mean annual
temperature at the study site was 9.4oC, with an average monthly temperature of
3.3oC in January and 15.6oC in July (Davis Instruments). Mean annual precipitation
in 1999 was 210 cm and occurred as rain mainly from October through March.
The study area was in the western hemlock (Tsuga heterophylla (Raf.) Sarg.)
zone described by Franklin and Dyrness (1973). The second-growth stand of planted
Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and naturally regenerated
western hemlock that existed on the study site had previously received a cumulative
urea fertilizer application of 1800 kg N ha-1 over the course of the second rotation.
The stand was clearcut harvested during April to July of 1999. Stand age was
36
approximately 47 years and estimated mean height was 40 m (Site Index II+ to I, site
class 41-43 m in 50 years; King 1966).
Control of competing vegetation was
performed on March 6-7, 2000 by herbicide (Oust and Accord) application. The site
was subsequently planted with 2-year-old (1+1) Douglas-fir seedlings selected for
minimum height variability on March 20-21, 2000 at 1600 trees ha-1 (2.5 * 2.5 m
spacing).
4.2 Experimental Design
The experiment was established in 1998 to examine the impacts of intensive
forest management practices on a high productivity coastal Douglas-fir stand. It was
installed as a randomized, complete block design experiment (Fig. 2) with a total of
twelve treatments (Table 1) replicated in four blocks at one study site. Treatment
plots were 0.25 ha (30 m x 85 m) in size, with central assessment plots of 0.10 ha (15
m x 70 m) surrounded by a buffer zone. The specific focus of this study was on three
organic matter removal levels only: bole-only harvesting, total-tree harvesting, and
total-tree plus harvesting. These treatments had all received herbicide application in
order to eliminate potential differences caused by competing vegetation, which might
confound the study results.
37
Figure 2. Map of study site showing four randomized blocks, three levels of
harvesting, boundaries of nonharvested reference stands, lysimeter sampling
points, and weather station locations.
38
Table 1. Experimental treatments in the complete randomized block design
study. Treatments 1, 5, and 7 were the focus of this investigation.
Treatment
Organic Matter & Nutrient Treatments
Compaction Treatments
Harvest
Nitrogen Vegetation
Soil
Soil
a
Level
Fertilization
Control Compaction Tillage
1
Bole-only
---b
+b
---
---
2
3
4
5
6
7
8
9
10
11
12
Bole-only
Bole-only to 5 cm top
Bole-only to 5 cm top
Total-tree
Total-tree
Total-tree plus
Total-tree plus
Bole-only
Bole-only
Bole-only
Bole-only
+
--+
--+
--+
--+
+
+
+
+
+
+
+
+
+
----+
+
------------------+
+
--------------------+
a
Fertilization treatment (rate and timing) to be determined when there is a difference
in tree growth among organic matter treatments or at time of crown closure,
whichever occurs first.
b
Treatment applied = (+); Treatment not applied = (---).
4.3 Treatment Installation
The second-rotation stand that existed at the study site was clearcut harvested
from April to July 1999 using a conventional cable system with a CAT 330L 2-drum
shovel yarder. All trees on the study site were hand-felled so that the tops remained
within the boundary area of the treatment plots. Harvesting treatments involved three
levels of organic matter removal:
39
(1) Bole-only (BO) harvest. Fig. 3. In the conventional, bole-only harvest
treatment, only the bolewood was removed from the plot. All butt cuts and nonmerchantable wood were left in place. Logging slash (foliage and branches) was
retained and distributed uniformly across the plot.
(2) Total-tree (TT) harvest. Fig 4. In the total-tree (or whole-tree) harvest
treatment, all aboveground tree biomass was removed from the plot, including all live
limbs and most large dead branches greater than 2.5 cm in diameter. The majority of
remnant old-growth logs, coarse woody debris and dead limbs were retained on the
plot.
(3) Total-tree plus (TP) harvest. Fig. 5. In the total-tree plus treatment, all
aboveground tree biomass was removed as in the TT harvest treatment. In addition,
all coarse woody debris, old-growth logs, and dead limbs greater than 0.6-cm
diameter were removed from the plot. Forest floor material was retained on all of the
plots, although there was some inadvertent displacement and mixing with mineral soil
on the TT and TP plots during mechanical removal of the logging slash and coarse
woody debris.
Total-tree and TP harvesting treatments were not expected to
represent standard operational harvesting practices but were designed to investigate
the relationships between organic matter and subsequent forest productivity. Future
rotations of intensively managed forests may lack substantial inputs of coarse woody
debris, and the role of this ecosystem component in forest productivity is not well
understood (Maser and Trappe 1984; Stevens 1998).
40
Figure 3. Conventional, bole-only (BO) harvesting retained branches, foliage,
and tops on site following removal of merchantable bolewood.
Figure 4. Total-tree (TT) harvesting removed the entire aboveground tree
biomass but retained most coarse woody debris.
41
Figure 5. Total-tree plus (TP) harvesting removed all aboveground tree
biomass in addition to most coarse woody debris.
Figure 6. Nonharvested forest (FT) where no harvesting treatment occurred
served as a reference stand. Note lysimeter sampling tubes.
42
In addition to the three harvesting treatments described above, four test plots
were located in the adjacent nonharvested forest (FT, Fig. 6) that surrounded the
south and west sides of the study site (Fig. 2). These stands were similar in age and
species composition to the study-site stand that was harvested. Although this was not
part of the original study design, it provided a useful comparison between harvested
and nonharvested treatments. This treatment is hereafter referred to as “nonharvested
forest” or “forest”.
4.4 Soil solution collection
Tension lysimeters (Lajtha et al. 1999) were used to collect and monitor soil
solution chemistry at 0.2-m and 1.0-m depths.
Tube lysimeters (Fig. 7) were
constructed from PVC pipes and porous ceramic cups (Soilmoisture Equipment
Company, Santa Barbara, CA, model 653X05-B01M3). The standard 0.1 MPa (1
bar), round-bottom, neck-top cups used had a maximum pore size of 2.5 m and were
6.5 cm long, with a 6.1 cm outside diameter.
Lysimeters were installed in October 1999 in nested pairs in the BO, TT, and
TP treatments. For each treatment plot, one lysimeter pair was randomly located
under organic matter coverage most frequently occurring within that treatment. Holes
were dug using either a hand or gas-powered auger such that lysimeters were angled
into the ground at approximately a 35o angle in order to minimize disturbance to the
soil profile above the ceramic cups. Soil was packed in around the sides of the tubes,
a plug of bentonite clay was poured around the tube at the soil surface to limit
43
subsurface flow into the lysimeter channel, and the forest floor was replaced. In
December 1999, a total of four more lysimeter “nests” were installed in the same
manner in the adjacent nonharvested stands (FT) and were located more than 40 m
inside the forest boundary in order to minimize potential edge effects. A total of 32
lysimeters were installed and monitored during the study period.
Figure 7. Lysimeter “nest.” Adapted from Harrison 1999 (not to scale).
44
During December 1999 to February 2000, lysimeters were prepared for
sampling by repeatedly pulling soil solution (approximately 1.0 L total) through the
ceramic cups in order to equilibrate their exchange sites with the soil-water solution.
Following lysimeter preparation, soil solution samples were collected on March 3,
May 4, and June 26, 2000.
Several days before each collection, a vacuum of
approximately 0.05 MPa (0.5 bar) was created in the tubes using a two-way hand
pump. Soil solution that accumulated in the tubes was collected and stored at 2oC for
up to several days until analyzed.
Soil solution pH was determined using a pH meter (Orion Research Model
230A, 1997).
Ammonium (NH4+-N) concentrations in the soil solutions were
determined colorimetrically using a modified indophenol blue method (Mulvaney
1996) and a spectrophotometer (Perkin-Elmer Model 55E, Norwalk, CT). Nitrate
(NO3--N) concentrations were determined on an autoanalyzer (Perstop Analytical 500
Series Flow-injection). Dissolved organic nitrogen (DON) was determined as the
difference in NH4+-N concentrations before and after digestion in a mixture of
H2SO4/H2O2 (Allen et al. 1974).
Lysimeters installed at the 0.2-m depth were approximately at the same level
as the seedling rooting zone, and therefore ionic mineral forms of N (NH4-N and
NO3-N) found in soil solutions collected here were assumed to be available for root
uptake. Lysimeters at a depth of 1.0 m in the soil profile were well below the
seedling root zone, and therefore nutrient elements sampled here were assumed to be
unavailable for seedling uptake.
Monthly nitrate and DON leaching rates (kg ha-1
45
mo-1) during the sampling period (February to July, 2000) were estimated based on
solution concentrations at the 1.0-m depth. Precipitation and air temperature were
recorded at 30-minute intervals by two weather stations (Davis Instruments, Inc.,
Weather Monitor II) located to the east and southwest of the study site (Fig. 2).
Thornthwaite’s equation, based primarily on mean monthly air temperatures, was
used to calculate net monthly evapotranspiration rates (Dunne and Leopold 1978).
Net monthly water flux was calculated by subtracting this evapotranspiration estimate
from mean monthly precipitation.
Monthly leaching rates were then calculated
according to the equation:
Leaching (kg ha-1 mo-1) = net water flux (cm mo-1) * [NO3--N] (mg L-1) *
water density (1000 kg m-3) * 10,000 m2 ha-1 * 10-6 L mg-1 * 10-2 m cm-1
4.5 Forest Floor Collection
On March 21, 2000, forest floor organic matter was sampled from each of the
four treatments (BO, TT, TP, and FT). Forest floor material was collected from a
0.25-m2 area (0.5 x 0.5 m) in three randomly located subplots in each plot. Only
undisturbed, intact forest floor was sampled.
Forest floor that had experienced
considerable mixing with mineral soil or that had large amounts of rotting wood
present were not sampled. Live vegetation and woody material greater than 0.6-cm
diameter was removed before collection. Forest floor samples were sieved with an
11.2-mm (7/16”) sieve to remove moss and twigs. Wet weights of sieved samples
were recorded in the laboratory. Moisture content (water weight * oven-dry weight-1)
46
of each sieved forest floor sample was determined by oven-drying a subsample at
70oC, and total forest floor dry weight (kg ha-1) was calculated. Field-moist forest
floor samples were then combined by treatment and carefully homogenized to obtain
a single, uniform composite sample. Three subsamples of each composite material
(BO, TT, TP, and FT) were then taken for determination of initial characteristics.
A pH meter (Orion Research Model 230A, 1997) was used to measure pH on
field-moist samples using a 5 parts (by weight) deionized water to 1 part forest floor
ratio. Concentrations of inorganic N were assessed on 15 g of field-moist material by
extraction with 100 mL 2M KCl. The resulting extract was analyzed for NH4+-N
colorimetrically using a modified indophenol blue method (Mulvaney 1996) and a
spectrophotometer (Perkin-Elmer Model 55E, Norwalk, CT). Nitrate concentrations
were determined on an autoanalyzer (Perstop Analytical 500 Series Flow-injection).
Subsamples were oven dried at 70oC, ground to 1 mm in a Wiley mill, and analyzed
for total C and N contents using a CHN analyzer (Perkin Elmer 2400, Norwalk, CT).
Percent organic matter (volatile solids) was determined by weight change of ovendried material following combustion in a muffle furnace at 500oC for 4 hours.
Estimates of water holding (field) capacity were determined on cores of field-moist
forest floor samples using a pressure plate apparatus as described by Cassell and
Nielsen (1986).
47
4.6 Decomposition and net N mineralization rates of forest floor organic matter
In order to examine rates of decomposition and net N mineralization of forest
floor organic matter in the field, an incubation technique using porous ceramic cups
was utilized (Henry et al. 2000). This method has been used extensively in biosolids
mineralization studies and the hydrophilic porous ceramic maintains moisture
equilibrium with the soil, as well as allowing for the transfer of gases and leachates.
However, the small pore size of the ceramic cups (2.1 m) has been found to limit
microbial exchange with the surrounding environment (Krejsl et al. 1994).
Because of the ability to transfer water and dissolved compounds,
mineralization estimates from ceramic cup incubations were expressed as a change in
the mass of organic nitrogen, rather than as a change in inorganic N concentrations.
The standard 0.1 MPa (1 bar) cups (Soilmoisture Equipment Company, Santa
Barbara, CA, model 653X06-B01M1) used in this study measured 3.6 cm outside
diameter by 9.5 cm in length. Cups were partially filled with 8 g of field-moist
material (approximately 3 g oven-dry weight basis), capped, weighed, and installed
on March 28, 2000 in the same treatment from which the material originated. In each
plot, a set of 3 cups were randomly placed beneath the organic layer in close contact
with mineral soil at a representative site within a 10-m radius of the lysimeter nests.
In addition, blank cups were also included to adjust for weight changes to the cups
themselves due to soil adhesion or precipitation. For each treatment, two temperature
data loggers (Hobo H8, Onset Computer Corp., Pocasset, MA) placed adjacent to the
48
cups monitored temperatures at the forest floor/mineral soil interface (approximately
3-cm depth).
At intervals of 4, 8, and 12 weeks (on April 25, May 26, and June 19, 2000),
one set of cups was removed from each plot (n=4), cleaned, and dried to constant
weight at 70oC. Dry weight of incubated material was recorded for each collection
period. In addition, changes in moisture content (water weight * oven-dry weight-1)
of forest floor organic matter over time were determined on additional samples
collected adjacent to the cups at each collection period by drying them to constant
weight at 70oC. For incubated samples collected at 12 weeks, a subsample of fieldmoist material was removed from the cup before drying, extracted in 2M KCl and
analyzed for NH4+-N and NO3--N, as above. Oven-dried material was ground to 1
mm and analyzed for total C and N contents on a CHN analyzer (Perkin Elmer 2400,
Norwalk, CT).
Decomposition rates for each collection period were calculated as the percent
of initial mass lost, according to the following equation:
Percent mass loss = (Initial weight - Final weight)/(Initial weight) * 100
Net N mineralization rates were calculated as the change in organic nitrogen
content after 12 weeks of incubation, according to the following equation:
49
ON mineralization = [(Mi * ONi) - (Mf * ONf)] / (Mi * ONi) * 100
Where: Mi = initial mass of material
Mf = final mass of material
ONi = initial concentration of organic N
ONf = final concentration of organic N
[Where ON = total N – inorganic N (NH4+-N + NO3--N)]
4.7 Statistical Analyses
The overall effects of harvesting treatment and collection date (and their
interaction) on concentrations of N species in soil solution were tested for each
sampling depth separately using a two-way multivariate ANOVA. The overall effects
of harvest intensity and sampling depth (and their interaction) on N concentrations
were also tested for each collection date separately by using a two-way multivariate
ANOVA. Where significant differences occurred (p<0.10), a univariate ANOVA for
each treatment, depth, and date combination followed by a Tukey’s Honestly
Significant Difference (HSD) post-hoc test was used to separate effects.
A
significance level of 0.10 was chosen due to the low number of replications in this
study and the high variability expected, and this level may be more appropriate than
0.05 for biological investigations (Conquest 1998).
The effect of sampling depth was also tested for each treatment and date
combination separately using an independent samples t-test. In addition, comparisons
between the nonharvested forest (FT) and each harvested treatment were performed
separately for each date and depth combination using an independent samples t-test
since FT was not part of the randomized complete block design. It is recognized that
50
the FT treatment was not blocked as the harvesting treatments were, but stand
conditions in FT were similar in species composition and age relative to the study
site.
The initial characteristics of the forest floor organic matter collected from the
harvesting treatments were compared using a one-way multivariate ANOVA statistic,
and where significant differences occurred (p<0.10), a Tukey’s HSD post-hoc test
was used to separate effects. The results of these analyses were then used as the
initial conditions of the forest floor materials in the field incubation.
The overall effects of harvesting treatment and time (and their interaction) on
decomposition rates of forest floor organic matter and moisture content of fieldincubated materials were analyzed by two-way ANOVA.
Where significant
differences occurred (p<0.10), a one-way univariate ANOVA for each treatment and
time combination followed by a Tukey’s HSD post-hoc test was used to separate
effects. The effects of harvest intensity on field net N mineralization rates after 12
weeks were analyzed with a one-way univariate ANOVA. In addition, comparisons
between nonharvested forest (FT) and each harvested treatment were performed
separately using an independent samples t-test.
All statistical analyses were
conducted using the Statistical Package for the Social Sciences (SPSS v. 8.0) (Nie et
al. 1975).
51
Chapter 5. Results and Discussion
5.1 Soil solution chemistry
5.1.1 Nitrogen availability at 0.2-m depth
5.1.1.1 Forms of nitrogen
In soil solutions collected from lysimeters at 0.2-m depth on March 3, May 4,
and June 26, 2000, nitrate (NO3-N) was the dominant form of N in the harvested
treatments and represented from 55 to 96% of the total N in solution (Fig. 8). The
remainder existed mostly as dissolved organic N (DON), with ammonium (NH4-N)
representing only a few percent of the total. Large concentrations of NH4-N were not
expected to be present in soil solution. The positively charged NH4+ ion is typically
bound to negatively charged particles in the soil exchange complex and exists in
equilibrium with soil solution concentrations.
For this reason, soil solution
concentrations of ammonium reflected this equilibrium rather than representing an
absolute amount of exchangeable or available ammonium.
Competition for
ammonium between plant roots, heterotrophic microorganisms, and nitrifying
bacteria further ensures that levels generally remain low.
As an anion, NO3- is not held strongly in the soil exchange complex and
typically exists as a free ion in solution. Although NO3-N concentrations reached
rather high levels (about 7 to 9 mg L-1) during the June 26, 2000 sampling date in the
TT and TP treatments, the mean concentration was still below the drinking water
quality standard of 10 mg L-1 (FWPCA 1970).
52
12.00
NH4-N
DON
NO3-N
N concentration (mg L -1)
10.00
8.00
*
*
6.00
*
4.00
2.00
*
*
*
b
ab
a
*
0.00
3/3/00
5/4/00
Bole-only
6/26/00
3/3/00
5/4/00
Total-tree
6/26/00
3/3/00
5/4/00
6/26/00
Total-tree plus
3/3/00
5/4/00
6/26/00
Forest
Figure 8. Concentrations (mg L-1) and forms of N in soil solution collected at 0.2-m
depth. Error bars represent one standard deviation from the mean. Different letters
represent significant differences between harvesting treatments (p<0.10, ANOVA,
Tukey's). An asterisk (*) represents a significant difference from nonharvested forest
(p<0.10, t-test).
5.1.1.2 Effect of harvest intensity
Results from the two-way ANOVA analyzing the effects of harvesting
treatment and time on soil solution chemistry at 0.2 m (Table 2) showed that there
was a significant overall treatment effect on concentrations of NO3-N (p=0.06), total
inorganic N (p=0.07), total N (p=0.05), and pH (p=0.04). There were no significant
effects of harvesting treatment on ammonium or DON concentrations, but there was a
significant interaction effect between these two variables.
53
Table 2. Overall effects of harvesting treatment, time, and their interaction on N
concentrations (mg L-1) and pH of soil solution by sampling depth. Values are
significance levels from multivariate two-way ANOVA.
Harvesting
Treatment
Effect
Sampling
Depth
a
Time
Effect
Trt * time
Interaction
0.2 m
NO3-N
NH4-N
total inorganic N
DONa
total N
pH
0.06+
0.28
0.07+
0.21
0.05+
0.04*
0.00**
0.27
0.00**
0.59
0.00**
0.04*
0.89
0.08+
0.88
0.06+
0.57
0.99
1.0 m
NO3-N
NH4-N
total inorganic N
DONa
total N
pH
0.27
0.75
0.26
0.62
0.24
0.95
0.87
0.20
0.85
0.78
0.76
0.36
0.76
0.84
0.76
0.76
0.62
0.67
DON=dissolved organic nitrogen
+ denotes significance at p < 0.10 level, ANOVA
* denotes significance at p < 0.05 level, ANOVA
** denotes significance at p < 0.01 level, ANOVA
There was a very clear trend between nitrate levels and harvesting treatments
(Fig. 8). For a given sampling date, nitrate concentrations typically followed the
pattern BO<TT<TP. However, one-way ANOVA results (Table 3) showed that this
treatment effect was only significant on March 3, 2000 (p=0.04). This was mainly
due to the greater variability of NO3-N concentrations over time and low sample size.
It was determined that increasing the number of lysimeters to between 8 and 10 per
treatment would allow for the detection of most of the differences between treatments
54
Table 3. N concentrations (mg L-1) and pH of soil solution collected at 0.2-m depth from harvested treatments by sampling date.
Values are means followed by standard deviation in parenthesis.
Date
NO3-N
NH4-N
total
inorganic N
DONa
total N
pH
a
Bole-only
Total-tree
Total-tree plus
significance
level
March 3, 2000
May 4, 2000
0.93a
2.85
(0.29)
(1.02)
2.00ab
3.12
(0.99)
(0.24)
2.30b
3.87
(0.33)
(0.54)
0.04*
0.17
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
5.28
0.05
0.08
0.04
0.97a
2.92
5.32
0.73
0.46
0.26
1.71a
3.38
5.58
5.59
4.78
5.49
(1.61)
(0.01)
(0.07)
(0.02)
(0.30)
(0.96)
(1.62)
(0.27)
(0.29)
(0.19)
(0.19)
(1.00)
(1.68)
(0.66)
(0.94)
(0.61)
7.14
0.07
0.02
0.04
2.99ab
3.14
7.18
0.54
0.55
1.05
2.61ab
3.69
8.23
5.13
4.45
5.31
(3.04)
(0.02)
(0.03)
(0.00)
(2.68)
(0.21)
(3.04)
(0.31)
(0.27)
(1.02)
(1.03)
(0.48)
(2.02)
(0.78)
(0.32)
(0.02)
6.65
0.05
0.00
0.05
2.35b
3.87
6.70
0.61
0.59
0.14
2.96b
4.46
6.83
4.64
4.05
4.91
(2.05)
(0.03)
(0.00)
(0.00)
(0.33)
(0.54)
(2.05)
(0.15)
(0.24)
(0.16)
(0.45)
(0.49)
(2.20)
(0.61)
(0.86)
(0.61)
0.38
0.60
0.18
0.68
0.04*
0.17
0.38
0.51
0.63
0.15
0.02*
0.23
0.22
0.25
0.51
0.27
DON=dissolved organic nitrogen
+ denotes significance at p < 0.10 level
* denotes significance at p < 0.05 level
** denotes significance at p < 0.01 level
Values in each row followed by different letters are significantly different (p < 0.10) using ANOVA and
Tukey's HSD test.
55
that were seen in May and June (at =0.10 level). Since nitrate was the predominate
form of N in soil solution, the same pattern of treatment effect existed for
concentrations of total inorganic N as well as total N in soil solution, but was also
only significant for the March sampling date (p=0.04 and p=0.02, respectively).
Soil solution pH values appeared to be inversely related to nitrate
concentrations and followed the pattern BO>TT>TP (Fig. 9), but this relationship was
not very strong over all sampling dates (r2=0.15). While there was a significant
overall treatment effect on pH (p=0.04, Table 2), for a given sampling date this trend
was not statistically significant (Table 3).
7.00
Bole-only
Total-tree plus
6.00
*
*
*
*
pH
Total-tree
Forest
5.00
*
*
*
4.00
3.00
March 3, 2000
May 4, 2000
June 26, 2000
Figure 9. Soil solution pH at 0.2-m depth. Error bars represent one standard
deviation from the mean. Different letters represent significant differences
between harvesting treatments (p<0.10, ANOVA, Tukey's). An asterisk (*)
represents a significant difference from nonharvested forest (p<0.10, t-test).
56
Lower pH values in the soil solution resulted from nitrification in which two
hydrogen ions were produced for every molecule of ammonium that was oxidized.
The higher nitrate concentrations and lower pH values associated with increasing
harvest intensity were evidence of increased nitrification rates in these treatments.
Although nitrifying bacteria are generally believed to be poor competitors for NH4+
(Fisk and Fahey 1990), a reduction in plant nutrient uptake following harvesting
decreases competition for ammonium, allowing populations of nitrifiers to multiply
(Jones and Richards 1977).
The results of this study were in direct contrast to the theory proposed by
Wells and Jorgensen (1979). In the absence of nutrient-rich logging residues, wholetree harvested plots were expected to experience reduced nutrient availability as a
result of reduced mineralization rates. Vitousek and Matson (1985b) reported that
intensive harvesting and site preparation practices decreased potential soil N
availability in 1- to 5-year-old loblolly pine plantations. Smethurst and Nambiar
(1990) also found that concentrations of soil mineral N after clearfelling a Radiata
pine plantation were lower where slash was removed.
The alternative theory proposed by Vitousek (1981) suggested that in the
absence of organic matter inputs in logging residues or litterfall, microbial
populations would be limited by the supply of available C, therefore reducing their
capacity to immobilize N and leading to greater nutrient availability in whole-tree
harvested treatments. Emmett et al. (1991a) observed in a Sitka spruce plantation
that inorganic N concentrations were initially lower where slash was retained
57
(supporting Vitousek’s theory), but after 6 months this pattern was reversed
(supporting Wells and Jorgensen’s theory). It therefore seems important to consider
the timing of when nutrient availability assessments are made when evaluating the
impacts of intensive harvesting.
Observations in this study occurred from approximately 8 to 12 months
(March to June 2000) following harvesting and treatment installation. During this
time, logging residues were likely supplying a source of labile C to growing microbial
populations and N immobilization was expected as predicted by Vitousek (1981).
Immobilization of N would explain the lower nutrient availability observed in the BO
treatment and evidence of this was found in this study (see Section 5.2.5. Net organic
N mineralization rate). In addition, observations of increased nitrification due to
increasing harvest intensity may also be attributed to the reduced capacity for N
assimilation by microbes.
5.1.1.3 Effect of time
Results from the two-way ANOVA (Table 2, page 48) showed that there was
a very significant overall effect of time on concentrations of nitrate (p=0.00), total
7inorganic N (p=0.00), total N (p=0.00), and pH (p=0.04). There were no significant
effects of time on ammonium or DON concentrations, but there were significant
interaction effects between treatment and time for these two variables.
Concentrations of nitrate ranged from about 1 to 7 mg L-1 over the entire study
period and increased linearly over time (Fig. 8, page 47). An analysis of variance for
58
Table 4. N concentrations (mg L-1) and pH of soil solution collected at 0.2-m depth with respect to time. Significance levels are
from one-way ANOVA results for each treatment.
total
Treatment
Date
NO3-N
NH4-N
---------------------------------
inorganic N
mg L-1
DONa
total N
pH
-----------------------------------
Bole-only
March 3, 2000
May 4, 2000
June 26, 2000
sig. level
0.93a
2.85b
5.28c
0.00**
0.05
0.08
0.04
0.60
0.97a
2.92b
5.32c
0.00**
0.73
0.46
0.26
0.15
1.71a
3.38a
5.58b
0.00**
5.59a
4.78b
5.49a
0.06+
Total-tree
March 3, 2000
May 4, 2000
June 26, 2000
sig. level
2.00
3.12
7.14
0.18
0.07
0.02
0.04
0.12
2.99
3.14
7.18
0.18
0.54
0.55
1.05
0.73
2.61a
3.69a
8.23b
0.08+
5.13
4.45
5.31
0.12
Total-tree plus
March 3, 2000
May 4, 2000
June 26, 2000
sig. level
2.30a
3.87a
6.65b
0.00**
0.05a
0.00b
0.05a
0.02*
2.35a
3.87a
6.70b
0.00**
0.61a
0.59ab
0.14b
0.03*
2.96a
4.46ab
6.83b
0.01*
4.64a
4.05b
4.91a
0.01*
Forest
March 3, 2000
May 4, 2000
June 26, 2000
sig. level
0.85
0.31
0.38
0.36
0.06
0.01
0.04
0.44
0.91
0.32
0.51
0.30
1.00
0.92
0.32
0.34
1.92
1.25
0.82
0.19
6.23a
5.51b
6.48a
0.00**
a
DON=dissolved organic nitrogen
+ denotes significance at p < 0.10 level
* denotes significance at p < 0.05 level
** denotes significance at p < 0.01 level
For each treatment, values in each column followed by different letters are significantly different (p < 0.10) using ANOVA and
Tukey’s HSD test
59
each treatment separately (Table 4) showed that nitrate concentrations increased
significantly over time in the BO (p=0.00) and TP (p=0.00) harvesting treatments.
The same trend was apparent but not significant in the TT treatment (p=0.18) due to
high variability and small sample size (n=2). Variability also increased over time
(Table 3, page 49). The increase in nutrient concentrations in solution over time may
be explained by increased rates of nitrification (and mineralization) in response to
increased soil temperatures as soils warmed up over the spring (see Section 5.2.4.
Temperature).
However, it was not expected that nitrate concentrations would
continue to increase over time indefinitely as dry soil conditions in the summer
months may inhibit nitrification (see Section 5.2.3. Moisture content).
5.1.1.4 Comparing harvested treatments to nonharvested forest
Process rates were different in the forested stand in relation to the harvested
treatments. Concentrations of total N in solution were typically about 2 to 8 times
lower in the forest (Fig. 8, page 47). There were significant differences in nitrate and
total N concentrations between FT and BO and between FT and TP and these
differences increased over time (Table 5). In the nonharvested stands, total N levels
tended to decrease over time presumably as a result of tree uptake, but this decrease
was not significant (Table 4).
Another difference between harvested treatments and nonharvested stands was
that DON represented an important form of N in the forest, accounting for about 50%
of total N found in solution, whereas in the harvested treatments, relative amounts of
60
Table 5. N concentrations (mg L-1) and pH of soil solution collected at 0.2-m depth from all treatments by sampling date.
Values are means followed by standard deviation in parenthesis. Significance levels are from separate t-test comparisons
of harvesting treatments to nonharvested forest.
Forest
mean
stdev
mean
Bole-only
stdev sig. level
mean
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
0.85
0.31
0.38
0.06
0.01
0.04
(0.66)
(0.39)
(0.50)
(0.07)
(0.02)
(0.02)
0.93
2.85
5.28
0.05
0.08
0.04
(0.29)
(1.02)
(1.61)
(0.01)
(0.07)
(0.02)
0.84
0.00**
0.00**
0.67
0.14
0.95
2.00
3.12
7.14
0.07
0.02
0.04
(0.99)
(0.24)
(3.04)
(0.02)
(0.03)
(0.00)
0.17
0.00**
0.01*
0.88
0.63
0.79
2.30
3.87
6.65
0.05
0.00
0.05
(0.33)
(0.54)
(2.05)
(0.03)
(0.00)
(0.00)
0.01*
0.00**
0.00**
0.75
0.39
0.53
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
0.91
0.32
0.51
1.00
0.92
0.32
1.92
1.25
0.82
6.23
5.51
6.48
(0.71)
(0.39)
(0.57)
(0.39)
(0.66)
(0.41)
(0.86)
(0.59)
(0.43)
(0.12)
(0.21)
(0.16)
0.97
2.92
5.32
0.73
0.46
0.26
1.71
3.38
5.58
5.59
4.78
5.49
(0.30)
(0.96)
(1.62)
(0.27)
(0.29)
(0.19)
(0.19)
(1.00)
(1.68)
(0.66)
(0.94)
(0.61)
0.88
0.00**
0.01*
0.32
0.24
0.81
0.65
0.01*
0.01*
0.16
0.22
0.04*
2.99
3.14
7.18
0.54
0.55
1.05
2.61
3.69
8.23
5.13
4.45
5.31
(2.68)
(0.21)
(3.04)
(0.31)
(0.27)
(1.02)
(1.03)
(0.48)
(2.02)
(0.78)
(0.32)
(0.02)
0.17
0.00**
0.19
0.18
0.51
0.49
0.42
0.01*
0.11
0.01*
0.08+
0.00**
2.35
3.87
6.70
0.61
0.59
0.14
2.96
4.46
6.83
4.64
4.05
4.91
(0.33)
(0.54)
(2.05)
(0.15)
(0.24)
(0.16)
(0.45)
(0.49)
(2.20)
(0.61)
(0.86)
(0.61)
0.02*
0.00*
0.00*
0.12
0.39
0.44
0.09
0.00**
0.01*
0.01*
0.02*
0.01*
Date
NO3-N
NH4-N
total
inorganic N
DONa
total N
pH
a
DON=dissolved organic nitrogen
+ denotes significantly different from Forest at p < 0.10 level, t-test
* denotes significantly different from Forest at p < 0.05 level, t-test
** denotes significantly different from Forest at p < 0.01 level, t-test
Total-tree
stdev sig. level
Total-tree plus
mean stdev
sig. level
61
DON ranged from about 5% to about 50% of total N (Fig. 8, page 48). However,
absolute levels of DON were not significantly different between treatments (Table 5).
The nonharvested forest may have also had additional sources of DON from the
leaching of foliage in throughfall and root turnover.
The lower concentrations of inorganic N (particularly NO3-N) in the
nonharvested forest were no doubt due to vegetative uptake and assimilation. Rates
of nitrification were lower in the forest, presumably due to rapid uptake and
assimilation of ammonium (the energy supply for nitrifying bacteria), and were
supported by the consistently higher solution pH ranges (Fig. 9). The elevated pH of
soil solution in the nonharvested forest was generally statistically different from
harvested treatments during the study period (Table 5).
5.1.1.5 Hypothesis testing
Null Hypothesis 1: Increasing the intensity of harvesting and organic matter removal
will have no significant effect on the availability of N during the 8- to12-month
period (March to June, 2000) following harvest.
Results from soil solution sampling at 0.2-m depth from March to June 2000
led to the rejection of this hypothesis. Increasing the intensity of harvesting and
organic matter removal was found to significantly affect N availability during this
period (= 0.10 level). In particular, concentrations of NO3-N increased significantly
(p=0.06) over bole-only harvesting with increasing harvest intensity. There were no
significant differences between treatments for ammonium or DON concentrations
however. Increased rates of nitrification were also supported by decreases in pH
associated with increasing harvest intensity. Vegetative uptake of inorganic N led to
62
significantly lower levels in solution and were associated with consistently higher soil
solution pH values. Increased mineralization rates in response to warmer and wetter
soil conditions and/or decreased capacity for microbial immobilization of N may
explain the increased availability of N following intensive harvesting.
5.1.2 Nitrogen leaching at 1.0-m depth
5.1.2.1 Forms of nitrogen
In the harvested treatments, nitrate was the predominate form of N in solution
at 1.0-m depth (Fig. 10) representing from 52 to 79% of total N, while DON
comprised most of the rest. Concentrations of ammonium were low and represented
only a few percent of total N in solution. While this pattern was similar to that
observed at 0.2-m depth (Fig. 8), the magnitude was quite different, with
concentrations of total N in solution on the order of 0.5 to 4 times less at 1.0 m than at
0.2 m.
5.1.2.2 Changes with depth
The decrease in soil solution N concentrations with depth was supported by a
two-way AVOVA for the overall effect of treatment and depth by sampling date
(Table 6). Over all three sampling dates, there was generally no overall treatment
effect on N concentrations when both sampling depths were combined, nor were there
generally any significant interaction effects between treatment and depth. There was
63
3.50
NH4-N
DON
NO3-N
-1
N concentration (mg L )
3.00
2.50
2.00
*
1.50
1.00
*
*
*
*
0.50
*
0.00
* Bole-only
3/3/00
5/4/00
6/26/00
3/3/00
5/4/00
Total-tree
6/26/00
*
Total-tree
* plus
3/3/00
5/4/00
6/26/00
3/3/00
5/4/00
6/26/00
Forest
Figure 10. Concentrations (mg L-1) and forms of N in soil solution collected
at 1.0-m depth. Error bars represent one standard deviation from the mean.
Different letters represent significant differences between harvesting
treatments (p<0.10, ANOVA, Tukey's). An asterisk (*) represents a
significant difference from nonharvested forest (p<0.10, t-test).
however a highly significant effect of sampling depth for nearly every variable
measured.
Concentrations of nitrogen in soil solution typically decreased from 0.2- to
1.0-m sampling depths.
For each treatment and sampling date, an independent
samples t-test was performed to evaluate differences in N concentrations between
depths (Table 7). In the BO and TP treatments there were significant differences
between 0.2 and 1.0 m for NO3-N, DON, and total N concentrations during some or
64
all of the sampling dates. These differences were not significant in the TT and FT
treatments however.
A decrease in nutrient concentrations as soil solution passes through the soil
profile was consistent with the findings of many studies. Qualls et al. (2000) reported
that over 99% of the organic and inorganic N released from a deciduous forest floor
after clearcutting was removed from solution by the time it had reached the C horizon
(90 to 120-cm depth). It was unlikely that this was due to decomposition in the
dissolved phase since DON was found to be very slow to mineralize (Qualls and
Haines 1991). Many well-known mechanisms have been described to explain the
efficient buffering capacity of soils against nutrient losses through leaching. Uptake
by plant roots, biological immobilization by microbes, and geochemical adsorption to
iron and aluminum oxyhydroxides and clays can all act to remove nutrients from soil
solution as it moves through the soil profile (Johnson and Cole 1977; Fahey et al.
1985).
65
Table 6. Overall effects of harvesting treatment, depth, and their interaction on N
concentrations (mg L-1) and pH of soil solution collected from 0.2- and 1.0-m depths
by sampling date. Values are significance levels from multivariate two-way
ANOVA.
Date
March 3, 2000
May 4, 2000
June 26, 2000
a
NO3-N
NH4-N
total inorganic N
Harvesting
Treatment
effect
0.09+
0.77
0.09+
DONa
total N
pH
NO3-N
NH4-N
total inorganic N
0.98
0.14
0.20
0.28
0.15
0.29
0.01*
0.00**
0.11
0.00**
0.67
0.00**
0.42
0.17
0.60
0.17
0.87
0.18
DONa
total N
pH
NO3-N
NH4-N
total inorganic N
0.95
0.32
0.56
0.73
0.78
0.73
0.07+
0.00**
0.01*
0.00**
0.16
0.00**
0.59
0.11
0.56
0.36
0.27
0.35
DONa
total N
pH
0.04**
0.38
0.96
0.87
0.00**
0.05+
0.99
0.33
0.15
DON=dissolved organic nitrogen
+ denotes significance at p < 0.10 level, ANOVA
* denotes significance at p < 0.05 level, ANOVA
** denotes significance at p < 0.01 level, ANOVA
Depth Trt * Depth
effect
Interaction
0.01*
0.06+
0.02**
0.77
0.01*
0.05+
66
Table 7. N concentrations (mg L-1) and pH of soil solution collected from each treatment with respect to depth.
Significance levels are from t-test comparison between 0.2- and 1.0-m sampling depths.
NO3-N
Trt Depth (m)
March 3
0.93
BO
0.2
0.95
1.0
sig. level 0.93
May 4
DONa
NH4-N
June 26
March 3
May 4
June 26
2.85
1.49
0.10+
5.28
1.37
0.01*
0.05
0.09
0.01*
0.08
0.07
0.93
total N
June 26 March 3
May 4
pH
March 3
May 4
June 26 March 3 May 4
June 26
0.04
0.04
0.82
0.73
0.15
0.01*
0.46
0.31
0.50
0.26
0.23
0.79
1.71
1.19
0.09+
3.38
1.88
0.08+
5.58
1.63
0.01*
5.59
5.84
0.52
4.78
5.44
0.26
5.49
5.72
0.57
5.31
-----
TT
0.2
1.0
sig. level
2.00
0.57
0.15
3.12
0.46
0.08+
7.14
-----
0.07
0.08
0.79
0.02
0.07
0.30
0.04
-----
0.54
0.34
0.64
0.55
0.33
0.63
1.05
-----
2.61
0.90
0.29
3.69
0.37
0.11
8.23
-----
5.13
5.57
0.48
4.45
5.61
0.21
TP
0.2
1.0
sig. level
2.30
0.83
0.00**
3.87
1.12
0.00**
6.65
1.52
0.04*
0.05
0.07
0.51
0.00
0.03
0.34
0.05
0.03
0.61
0.23
0.05*
0.59
0.20
0.02*
0.14
0.09
0.82
2.96
1.32
0.02*
4.46
1.35
0.00**
6.83
0.48
0.08+
4.64
5.68
0.10+
4.05 4.91
5.44 5.68
0.04* 0.25
FT
0.2
1.0
sig. level
0.85
0.05
0.17
0.31
0.04
0.26
0.38
0.06
0.30
0.06
0.11
0.34
0.01
0.09
0.44
0.04
0.05
0.55
1.00
0.82
0.77
0.92
1.16
0.59
0.32
0.43
0.77
1.92
0.98
0.22
1.25
1.29
0.92
0.82
0.54
0.46
6.23
5.60
0.42
5.51
5.30
0.71
a
DON=dissolved organic nitrogen
+ denotes significance at p < 0.10 level, t-test
* denotes significance at p < 0.05 level, t-test
** denotes significance at p < 0.01 level, t-test
BO= bole-only harvest; TT= total-tree harvest; TP= total-tree plus harvest; FT= forest.
6.48
5.88
0.33
67
5.1.2.3 Effect of harvest intensity and time
Results from the two-way multivariate ANOVA testing the overall effects of
harvesting treatments and time on N concentrations at 1.0-m showed that there was
no significant effect of treatment or time, nor were there any significant interactions
between the two (Table 2, page 48). Clearly, differences seen in the surface horizon
were reduced by the time soil solution had passed though the soil profile to a depth of
1.0 m.
7.00
Bole-only
Total-tree plus
Total-tree
Forest
pH
6.00
5.00
4.00
March 3, 2000
May 4, 2000
June 26, 2000
Figure 11. Soil solution pH at 1.0-m depth. Error bars represent one standard
deviation from the mean.
68
There were no significant effects of harvesting treatment when sampling dates
were looked at separately (Appendix A).
There was a weak trend of nitrate
concentrations increasing over time, similar to what was observed at the 0.2-m depth,
but this was not significant (Appendix B). The patterns in solution pH seen at 0.2-m
depth also seemed to be absent at 1.0 m (Fig. 11).
5.1.2.4 Comparing harvested treatments to nonharvested forest
In general, there were few significant differences in N concentrations at 1.0-m
depth between the harvested treatments and nonharvested forest (Table 8).
Concentrations of total N in solution were similar across all treatments and ranged
between 0.6 and 1.8 mg L-1 (Fig. 10, page 58). Nitrate concentrations appeared to be
much greater in harvested treatments, representing from 50 to 80% of total N,
compared to less than 15% in the nonharvested forest. These differences in nitrate
concentrations between harvested treatments and nonharvested stands were only
significant in March and May 2000.
In June, the relative difference in nitrate
concentrations between harvested treatments and nonharvested forest was much
greater (harvested treatments being about 20 times higher), but variability was also
much higher and differences were therefore not significant.
As was observed in the 0.2-m depth samples, DON was a significant
component of the total N in the nonharvested forest and represented about 80% of
total N at 1.0 m (Fig. 10). While both relative and absolute concentrations of DON
appeared to be much greater in the nonharvested stands than in harvested treatments,
69
these differences were only significant in May 2000 in the BO (p=0.03) and TP
(p=0.03) treatments. DON losses have been reported to account for over 95% of total
N losses in streams draining from non-polluted old-growth conifer forests in Chile,
emphasizing the importance of DON in the movement and loss of N from forested
ecosystems (Hedin et al. 1995). Forest harvesting in general has also been reported to
increase losses of both inorganic N and DON to streamwater (Bormann and Likens
1979; Sollins and McCorison 1981; Qualls et al. 2000), but total N leaching was not
increased by harvesting during the period sampled in this study.
Several geochemical mechanisms leading to the retention of dissolved organic
matter in soils have been described and include: the release of potentially soluble
dissolved organic matter by slow dissolution, equilibrium controlled desorption from
organic surfaces, the gradual exposure of surfaces to percolating water during
fragmentation, and the equilibrium adsorption to Fe and Al oxyhydroxides and clays
(Qualls et al. 2000). The ability of a given soil to buffer against nutrient losses
depends to a large extent on the depth of the soil profile, the nature of the soil
(particularly texture and mineralogy), and the presence of organic matter and
vegetation. The Boistfort soil series found at this study site has a deep soil profile,
which may extend to greater than 2-m deep. In addition, it has a considerable amount
of volcanic ash, which contributes to its ability to adsorb nutrient ions. The presence
of noncrystalline imogolite clay minerals with a high surface area may also be
extremely important as a source of anion exchange capacity, but the ability of these
70
Table 8. N concentrations (mg L-1) and pH of soil solution collected at 1.0-m depth from all treatments by
sampling date. Values are means followed by standard deviation in parenthesis. Significance levels
are from t-test comparison of harvested treatments to nonharvested forest.
Forest
mean
stdev
mean
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
0.05
0.04
0.06
0.11
0.09
0.05
(0.08)
(0.04)
(0.05)
(0.03)
(0.19)
(0.02)
0.95
1.49
1.37
0.09
0.07
0.04
(0.45)
(0.98)
(1.28)
(0.02)
(0.08)
(0.01)
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
March 3, 2000
May 4, 2000
June 26, 2000
0.16
0.13
0.11
0.82
1.16
0.43
0.98
1.29
0.54
5.60
5.30
5.88
(0.11)
(0.18)
(0.03)
(0.83)
(0.49)
(0.48)
(0.88)
(0.52)
(0.48)
(1.20)
(1.07)
(0.92)
1.04
1.56
1.41
0.15
0.31
0.23
1.19
1.88
1.63
5.84
5.44
5.72
(0.44)
(1.04)
(1.27)
(0.10)
(0.28)
(0.19)
(0.47)
(1.00)
(1.12)
(0.31)
(0.19)
(0.44)
Date
NO3-N
NH4-N
total
inorganic N
DONa
total N
pH
a
Bole-only
stdev sig. level
Total-tree
mean stdev sig. level
Total-tree plus
mean stdev sig. level
0.03*
0.06+
0.13
0.46
0.83
0.32
0.57
0.46
--0.08
0.07
---
0.00**
0.53
--0.32
0.85
---
0.83
1.12
1.52
0.07
0.03
0.03
(0.44)
(0.66)
(1.63)
(0.04)
(0.04)
---
0.09+
0.02*
0.43
0.26
0.50
0.40
0.02*
0.07+
0.14
0.20
0.03*
0.47
0.69
0.35
0.15
0.71
0.81
0.76
0.64 (0.11)
0.52 (0.68)
----0.34
--0.33
------0.90
--0.37
------5.57
--5.61
-------
0.01*
0.57
--0.64
0.23
--0.94
0.22
--0.98
0.81
---
0.90
1.15
1.96
0.23
0.20
0.09
1.32
1.35
0.48
5.68
5.44
5.68
(0.48)
(0.63)
(2.21)
(0.18)
(0.08)
--(0.67)
(0.58)
--(0.74)
(0.58)
(0.78)
0.11
0.02*
0.45
0.25
0.03*
0.58
0.67
0.88
0.92
0.92
0.82
0.81
DON=dissolved organic nitrogen
+ denotes significantly different from Forest at p < 0.10 level, t-test
* denotes significantly different from Forest at p < 0.05 level, t-test
** denotes significantly different from Forest at p < 0.01 level, t-test
(0.14)
(0.64)
--(0.02)
(0.04)
---
71
soils to buffer against leaching losses, in particular by adsorbing anions (especially
NO3- and organic acids), needs to be investigated.
5.1.2.5
Estimates of N leaching at 1.0-m depth—nitrate and DON
Monthly estimates of nitrate and DON leaching were calculated based on
concentrations found at 1.0-m depth and an estimate of monthly water flux using
Thornthwaite’s equation (Dunne and Leopold 1977). This method for calculating
evapotranspiration utilized mean monthly air temperatures and monthly precipitation,
which are shown in Figure 12. Sampling events occurred approximately every two
months and concentrations of nitrate and DON from each sampling date were used to
estimate leaching over the two-month interval surrounding each sampling date. Since
there was no effect of harvesting intensity or time on N concentrations at 1.0-m depth
(see Section 5.1.2.3. Effect of harvest intensity and time), estimates of N leaching
also showed no statistical difference with harvesting treatment. However, there were
significant differences in N leaching between the harvested treatments and
nonharvested forest (Table 9).
72
20.0
Rainfall and net water flux (cm);
0
and mean temperature ( C)
Rainfall (cm)
Net water flux (cm)
15.0
Temperature (*C)
10.0
5.0
0.0
-5.0
Feb
March
May
April
June
July
2000
-10.0
Figure 12. Mean monthly rainfall (cm), net water flux (cm), and mean air
temperature (0C) during the study period.
73
Table 9. Monthly leaching estimates of NO3-N and DON (kg ha-1). Values are means followed by standard
deviations in parenthesis below. Harvested treatments were compared by ANOVA and were
compared separately to “Forest” with a t-test.
Bole-only
month, 2000
mean SD mean SD
Feb
March
April
May
June
July
Total
-1
(kg N ha *
6 mo
Total-tree
-1)
a
Total-tree
ANOVA
plus
significance
mean SD
level
Forest
mean
SD
t-test significance level
BoleTotalTotal-tree
only
tree
plus
NO3-N
0.8
(0.4)
0.5
(0.1)
0.7
(0.4)
0.82
0.0
(0.1)
0.03*
0.00**
0.09+
a
DON
NO3-N
DON
NO3-N
DON
0.1
1.3
0.2
1.0
0.2
(0.1)
0.3
0.8
0.5
0.3
0.2
---
0.2
1.1
0.2
0.8
0.1
(0.2)
0.96
0.82
0.96
0.65
0.72
0.7
0.1
1.1
0.0
0.8
(0.7)
0.20
0.03*
0.20
0.06+
0.03*
0.64
0.00**
0.64
0.53
0.23
0.25
0.09+
0.25
0.02*
0.03*
NO3-N
DON
NO3-N
DON
NO3-N
DON
0.8
0.2
0
0
0
0
(0.5)
0.6
0.1
0
0
0
0
(0.3)
0.65
0.72
---------
0.0
0.6
0
0
0
0
(0.0)
0.06+
0.03*
---------
0.53
0.23
---------
0.02*
0.03*
---------
NO3-N
3.9
1.8
3.2
0.1
DON
0.7
1.2
0.6
3.2
Total
4.6
3.0
3.8
3.3
(0.6)
(0.1)
(0.7)
(0.2)
(0.1)
---------
0.2
0.2
0
0
0
0
(0.2)
--(0.4)
--(0.3)
-----------
DON=dissolved organic nitrogen
+ denotes significance at p < 0.10 level, t-test
* denotes significance at p < 0.05 level, t-test
** denotes significance at p < 0.01 level, t-test
(0.6)
(0.2)
(0.4)
(0.1)
(0.0)
---------
(0.1)
(1.1)
(0.0)
(0.3)
(0.3)
---------
74
There were no significant differences in nitrate leaching between harvesting
treatments for any sampling date, but nitrate leaching was significantly greater in
harvested treatments than in nonharvested stands during the months February to May
2000 (Table 9). In the FT treatment, leaching of nitrate to 1.0-m depth was very low
and amounted to a total of only 0.1 kg NO3-N ha-1 over the 6-month study period
compared to 2 to 4 kg NO3-N ha-1 in the harvesting treatments (Table 9). Nitrate
leaching peaked in March 2000 when net water flux was greatest (Fig. 12). In June
and July, evapotranspiration exceeded precipitation and no leaching occurred.
Leaching of DON however followed a different pattern and was much higher
in the nonharvested forest (Table 9). During the 6-month study period, approximately
3.2 kg N ha-1 as DON was leached to 1.0-m depth in the FT treatment, compared to
only 0.6 to 1.2 kg ha-1 in the harvested treatments (Table 9). Differences in DON
leaching between the nonharvested forest and harvested treatments were significant
during April and May 2000 for the BO and TP treatments (p<0.03). When leaching
losses of nitrate and DON were combined, there was no difference in total N leaching
between harvesting treatments nor between the nonharvested forest and harvested
treatments (Table 9). Total N leaching to 1.0-m depth represented approximately 3 to
4.5 kg N ha-1 during the 6-month study period.
Leaching values over the 6-month study period were converted to annual
estimates based on the work of Sollins and McCorison (1981). After clearcutting an
old growth Douglas-fir watershed in Oregon, they observed that leaching losses of
nitrate and DON showed a bimodal pattern, where concentrations of N in soil solution
75
peaked in the early fall at the time of the first rains and again in the late spring and
early summer just before saturated flow ceased.
Cole and Gessel (1965) also
observed a similar bimodal pattern in N leaching. Approximately 53% of the annual
amount of nitrate leached to 1.0-m depth occurred during the same time period
investigated in this study (February to July), while DON leaching during this period
represented about 58% of the annual rate (Sollins and McCorison 1981). When these
percentages were used to estimate annual leaching rates in this study, annual nitrate
leaching was calculated to be 7.4, 3.4, 6.0, and 0.2 kg NO3-N ha-1 yr-1 for BO, TT,
TP, and FT treatments, respectively. Annual DON leaching was calculated to be 1.2,
2.1, 1.0, and 5.5 kg DON ha-1 yr-1 for BO, TT, TP, and FT treatments, respectively.
Combined annual leaching losses of total N therefore ranged from 5.5 to about 8.5 kg
N ha-1 yr-1.
Other studies have also reported that harvesting in general, or increasing
harvesting intensity in particular, did not affect N leaching rates. Johnson et al.
(1982) observed no difference in leaching for 1.5 years following bole-only and
whole-tree harvesting of an upland mixed-oak forest in Tennessee. Vitousek and
Matson (1985a) also observed no difference in leaching in intensively harvested
loblolly pine plantations despite increased mineralization and nitrification rates.
Clearcut harvesting of a Douglas-fir stand in the Cedar River watershed in western
Washington did not alter leaching losses (Cole and Gessel 1965). Silkworth and
Grigal (1982) observed no increase in streamwater nutrient losses following
clearcutting of quaking aspen (Populus tremuloides) watersheds and similar results
76
were found in mixed hardwood stands in West Virginia (Aubertin and Patric 1974).
In some cases forest cutting caused only modest increases in streamwater nitrate
concentrations with values averaging about 1 mg L-1 or less (Fredriksen 1971;
Fredriksen et al. 1975; Feller and Kimmins 1984).
In a study similar to this one investigating the effects of harvest intensity on
leaching losses in Douglas-fir stands, Bigger (1988) reported that harvesting did not
significantly alter leaching losses during the first three years regardless of harvesting
intensity. At a high productivity Douglas-fir site (site index 36 m at 50 years),
inorganic N losses were estimated to be less than 1 kg N ha-1 yr-1 during the first three
years following harvest while leaching of total N was only slightly higher (<5 kg N
ha-1 yr-1). At a low productivity site (site index 25 m at 50 years), nitrate leaching
increased with harvest intensity only during the first year (from <1 to 3 and 5 kg
NO3-N ha-1 yr-1 in bole-only, total-tree, and complete removal treatments,
respectively) but was not statistically significant.
In 50-year-old nonharvested
reference stands, annual N leaching losses were on the order of less than 3 kg N ha-1
yr-1 (Bigger 1988). Similar leaching rates of 0.6 to 4.0 kg N ha-1 yr-1 have been
reported for Douglas-fir stands ranging from 36 to 95 years old (Turner 1975).
Leaching losses estimated in this study ranged from approximately 5.5 to 8.5
kg N ha-1 yr-1 and were slightly higher compared to observations from other studies.
Estimates of leaching losses in this study may not accurately reflect actual losses in
the field. Lysimeters were installed at 1.0-m depth as this was the maximum practical
sampling depth. However, the soils at this study site may extend at least another 0.5-
77
m deeper or more before weathered parent material is reached (Steinbrenner and
Gehrke 1973). Nutrients may be removed from soil solution below 1.0-m depth due
to adsorption, immobilization, or uptake by plant roots. In addition, the Thornthwaite
equation tends to underestimate evapotranspiration compared to other methods
(Dunne and Leopold 1987), resulting in a higher estimate of net water flux.
Thornthwaite’s method uses air temperature as an index of the energy available for
evapotranspiration and does not correct for different vegetation types nor distinguish
between clearcut and forested sites. It is expected that water flux in the forest (FT)
treatment was actually lower than estimated due to evapotranspiration by the standing
trees and therefore leaching losses were most likely overestimated in this treatment.
On the other hand, the ability of soils to buffer against leaching losses is also
influenced by site quality (Vitousek 1981), and the higher leaching losses observed in
this study may be valid. In a regional study, Vitousek et al. (1982) reported that more
fertile sites lost nitrate most rapidly following trenching treatments. Greater leaching
losses might be expected at the site investigated in this study since it was more fertile
than Bigger’s (1988) high site. At this site, site index (tree height at 50 years) was
41-43 m compared to only 36 m at Bigger’s site. In addition, this study site had been
fertilized a total of four times over the previous rotation with a total of 1800 kg N ha-1
as urea, which no doubt had an important effect on the N status of the site.
78
5.1.2.6 Hypothesis testing
Null Hypothesis 2: Increasing the intensity of harvesting and organic matter removal
will have no significant effect on N leaching during the 8- to 12-month period (March
to June 2000) following harvest.
Based on the results from soil solution sampling at 1.0-m depth from March to
June 2000, this hypothesis could not be rejected.
Increasing the intensity of
harvesting and organic matter removal was found to have no significant effect on N
leaching during this period (= 0.10 level). Concentrations of inorganic N and DON
at 1.0-m depth were not affected by harvesting intensity. Statistically significant
decreases in nitrate and DON concentrations as soil solution passed though the soil
profile from 0.2- to 1.0-m depths provided evidence of the capacity of these volcanic
soils to buffer against N losses, but their specific ability to adsorb anions still needs to
be quantified. Harvesting in general did not affect total N leaching, but the relative
contribution of NO3-N and DON to total N leaching was different between the
harvested treatments and nonharvested forest. Nitrogen leaching in the harvested
treatments was dominated by nitrate, but DON dominated leaching in the forest. This
may be due to continual uptake of inorganic N by vegetation and inputs of DON in
throughfall and root turnover in the forest. From February to July 2000, estimates of
nitrate leaching in the harvested treatments ranged from 2 to 4 kg N ha-1 but only 0.2
kg N ha-1 in the forest. During the same period, DON leaching was about 1 kg N ha-1
in the harvested treatments compared to over 3 kg N ha-1 in the forest. Total N
leaching from February to July 2000 was between 3 and 4.5 kg N ha-1 for all
treatments. When these values were pro-rated based on the work of Sollins and
79
McCorison (1981), annual leaching of total N was estimated to be about 5.5 to 8.5 kg
ha-1 yr-1.
5.1
Forest floor organic matter decomposition and net N mineralization
5.1.1
Initial forest floor characteristics
The physical and chemical characteristics of intact forest floor organic matter
collected from the harvesting treatments are summarized in Table 10. Estimates of
forest floor biomass (<1-cm diameter) strongly reflected the intensity of harvesting
removals with the pattern BO>TT>FT>TP, but these differences were not significant.
In general, properties of organic matter were similar between the BO and FT
treatments and between the TT and TP treatments. Lower percent organic matter
(percent volatile solids) in the TT and TP materials (about 60 to 70%, compared to
about 80% in the BO and FT) was most likely due to mixing with mineral soil that
occurred during mechanical slash removal. This difference was also reflected in the
water holding (field) capacity of the materials. Statistical differences existed in N
concentrations, which followed the pattern FT>BO>TT>TP and ranged from about
1.4% in FT to about 0.9% in TP material. Carbon concentrations ranged from 29 to
37% among forest floor materials, but were not significantly different. C:N ratios
ranged from a low of 32% in FT to 45% in TP.
80
Table 10. Characteristics of forest floor materials collected on March 21,
2000 (< 1-cm diameter) prior to field incubation. Values are means of 3
subsamples, except for biomass (n=8) and field capacity (n=1), and are
followed by standard deviation in parenthesis below.
Bole-only
Total-tree
Total-tree
plus
Forest
significance
level
20.5
14.4
12.0
12.8
0.11
(Mg ha )
(5.5)
(6.8)
(11.2)
(4.8)
pH
4.17a
4.12b
3.67c
3.63c
(0.02)
(0.03)
(0.02)
(0.01)
% organic
matter
83.2a
59.2b
69.3c
78.9a
(3.8)
(2.7)
(1.0)
(0.8)
NH4-N
10.6ab
8.4b
13.1a
6.4b
(mg kg )
(2.68)
(2.77)
(0.71)
(0.93)
NO3-N
0.0
0.2
0.9
1.0
(mg kg )
(0.00)
(0.16)
(1.47)
(0.83)
%N
1.04ab
0.97ab
0.87b
1.42a
(0.13)
(0.06)
(0.02)
(0.41)
32.6
29.4
33.5
37.4
(5.75)
(4.37)
(2.26)
(2.28)
36.5ab
35.2ab
44.8b
32.2a
(2.61)
(3.22)
(2.74)
(8.34)
2.56
1.56
1.77
2.63
Forest floor
biomass
-1
-1
-1
%C
C:N ratio
field
0.00**
0.00**
0.02*
0.43
0.06+
0.18
0.06+
-1
capacity (g g )
+ denotes significance at p < 0.10 level
* denotes significance at p < 0.05 level
** denotes significance at p < 0.01 level
Values in each row followed by different letters are significantly different (p < 0.10) using ANOVA and
Tukey's HSD test.
81
5.2.2 Forest floor decomposition rate
After incubating organic materials in porous ceramic cups in the field for 12
weeks (March 28 to June 19, 2000), decomposition rates of forest floor followed the
pattern BO>TP>FT>TT (Fig. 13). Forest floor in the BO treatment lost about 8% of
its original mass after 12 weeks, compared to about 4% for TP, and 1.5 to 1% for FT
and TT respectively. The effect of harvesting treatment was significant (p=0.00) after
12 weeks of incubation (Table 11). At this time, only the decomposition rate of BO
was significantly different from that of FT (p=0.02). Based on the variability found in
this study, it was determined that the mean detectable difference in decomposition
rates was 1.7, 3.3, and 2.4% in April, May, and June 2000, respectively (at =0.10
level). In western Washington Douglas-fir stands ranging from 11 to 97 years of age,
needle litter lost between 18 to 22% of its initial mass during a time period similar to
this study (February to May) (Edmonds 1979).
In an intensive harvesting study similar to this one, decomposition rates of
Douglas-fir needles in a low site (Site Class IV) were highest in the nonharvested
reference stand during the first year, but after two years needles in the bole-only
treatment had the highest decomposition rates (Edmonds and Bigger 1983).
However, in an adjacent high site (Site Class II), decomposition rates after one year
were highest in the bole-only treatment but there was no difference between wholetree harvested plots and nonharvested reference stands, as was found in this study.
After 2 years, needles in the bole-only treatment still were decomposing at the fastest
rate, but this was no longer significantly different from whole-tree or nonharvested
82
treatments. Gadgil and Gadgil (1978) proposed that litter decomposition rates in
clearcuts are often greater than in nonharvested stands because of reduced
mycorrhizal competition for organic substrates. Mycorrhizae were found to inhibit
decomposition by accessing organic nutrients directly and bypassing microbial
mineralization processes.
12.0
a*
Bole-only
10.0
Total-tree
Total-tree plus
Percent mass loss
8.0
Forest
b
6.0
4.0
ab
a
b
--
--
b
2.0
0.0
-2.0
April 25, 2000
(4 weeks)
May 26, 2000
(8 weeks)
June 19, 2000
(12 weeks)
-4.0
Figure 13. Decomposition of forest floor during field incubation, March 28 to June
19, 2000. Error bars represent one standard deviation from the mean. Different
letters represent significant differences between harvesting treatments (p<0.10,
ANOVA, Tukey's). An asterisk (*) represents a significant difference from
nonharvested forest (p<0.10, t-test).
83
Table 11. Decomposition (as percent of initial mass lost) of forest floor during field
incubation period, March 28 to June 19, 2000. Values are means followed
by standard deviation in parenthesis below. Harvested treatments were
compared by ANOVA and were compared separately to nonharvested
forest with a t-test.
Date, 2000
April 25
(4 weeks)
Boleonly
Total- Total-tree
tree
Plus
3.3a
0.3b
2.2ab
(0.28)
(2.28)
(1.42)
May 26
(8 weeks)
4.5
0.8
2.3
(4.65)
(3.76)
(1.45)
June 19
(12 weeks)
7.6a
0.8b
3.8b
(2.22)
(2.46)
(1.77)
ANOVA
sig.
level
Forest
0.07+
2.3
t-test
significance
level
BoleTotal- Total-tree
only
tree
plus
0.21
0.17
0.87
0.41
0.50
0.98
0.02*
0.85
0.16
(1.76)
0.39
2.3
(1.27)
0.00**
1.7
(1.56)
+ denotes significance at p < 0.10 level
* denotes significance at p < 0.05 level
** denotes significance at p < 0.01 level
Values within a row followed by different letters are significantly different (p<0.10) using ANOVA and Tukey's
HSD test.
Decomposition rates are a function of substrate chemistry, temperature, and
moisture. Substrates with a low C:N ratio are expected to decompose more rapidly
(Edmonds 1991). Based on the initial C:N ratio of organic materials used in this
study, the pattern in decomposition rates expected was FT>TT>BO>TP,
corresponding to C:N ratios of 32, 35, 37, and 45:1, respectively. However, this
pattern was not observed in this study, and C:N ratio may not be a good indicator of
the inherent decomposability of a given substrate as C may exist in recalcitrant forms
84
(Paul and Clark 1996). It may be that other factors, such as temperature and moisture,
also had a significant influence on decomposition rates.
5.2.2.1 Hypothesis testing
Null Hypothesis 3: Increasing the intensity of harvesting and organic matter removal
will have no significant effect on decomposition rates of forest floor organic matter
during the 8- to 12-month period (March to June 2000) following harvest.
Decomposition rates of organic matter from the four treatments after
incubating the field for 12 weeks (March 28 to June 19, 2000) led to the rejection of
this hypothesis. After 12 weeks of incubation, there was a significant effect of
harvesting treatment on decomposition rates (p=0.00), with the pattern BO > TP >
TT. Only the decomposition rate of BO was significantly different from that in the
nonharvested reference stands (p=0.02) after 12 weeks. These decomposition rates
represented the combined effects of differing substrate quality as well as temperature
and moisture conditions. Patterns in decomposition as predicted by C:N ratio alone
did not explain the observations in this study. Moisture and temperature factors must
also be considered.
5.2.3
Moisture content
5.2.3.1 Forest floor
Moisture content (proportion of water weight to oven-dry forest floor weight)
of forest floor organic matter was determined on additional samples collected
adjacent to the incubation cups during each collection event. There was a statistically
significant effect of harvesting treatment during all sampling dates over the
85
incubation period (March 28 to June 19, 2000) (Table 12). Organic matter in the BO
treatment always had higher moisture contents than that in the TT and TP treatments.
Moisture content in FT closely resembled that in BO. Moisture contents
generally decreased for all treatments over time, but this decrease was most rapid in
the TT and TP treatments (Fig. 14). On April 25, 2000 (after 4 weeks), moisture
content for all treatments ranged from 2.4 to 1.6 g g-1. By the June 19 collection (12
weeks), forest floor in the TT and TP treatments had dried considerably and had very
Table 12. Moisture content (g g-1) of forest floor during field incubation period,
March 28 to June 19, 2000. Values are means followed by standard
deviation in parenthesis below. Harvested treatments were compared by
ANOVA followed by Tukey’s HSD post hoc test, and were compared
separately to nonharvested forest with a t-test.
Boleonly
Total- Total-tree
tree
plus
April 25
(4 weeks)
2.39a
1.65b
1.59ab
(0.43)
(0.26)
(0.46)
May 26
(8 weeks)
2.15a
1.28b
0.13c
(0.42)
(0.36)
(0.22)
June 19
(12 weeks)
1.61a
0.32b
0.36b
(0.73)
(0.11)
(0.28)
Date, 2000
ANOVA
sig.
level
0.03*
Forest
1.90
t-test sig. level
Bole- Total- Total-tree
only
tree
plus
0.11
0.45
0.39
0.18
0.00**
0.00**
0.28
0.00**
0.00**
(0.22)
0.01*
2.34
(0.23)
0.00**
1.27
(0.10)
+ denotes significance at p < 0.10 level
* denotes significance at p < 0.05 level
** denotes significance at p < 0.01 level
Values within a row followed by different letters are significantly different (p<0.10) using ANOVA and Tukey's
HSD test.
86
low moisture contents of only 0.3 g g-1, while that in the BO and FT remained
considerably higher (1.6 to 1.3 g g-1).
The treatment effect was statistically
significant over all sampling dates and may be attributed to increased evaporation
rates in the intensively harvested treatments (see Section 5.2.4. Temperature, below).
The thick layer of logging residues retained in the BO treatment (about 5- to 15-cm
deep) most likely acted as a mulch and buffered against evaporational water losses.
Litter thickness in the TT and TP treatments was much less (about 3 cm) and would
not have had as strong a buffering effect. Even despite water uptake by trees in the
FT treatment, moisture content in the forest floor remained about the same as in the
BO treatment due to the shading effect of the forest canopy in reducing evaporational
water losses and to fog drip from the forest canopy.
The drying out of the forest floor in the TT and TP treatments likely had an
important effect on the slower decomposition rates of these materials relative to the
BO treatment (Fig. 13). In another study, decomposition rates of organic matter were
primarily correlated with minimum litter moisture content (Fogel and Cromack
1977). In the Pacific Northwest, Edmonds (1979) felt that moisture would be a more
important factor than temperature in influencing litter decomposition rates because of
the summer drought conditions that typically occur. However, it is still not clear why
TP had a significantly faster decomposition rate than TT (3.8 versus 0.8% after 12
weeks; Table 11), despite a higher initial C:N ratio (45 versus 35:1) and similar
moisture contents.
87
3.00
a
Bole-only
Total-tree
Total-tree plus
Forest
a
Forest floor
moisture Content (g g -1)
2.50
a
ab
2.00
b
b*
1.50
1.00
c*
0.50
b*
b*
0.00
April 25, 2000
May 26, 2000
June 19, 2000
Figure 14. Moisture content of forest floor during field incubation (March 28
to June 19, 2000). Error bars represent one standard deviation from the mean.
Different letters represent significant differences between harvesting
treatments (p<0.10, ANOVA, Tukey's). An asterisk (*) represents a
significant difference from nonharvested forest (p<0.10, t-test).
88
5.2.3.2 Mineral soil (0- to 10-cm depth)
Samples of surface soil (0 to 10 cm) were also collected during each sampling
event to estimate changes in soil moisture content over the incubation period.
Gravimetric soil moisture content was about 0.9 g g-1 on April 25 and decreased to
about 0.7 to 0.8 g g-1 by June 19, 2000 (Fig. 15). This decrease over time was
consistent in all treatments and there were no significant treatment effects (see
Appendix C), implying that even small amounts of organic matter on the surface (as
in TT and TP treatments) could effectively buffer against evaporational losses from
the underlying soil.
1.20
Bole-only
Total-tree
Total-tree plus
Forest
Mineral soil
Moisture Content (g g-1)
1.10
1.00
0.90
0.80
0.70
0.60
0.50
April 25, 2000
May 26, 2000
June 19, 2000
Figure 15. Moisture content of mineral soil (0 - 10 cm) during field incubation
(March 28 to June 19, 2000). Error bars represent one standard deviation from the
mean.
89
5.2.4
Temperature (Forest floor/mineral soil interface)
Temperatures at the forest floor/mineral soil interface were recorded at 30-
minute intervals in the various treatments from March 29 to May 26, 2000. Mean
temperatures during this measurement period followed the pattern TP>BO>FT (Fig.
16). Data logger failure in the TT treatment eliminated the ability to compare this
treatment, but temperatures are assumed to be very similar to those observed in TP
since the depth of the residual forest floor was approximately the same (about 3 cm).
Mean temperatures over the study period were highest in the TP treatment (11oC),
intermediate in BO (10oC), and lowest in FT (70C).
Minimum and maximum
temperatures ranged from a low of 2oC to a high of 30oC in TP, from 3 to 18oC in
BO, and from 3 to 14oC in FT.
Mean daily temperatures increased more rapidly in the more intensive
harvesting treatments (Fig. 16), indicating that the soils in these treatments warmed
up more quickly in the spring.
In the TP treatment, mean daily temperatures
increased from about 5oC in late March to about 15oC by late May. During this same
time period, mean daily temperature in the BO treatment increased from 5 to 12oC
and in FT from 4 to 10oC.
While mean temperatures only varied several degrees between treatments, a
closer look at diurnal fluctuations revealed a more profound treatment effect (Fig.
17). Over a three-day period from May 20 to 22, 2000, temperatures reached a
maximum at about 1 p.m., and were about 5 - 8oC warmer in the TP than in the BO,
Temperature ( 0C)
1
0
89
1805
1849
1893
1937
1981
2025
2069
2113
2157
2201
2245
2289
2333
2377
2421
2465
2509
2553
2597
2641
2685
2729
2773
1805
1849
1893
1937
1981
2025
2069
2113
2157
2201
2245
2289
2333
2377
2421
2465
2509
2553
2597
2641
2685
2729
2773
1629
1585
1541
1497
1453
1409
1365
1321
1277
1233
1189
1145
1101
1057
1013
969
925
881
837
793
749
705
661
617
573
529
485
441
397
353
309
265
221
177
1761
10
1761
20
1717
30
1717
mean 7.2 (SD 1.9)
1673
Forest (block 2)
1673
1629
1585
1541
1497
1453
1409
1365
1321
1277
1233
1189
1145
1101
1057
1013
969
925
881
837
793
749
705
661
617
573
529
485
441
397
353
309
265
221
177
133
89
45
1
0
45
1
89
45
2773
2729
2685
2641
2597
2553
2509
2465
2421
2377
2333
2289
2245
2201
2157
2113
2069
2025
1981
1937
1893
1849
1805
1761
1717
1673
1629
1585
1541
1497
1453
1409
1365
1321
1277
1233
1189
1145
1101
1057
1013
969
925
881
837
793
749
705
661
617
573
529
485
441
397
353
309
265
221
177
133
0
Temperature ( C)
0
133
Temperature ( 0C)
90
Bole-only harvest (block 2)
mean 9.3 (SD 2.8)
30
20
10
Total-tree plus harvest (block 4)
mean 11.1 (SD 5.1)
30
20
10
Figure 16. Temperatures at forest floor/mineral soil interface during field
incubation (March 29 - May 26, 2000).
91
and about 8 – 13oC warmer in the TP than in the FT treatment. Differences between
BO and FT were about 3 - 5oC during this same time period.
This pattern was even more apparent when a temperature histogram was
plotted for the treatments (Fig. 18). Only about 8% of the time during March 29 to
May 26, 2000 did temperatures exceed 100C in the FT treatment. In the BO and TP
treatments however, this frequency was about 50% and 55% respectively.
Temperatures in the TP treatment also had greater fluctuations, with frequencies in
both the highest and lowest temperature classes.
Temperature has an important effect on the activity of microorganisms and
therefore on rates of decomposition and mineralization. The ideal temperature range
for microbial activity is between 5 and 35-400C, and rates of biochemical reactions
exponentially increase with rising temperature (approximately doubling for every
10oC increase) until the upper temperature limit is reached (Brady and Weil 1999).
The cooler temperatures (4 to 10oC) found in the FT treatment may have kept
decomposition rates low despite favorable moisture conditions and a relatively low
C:N ratio (32:1). On the other hand, the stimulation of decomposition by the warmer
conditions in the TP and the TT was most likely offset by unfavorable moisture
conditions. Warm temperatures combined with favorable moisture and a source of
labile C in fresh logging residues most likely accounted for the higher decomposition
rates in the BO. Other studies reported that whole-tree harvesting caused reduced
microbial biomass pools and organic matter decomposition, which was attributed to
decreased moisture content of underlying soil (Edwards and Ross-Todd 1983;
92
25
Temperature ( C)
B o le-o nly
To tal-tree plus
Fo rest
0
20
15
10
5
00 :30 :00 :30 :00 :30 :00 :30 :00 :30 :00 :30 :00 :30 :00 :30
0:
4
9 13 18 22
3
7 12 16 21
1
6 10 15 19
Figure 17. Diurnal temperature fluctuations during May 20 - 22, 2000.
45
Bole-only
Total-tree plus
Forest
40
Frequency (%)
35
30
25
20
15
10
5
0
0.0 2.5
2.6 5.0
5.1 7.5
7.6 10.0
10.1 12.5
12.6 15.0
15.1 17.5
17.6 20.0
20.1 22.5
22.6 25.0
Tem perature class ( 0C)
Figure 18. Percent of time forest floor/mineral soil interface temperature was
in a given temperature class (by 2.50C classes) during the measurement period
March 29 to May 26, 2000.
93
Jansson 1987) and/or temperature fluctuations (Bjor 1972). Entry et al. (1986) also
found that whole-tree harvesting reduced microbial biomass pools in summer and
winter, and this was positively correlated with moisture and negatively correlated
with soil temperature.
5.2.5 Net organic N mineralization rate
After 12 weeks of incubating forest floor in the field (March 28 to June 19,
2000), there was a significant effect of harvest intensity (p=0.01) on net organic N
mineralization rates (Table 13). In the TT and TP treatments, initial organic N mass
decreased (mineralized) by approximately 12 and 8% respectively, but these
differences were not significant. However, in the BO treatment the mass of organic N
increased by about 18% relative to the initial mass and was significantly different
from TT and TP. Thus, net N immobilization rather than mineralization characterized
the behavior of the organic matter in the BO treatment.
In the nonharvested stands net N mineralization during the incubation period
was about 31% of initial organic N, and this was significantly higher than all three
harvesting treatments (p<0.01, Table 13). In a similar study of the effects of intensive
harvesting on Douglas-fir stands in Washington, the percent mass of N mineralized
after two years of needle decomposition in a high productivity site (Site Class II) was
reported to be 20, 13, and 23% for BO, TT, and FT treatments, respectively
(Edmonds and Bigger 1983). Faster mineralization rates in the nonharvested stand
were attributed to more intact microbial and mycorrhizal communities in the forest in
94
Table 13. Net organic N mineralization of forest floor after 12 weeks of field
incubation (March 28 to June 19, 2000). Values are means followed by standard
deviation in parenthesis below. Harvested treatments were compared by ANOVA
followed by Tukey’s post hoc test and were compared separately to nonharvested
forest using a t-test.
Boleonly
Total- Total-tree
tree
plus
mineralization -17.7a
(% organic N) (9.4)
12.0b
7.9b*
(6.6)
(8.1)
40.5a
25.4b
35.4c
(2.11)
(3.79)
(1.75)
1.33a
0.87b
0.84b
(0.13)
(0.09)
(0.07)
35.6a
33.8a
49.0b
(1.9)
(1.6)
(2.3)
%C
%N
C:N ratio
ANOVA
sig.
level
Forest
0.01*
30.9
t-test sig. level
BoleTotal- Total-tree
only
tree
plus
0.00**
0.00**
0.01*
0.01*
0.00**
0.80
0.00**
0.04*
0.01*
0.00**
0.00**
0.00**
(1.5)
0.00**
35.0
(1.744)
0.00**
1.00
(0.04)
0.00**
41.0
(1.5)
+ denotes significance at p < 0.10 level
* denotes significance at p < 0.05 level
** denotes significance at p < 0.01 level
Values within a row followed by different letters are significantly different (p<0.10) using ANOVA and Tukey's
HSD test.
95
combination with wetter surface soil conditions and less extreme temperature
fluctuations.
The high estimate (31%) observed in this study was not expected, particularly
since the decomposition rate in FT was relatively low (1.7%), and is not believed to
reflect an actual in situ field mineralization rate. The process of collecting forest
floor material and incubating it in ceramic cups was a dramatic disturbance that
severed the supply of photosynthate to root-associated microorganisms.
In the
absence of an energy source, it is hypothesized that microbial biomass could not be
sustained and N was released following microbial death and decay. In addition, the
sudden death of active fine roots also contributed to a source of readily
mineralizeable N. This effect was not as important in the harvested treatments since
harvesting occurred more than 6 months prior to the incubation experiment, giving
microbial communities more time to adjust to the altered organic matter inputs and
microclimatic changes.
The initial stages of litter decomposition are typically associated with an
increase in N content of the decomposing substrate (Aber and Melillo 1980). N
immobilization may occur via translocation by fungal hyphae, but since this could not
occur through the walls of the ceramic cups, increases in N in BO organic matter
were attributed to leachate from the forest floor carrying dissolved N into the cups.
Douglas-fir needles generally immobilize N during the first three months of
decomposition, after which time they begin mineralizing N (Edmonds 1991).
However, in this study N immobilization by logging residues in the BO treatment was
96
still occurring from 8 to 12 months after harvesting. Percent N in organic matter
increased in the BO treatment over the 12 week incubation (from 1.0 to 1.3%), but
decreased in the other treatments (from 1.0 to 0.9, 0.9 to 0.8, and 1.4 to 1.0% for TT,
TP, and FT respectively, Table 14). Percent C also increased in the BO treatment
(from 33 to 41%) but generally decreased in the other treatments. The C:N ratio
decreased in BO organic matter as microbes respired CO2 and immobilized N, but
generally increased in the other treatments as N was mineralized.
Such results
indicated that microbes were utilizing the logging slash retained in the BO treatment
as a source of C and that the microbial biomass pool was increasing in size, although
this was not directly measured. The rapid decomposition of organic matter in the BO
treatment further supports the assumption of more active microbial populations acting
on a favorable substrate. Lundgren (1982) also reported that bacterial biomass tends
to increase during the first two years following clearcutting.
The C:N ratio of organic matter is often used to predict whether net
mineralization or immobilization will occur, with net mineralization generally
occurring in Douglas-fir needles when C:N ratios are between 20 and 30:1 and
immobilization occurring at ratios greater than 30:1 (Edmonds and Bigger 1983).
Based on this assumption, the pattern in mineralization expected in this study was FT
> TT > BO > TP (corresponding to C:N ratios of 32, 35, 37, and 45:1, respectively).
In general, this pattern was observed in this study with the exception of the BO
treatment, which immobilized rather than mineralized N. The absolute C:N ratio does
not consider the relative decomposability of the C substrate to microbes, and for this
97
reason forest residues may exhibit mineralization at C:N ratios as high as 50 to 100:1
(Paul and Clark 1996). Therefore, the C:N ratio may be of little value in predicting
when the switch from mineralization to immobilization will occur. For example, net
N mineralization from needles occurred in 24- and 97-year-old Douglas-fir stands
when the C:N ratio was greater than 34:1, while it occurred in a 75-year-old stand
when the C:N ratio was 23:1 (Edmonds 1979).
Other studies have reported increased microbial biomass and microbial
activity under slash (Hendrickson et al. 1985; Entry et al. 1986; Emmett et al. 1991b)
due to the large amount of easily degradable organic matter remaining. Removal of
the organic matter substrate in whole-tree harvesting has been found to reduce the
biomass of both fungi (Baath 1980) and bacteria (Lundgren 1982).
The
immobilization of N in microbial biomass is an important mechanism for the
retention of N on site (Vitousek 1981). In a loblolly pine ecosystem that had been
bole-only harvested, more than 90% of added
15
N was immobilized by microbial
biomass in 28 days, while only 70% was immobilized in whole-tree harvested plots
(Vitousek 1984). Emmett et al. (1991b) found that microbial biomass at a Sitka
spruce site represented 2.1% of the total organic N pool where slash was removed,
but 3.2% where slash was retained. In addition to a reduced microbial biomass pool,
organic matter decomposition rates and N losses were also reduced following wholetree harvesting.
During the 12-week incubation period (March 29 to June 19, 2000) it was
estimated that forest floor organic matter from the TT treatment mineralized
98
approximately 17 kg N ha-1, compared to 8 kg N ha-1 in the TP treatment, and 56 kg
N ha-1 in the FT treatment.
The BO treatment was estimated to immobilize
approximately 38 kg N ha-1 (Table 14). Nitrogen mineralization is a very dynamic
process and throughout the course of a year may show periods of net immobilization
as well as net mineralization. Seasonal patterns in mineralization may be a function
of changing moisture and temperature conditions, but data describing seasonal
mineralization rates in Douglas-fir clearcuts is rare.
Barg and Edmonds (1999)
estimated seasonal soil N mineralization rates in a Douglas-fir clearcut in western
Washington over the course of a year using consecutive 2-week buried bag
incubations. Net N mineralization rates were positive during July to August but were
followed by a period of net immobilization in September. Net immobilization also
occurred during the late spring and early summer seasons, with the highest
mineralization occurring during the winter. White et al. (1988) observed that soil
from two Douglas-fir stands in New Mexico showed net mineralization over a twoyear period. However, forest floor was found to act as both a potential source as well
as a sink for inorganic N at different times in the year, but seasonal data was not
presented. Vitousek and Matson (1985a) reported that in situ mineralization rates in a
loblolly pine plantation reflected seasonal variations in climate and increased from
March to June as soil warmed up but then declined as soil dried out during the
summer.
99
Table 14. Forest floor chemical characteristics before and after 12-week field incubation (March 28 to June 19, 2000).
%N
Treatment
%C
initial final
C:N ratio
percent
mineralization
percent
mass loss
initial forest
floor N pool
net N
mineralization
initial
final
initial
final
after 12 weeks
after 12 weeks
(kg ha-1)
(kg ha-1 *6 mo-1)
Bole-only
1.04
1.33
32.6
40.9
36.5
35.6
-17.7
7.6
213
-38
Total-tree
0.97
0.87
29.4
25.4
35.2
33.8
12.0
0.8
140
17
Total-tree plus
0.87
0.84
33.5
35.4
44.8
49.0
7.9
3.8
104
8
Forest
1.42
1.00
37.4
35.0
32.3
41.0
30.9
1.7
182
56
100
It was difficult to estimate annual mineralization rates for this study due to the
highly variable results from other mineralization studies. As Barg and Edmonds
(1999) showed, the patterns of net mineralization or immobilization observed during
the spring season cannot necessarily be assumed to continue throughout the entire
year. Frazer et al. (1990) reported that about 14% of total annual mineralization from
soil in mixed-conifer stands in northern California occurred during March to June (the
same time period investigated in this study). When this value was used to pro-rate the
mineralization data reported in this study, annual mineralization rates of 121, 57, and
400 kg N ha-1 were estimated for TT, TP, and FT, respectively, but these rates seem
high.
In a loblolly pine plantation in North Carolina, about 56% of annual
mineralization occurred during the spring season (March to June), which would
correspond to 30, 14, and 100 kg N ha-1 yr-1 for TT, TP, and FT, respectively. These
latter estimates appear to be more realistic, but are probably not very meaningful
since they are based on the assumption that net mineralization will continue
throughout the entire year.
Average annual rates of N mineralization typically range from 30 to 120 kg N
ha-1 yr-1 (Barnes et al. 1998), but it was difficult to estimate annual rates in this study.
This study was conducted during a 12-week period in the spring (March 28 to June
19, 2000) when moisture and temperature conditions were favorable for
mineralization. As organic matter began to dry out in the summer (see Section 5.2.3.
Moisture content, above) mineralization may be inhibited, but may increase again in
the fall with the onset of more frequent precipitation. In addition, organic matter
101
decomposes and mineralizes differently at different depths in the forest floor (Binkley
1984).
Therefore it is not reliable to extrapolate rates measured at the forest
floor/mineral soil interface to the entire mass of logging residues that remain on site
since surface conditions may be much drier and warmer than deeper down. Also, the
estimates in this study were based only on material less than 1-cm in size and ignored
interactions with branches and larger coarse woody debris, which are expected to
immobilize substantial amounts of N as they decompose.
5.2.5.1 Hypothesis testing
Null Hypothesis 4: Increasing the intensity of harvesting and organic matter removal
will have no significant effect on net N mineralization rates of forest floor organic
matter during the 8- to 12-month period (March to June, 2000) following harvest.
Net N mineralization rates of organic matter from the four treatments after
incubating the field for 12 weeks (March 28 to June 19, 2000) led to the rejection of
this hypothesis. After 12 weeks of incubation, there was a significant effect of
harvesting treatment on mineralization rate (p=0.01), with the pattern TT > TP > BO.
The FT treatment showed the highest mineralization rate (31%), while organic matter
in the BO treatment actually showed net immobilization of N. Even though there
were differences in temperature and moisture conditions experienced by the various
treatments, it appears that substrate quality was the most important variable in
explaining the mineralization results. However, C:N ratio did not seem to be an
effective measure of substrate quality since immobilization was observed in the BO
treatment, which had a C:N ratio that was intermediate (37:1). Mineralization in the
102
other treatments did seem to be consistent with the pattern predicted by C:N ratio
however, with FT > TT > TP (corresponding to C:N ratios of 32, 35, and 45:1).
5.3 Net N mineralization and nitrogen availability
The field incubation experiment provided additional support for the
observations made during the soil solution monitoring. From 8 to 12 months after
harvesting, the availability of nitrogen in soil solution at 0.2-m depth increased with
increasing harvest intensity (TP>TT>BO).
Immobilization of N by microbial
biomass in decomposing foliage in the BO treatment contributed to the lower N
availability and reduced rates of nitrification compared to TT and TP treatments.
These results supported the theory of Vitousek (1981) who proposed that slash
removal would decrease the potential for microbial immobilization, thereby
increasing net N mineralization and N availability. This was in contrast to the
alternative theory of Wells and Jorgensen (1979) who proposed that the removal of
nitrogen-rich logging residues in whole-tree harvesting would lead to decreased N
mineralization rates relative to bole-only harvesting and therefore decreased N
availability.
Results from a number of studies have suggested that patterns in nutrient
availability are highly variable over time and reflect the changing dynamics of
decomposing organic matter.
In a Sitka spruce plantation, concentrations of
inorganic N following harvest were initially lower where slash was retained, but after
6 months organic matter began mineralizing N and this pattern was reversed (Emmett
103
et al. 1991a). In 1- to 5-year-old loblolly pine plantations, the N mineralization
potential of the top 15 cm of soil was 117 kg N ha-1 in the first year following a boleonly harvest compared to 107 kg ha-1 in a whole-tree harvest treatment. During the
second year, this pattern was reversed and N mineralization potential was 127 and 92
kg ha-1 in bole-only and whole-tree harvested treatments respectively (Vitousek and
Matson 1985b).
Retaining slash after harvesting supplies a source of labile C to microbial
populations, which immobilize N as they decompose the residual organic matter. As
heterotrophic decomposers continue to utilize the available C in the logging residues
over time, at some point they will eventually run out of this energy supply and will
begin to release N. It therefore seems plausible that the theories put forth by Wells
and Jorgensen (1979) and Vitousek (1981) to predict the consequences of slash
retention on nutrient availability may both be correct depending on when assessments
are made.
The timing of when the transition from net immobilization to mineralization
will occur depends on the dynamics of the decomposing organic matter. It may occur
after a short period of time, such as 6 months to one year (Emmett et al. 1991a;
Vitousek and Matson 1985b) or may take much longer and will depend on the
quantity and quality of the C substrate remaining after harvest and the prevailing
environmental conditions.
Edmonds (1979) found that Douglas-fir needles in a
number of stands ranging from 11- to 97-years-old typically immobilize N for about
the first three months (February to May) following litterfall. In a 44-year-old stand,
104
mineralization occurred between 6 and 12 months, and needles in all the stands
showed an absolute N loss by 24 months.
Covington (1981) investigated the
dynamics of forest floor organic matter in a clearcutting chronosequence of northern
hardwoods. In this ecosystem, the forest floor acted as a major source of nutrients
during the first 15 years after harvesting, releasing a net amount of 800 kg N ha-1, but
as much as half of this was immobilized by microbes in decaying logging residues.
After 15 years the forest floor began to act as a sink for nutrients as organic matter
began to accumulate on the soil surface again and logging slash shifted from being a
sink to a source of nutrients.
At the site investigated in this study, logging residues in the BO treatment
immobilized N for a period of at least one year following harvesting, and it is not yet
clear when this N will begin to be mineralized. The peak nutrient demand for
regenerating Douglas-fir stands reaches a maximum at or shortly after crown closure
(typically at 20- to 30-years-old or earlier depending on stand density) (Cole and
Bledsoe 1976), and therefore the timing of when slash begins to release N has
important management implications. Nitrogen fertilization is typically applied at the
time of crown closure, and if the net mineralization phase coincides with this peak
nutrient demand then slash retention may provide a cost-effective nutrient
management option.
On the other hand, if the net mineralization phase occurs when nutrient
demand is low, such as in the first few years of stand establishment, then increased
leaching losses may result. In this study there were to date no differences in N
105
leaching attributable to harvesting treatments, but it is not clear how this may change
in the future. Vitousek and Matson (1985a) also observed no difference in leaching
during the first few years following bole-only and whole-tree harvesting of loblolly
pine plantations in North Carolina, despite higher mineralization and nitrification
rates associated with bole-only harvesting. It seems that the ability of soil to buffer
against leaching losses is an effective mechanism to retain nutrients on site.
Continued monitoring of soil solution chemistry and the N dynamics of decomposing
logging residues are necessary to evaluate the potential contribution of organic matter
retention to sustained forest productivity.
106
Chapter 6. Summary and Conclusions
6.1 Summary
The relationships between organic matter and sustained forest productivity are
not well understood (Edmonds and Chappell 1994). Nitrogen is the nutrient element
most limiting to tree growth in Pacific Northwest forests (Gessel et al. 1973), and
large amounts accumulate in soil organic matter in a form that is unavailable to
plants. Intensive harvesting practices may alter the inputs of organic matter into
forest ecosystems by removing large amounts of organic matter and nutrients from a
site.
Such practices may also alter the environmental conditions that influence
nitrogen cycling processes, thereby affecting the rates at which N is made available to
plants or lost from ecosystems. This study investigated the influence of intensive
harvesting practices on soil nitrogen dynamics at a high productivity (Site Index II+)
coastal Douglas-fir site in western Washington. The specific focus was on the effects
of bole-only (BO) harvesting, total-tree (TT) harvesting, and total-tree plus (TP)
harvesting on N availability, N leaching, organic matter decomposition, and N
mineralization during the 8- to 12-month period following harvesting. In addition,
comparisons were made with adjacent areas representative of the original stand
conditions that were left as nonharvested reference stands (FT).
Analysis of soil solutions collected from suction lysimeters at 0.2-m depth
from March to June 2000 revealed that nitrate was the dominant form of N in the
107
harvesting and organic matter removal treatments.
Concentrations of dissolved
organic N (DON) represented a small but statistically significant (α= 0.10 level)
portion of total N, while ammonium concentrations were negligible. There was a
significant treatment effect with concentrations of nitrate increasing with increasing
intensity of harvesting and organic matter removal.
Nitrate concentrations also
increased over time, presumably due to increased microbial activity and nitrification
in response to increasing soil temperatures. Evidence of nitrification in the harvesting
treatments was also supported by changes in soil solution pH, which decreased with
increasing harvest intensity.
In the nonharvested reference forest, nitrate
concentrations were 2 to 8 times lower than in the harvested plots and decreased over
time, presumably due to vegetative uptake and assimilation of ammonium. Higher
pH values in soil solution from the forest also supported the observations that
nitrification rates were lower compared to those in harvested treatments. In the
forest, DON was the dominant form of N rather than nitrate.
However, DON
concentrations in solutions collected at 0.2-m depth were not significantly different
among all treatments. These findings support the theory of Vitousek (1981) in which
increasing harvest intensity leads to increased N availability. This has been attributed
to the reduced capacity of microbes to immobilize N in the absence of logging slash.
Soil solutions collected from suction lysimeters located at 1.0-m depth from
March to June 2000 showed a somewhat different pattern to that found at the 0.2-m
depth.
Nitrate was still the dominant form of N in the harvested plots, but
concentrations of total N were 0.5 to 4 times less at 1.0-m depth. Concentrations of
108
both nitrate and DON decreased significantly from 0.2- to 1.0-m depths in the
harvested plots, presumably due to the removal of N from soil solution via plant
uptake, microbial immobilization, and/or geochemical adsorption to iron and
aluminum oxyhydroxides and clays. However, nitrate and DON concentrations in the
forest did not change significantly with depth. There were no overall effects of
harvesting treatment or time on N concentrations in soil solution at 1.0 m, indicating
that the slight treatment differences observed at 0.2-m depth were eliminated by the
time soil solution reached 1.0-m depth. While nitrate levels at 1.0-m depth were
significantly less in the forest than in harvested plots, concentrations of DON were
several times greater in the nonharvested stands.
Nitrogen concentrations in solution collected at 1.0-m depth and an estimate
of monthly water flux were used to calculate estimates of N leaching on a unit-area
basis. Since there were no statistical differences in N concentrations at 1.0 m, there
were also no corresponding statistical differences in leaching estimates.
From
February to July 2000, a total of 3 to 4.5 kg N ha-1 were leached below the seedling
rooting zone to 1.0-m depth. Leaching of nitrate in the harvested treatments peaked
in March when net water flux was the greatest. The forest experienced very low
levels of nitrate leaching and quantities were significantly less than in the harvested
treatments. However, this pattern was completely reversed with respect to DON
leaching, which was greater in the forest but consistently lower in the harvested
treatments. The combined result was that there was no treatment effect on total N
109
leaching losses during the sampling period.
Annual estimates represented total
leaching losses of about 5.5 to 8.5 kg N ha-1 yr-1.
Forest floor organic matter was collected from the harvesting treatments and
incubated in porous ceramic cups in the field for 12 weeks from March to June 2000.
Initial characteristics, such as percent organic matter and water holding capacity,
were similar between organic matter from BO and FT treatments and between TT and
TP treatments. After 12 weeks of incubation, decomposition was greatest in the BO
treatment, followed by TP, FT, and TT and corresponded to a mass loss of 8, 4, 1.5,
and 1% respectively.
This pattern was not explained by differences in initial
chemical characteristics (C:N ratio). Differences in gravimetric moisture contents
may help explain the slow decomposition rates observed in TP and TT since these
treatments dried out more quickly during the spring. In addition, mean temperatures
at the forest floor/mineral soil interface were warmest in the TP treatment (11oC) and
were on average about 2oC warmer than BO and over 3oC warmer than FT. The
range of temperature extremes was from 2 to 30oC in the TP treatment but only 3 to
18oC and 3 to 14oC for BO and FT, respectively.
Net N mineralization rates from forest floor organic matter were also
estimated after 12 weeks of field incubation in porous ceramic cups and showed a
significant treatment effect. During the period of March to June 2000, organic matter
in FT lost 31% of initial organic N, while TT and TP lost 12 and 8%, respectively. In
contrast, the quantity of organic N in forest floor material in the BO treatment
increased by an estimated 18% of the initial amount. In this study, forest floor
110
organic matter immobilized N during the 8- to 12-month period following harvesting,
supporting the theory of Vitousek (1981). Retaining logging slash on site provided a
source of carbon for decomposer organisms, which assimilated N into microbial
biomass. Organic matter retention has been proposed as a means of conserving N on
site and evidence of this effect was found in this study. Nitrogen immobilization by
microbes in decomposing logging residues also supports the observation of reduced N
availability as seen in the lysimeter sampling of soil solution from 0.2-m depth. Once
the labile C in logging slash is completely utilized by microbes, it is expected that N
mineralization will begin to occur and N availability will increase in the BO treatment
relative to the more intensive harvesting removals. The timing of when the transition
from net N immobilization to mineralization will occur remains uncertain and will
depend on environmental factors and the quantity and quality of organic matter that is
retained, as well as interactions with coarse woody debris and other high C:N ratio
organic matter that remains on site.
6.2 Management Implications and Future Work
Intensive harvesting of the high productivity coastal Douglas-fir site
investigated in this study appeared to have only minimal effects on soil N dynamics
during the 8- to 12-month period (March to June 2000) following harvesting. While
increasing harvest intensity caused statistically greater nitrate concentrations in the
surface soil (0.2-m depth), the magnitude of these differences was rather small (i.e., 1
to 2 mg L-1), and their effect on seedling growth rates and foliar N concentrations is
111
not clear. In addition, elevated nitrification rates and lower soil solution pH values
were associated with intensive harvesting, but the persistence and duration of these
changes is uncertain. These small differences should also be seen in the context that
the experimental treatments investigated in this study do not mimic operational
harvesting practices. For example, in typical whole-tree yarding practices, branches
and foliage frequently break off and remain on the site, while much effort was made
to manually remove them for the purposes of this study.
The short-term effects of removing logging residues did not appear to be
detrimental, but instead actually increased N availability to seedlings during the study
period. Since it is the availability of N that typically limits forest growth in the
Pacific Northwest, whole-tree harvesting may improve short-term N availability by
removing the organic barrier on the soil surface, leading to warmer soil temperatures
and increased rates of mineralization. Valentine (1975) reported that Douglas-fir
seedlings grown in scarified plots where logging slash was removed grew better after
the first growing season compared to where this material was retained. This was
attributed to better root growth in response to warmer soil temperatures in the
scarified plots. Tree growth is also dependent on water availability in addition to
nutrients and temperature, and removing logging residues was found to lead to much
drier forest floor conditions. Soil moisture retention may be of particular importance
in the Pacific Northwest, which typically experiences summer drought conditions that
may limit tree growth. However, it was found in this study that even small amounts
of organic matter on the surface, such as that present in whole-tree harvested
112
treatments, could effectively buffer against soil moisture losses. In evaluating the
potential effects of intensive harvesting on sustained productivity, it is therefore
important to consider the combined effects of slash retention on nutrient cycling and
availability, and temperature and moisture conditions.
The leaching of total N to 1.0-m depth was not significantly affected by
intensive harvesting or from harvesting in general. Quantities of N that leached to
1.0-m depth were estimated to range from 5.5 to about 8.5 kg N ha-1 yr
–1
and losses
of this magnitude may be replaced over the course of a rotation through natural
atmospheric inputs and free-living N2 fixation (from 1 to 6 kg N ha-1 yr-1) (Vitousek
and Melillo 1979). However, nitrate leaching may experience a lag period and may
be substantially greater during the second year following clearcutting (Sollins and
McCorison 1981; Feller and Kimmins 1984). This lag period is frequently attributed
to low summer rainfall, decomposition products have time to build up in the soil but
are not flushed out until midwinter, and because it usually takes at least one summer
before appreciable mineralization can occur (Vitousek et al. 1982).
While total N leaching did not differ between the harvested treatments and
nonharvested forest, the proportion of N leaching as NO3-N was much greater in the
harvested treatments. Nitrate leaching has important implications in terms of water
quality, cation leaching, soil acidification, and downstream eutrophication. However,
NO3-N leaching at this site was quite low (about 3 to 7 kg ha-1 yr-1) and therefore not
expected to pose a significant detrimental effect. The ability of a given soil to buffer
against leaching losses depends to a large extent on the soil profile depth and nature
113
and mineralogy of the soil. The Boistfort soil series at this site is greater than two
meters deep, so estimates of leaching taken from 1.0-m depth samples are not
considered to be lost from the system as nutrients may be stored at depth and taken up
by roots at a later date. In addition, the presence of volcanic ash in this soil gives it
an unique anion exchange capacity, but further work needs to be done to quantify
this.
Even if leaching losses were greatly elevated, they rarely approach the
magnitude of losses associated with the removal of harvested biomass. For example,
bole-only harvesting of a high productivity 50-year-old Douglas-fir site (Site Index
36 m at 50 years) removed 478 kg N ha-1, while whole-tree harvesting increased this
to 728 kg N ha-1 (Bigger 1988). Nutrient losses in harvested biomass were therefore
two orders of magnitude greater than that of leaching losses.
These harvesting
removals represented losses of 15 and 22% of total ecosystem N for bole-only and
whole-tree harvesting, respectively (Bigger 1988). The proportion of total ecosystem
nutrients that are removed under various harvesting strategies depends to a large
extent on the proportion of nutrients that are retained in aboveground tree biomass
and this varies with the productivity of the site. Calculations of nutrient budgets for a
given site are useful in evaluating the impacts of nutrient removals (e.g. harvesting or
leaching) on total ecosystem nutrient pools, and a nutrient budget is currently being
developed for the site investigated in this study so that similar comparisons may be
made.
114
The mineral soil pool often contains much larger stores of nutrients than in
tree biomass and may represent up to 85% of total ecosystem N (Cole et al. 1967).
This corresponds to about 5000 to 9000 kg N ha-1 in coastal Douglas-fir forests
(Edmonds and Chappell 1993), but this generally tied up in soil organic matter, which
is only made available to plants on the order of 1 to 2% annually. This phenomenon
explains why N is the element most often limiting plant growth in the Pacific
Northwest, and it is for this reason that N fertilization of forests is a common practice.
The stand investigated in this study was fertilized a total of four times over the
previous rotation with a cumulative total of 1800 kg N ha-1 as urea. Practices such as
clearcutting and whole-tree harvesting often lead to increased N availability in the
short-term. The impact of such practices on long-term sustained productivity is
particularly important to address in relation to high productivity sites since it is these
areas where management practices are likely to intensify in the future.
In order to evaluate the impacts of intensive harvesting on soil N dynamics
and sustained forest productivity in a meaningful way, it is important that continued,
long-term monitoring occur. In particular, the role of logging slash as a source or a
sink for nutrients has not been fully evaluated in this study. It was determined that
retaining logging slash on site temporarily immobilized N for up to one year
following harvesting as microbes utilized the available carbon in the organic matter.
This period of immobilization was still occurring from 8 to 12 months after
harvesting, but it is not clear when the source of available C will be completely
utilized. Once this happens, it is expected that microbial biomass will begin to
115
mineralize and release N, and slash will start to act as a source of nutrients. The
timing of this transition should also have important implications for seedling growth,
particularly if it coincides with periods of high seedling nutrient demand.
Measurements of seasonal changes in N mineralization rates combined with more
intensive temperature and moisture monitoring over a longer time period will provide
valuable information to evaluate the role of slash retention on tree growth.
In
addition, the role of high carbon woody substrates and decomposing logs was not
addressed in this study, but may have a considerable influence on dynamics of N
immobilization, N fixation, and moisture retention.
The mineralization of soil
organic matter has the potential to contribute substantially more inorganic N to the
regenerating stand than the forest floor does (White et al. 1988), and current efforts at
the study site should supply valuable information concerning the effects of intensive
harvesting on soil nutrient availability.
Lastly, there is a substantial lack of
information concerning the effects of intensive harvesting on the cycling of other
nutrient elements and on populations of soil organisms, warranting further
investigation.
116
6.3 Conclusions
Results from soil solution monitoring and field incubations of forest floor during
March to June 2000 (8 to 12 months following harvesting) indicated that increasing
harvest intensity from bole-only to total-tree harvesting affected the N dynamics of a
high productivity Douglas-fir site in the following ways:

Increased N availability (nitrification) at 0.2-m soil depth as harvest intensity
increased from bole-only to total-tree to total-tree plus harvesting.

No difference between treatments in total N leaching at 1.0-m soil depth.
Total N leaching was estimated at 3 to 4.5 kg ha-1 6 mo-1 and existed
predominately as NO3-N in the harvested treatments but as DON in the
nonharvested forest.

Soil solution N concentrations decreased on the order of 1 to 4 times as
solution flowed from 0.2-m to 1.0-m depths, stressing the high buffering
capacity of these volcanic Boistfort series soils.

Logging slash retained in the bole-only harvest decomposed faster than forest
floor organic matter in the other treatments, but was a net sink for N
(immobilization).

Forest floor C:N ratios did not explain observed patterns in decomposition or
mineralization.

Low summer moisture in the total-tree and total-tree plus harvesting
treatments likely inhibited microbial activity in decomposing forest floor
organic matter.

Low temperatures in the nonharvested forest led to slower forest floor
decomposition rates.
117
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Appendix A. N concentrations (mg L-1) and pH of soil solution collected at 1.0-m depth from harvested treatments
by sampling date. Values are means followed by standard deviation in parenthesis.
Date
Bole-only
Total-tree
Total-tree plus
significance
level
NO3-N
March 3, 2000
May 4, 2000
June 26, 2000
0.95
1.49
1.37
(0.45)
(0.98)
(1.28)
0.57
0.46
---
(0.14)
(0.64)
---
0.83
1.12
1.52
(0.44)
(0.66)
(1.63)
0.82
0.65
0.94
NH4-N
March 3, 2000
May 4, 2000
June 26, 2000
0.09
0.07
0.04
(0.02)
(0.08)
(0.01)
0.08
0.07
---
(0.02)
(0.04)
---
0.07
0.03
0.03
(0.04)
(0.04)
---
0.62
0.13
---
total
inorganic N
March 3, 2000
May 4, 2000
June 26, 2000
1.04
1.56
1.41
(0.44)
(1.04)
(1.27)
0.64
0.52
---
(0.11)
(0.68)
---
0.90
1.15
1.96
(0.48)
(0.63)
(2.21)
0.79
0.66
0.86
DONa
March 3, 2000
May 4, 2000
June 26, 2000
0.15
0.31
0.23
(0.10)
(0.28)
(0.19)
0.34
0.33
---
-------
0.23
0.20
0.09
(0.18)
(0.08)
---
0.96
0.72
---
total N
March 3, 2000
May 4, 2000
June 26, 2000
1.19
1.88
1.63
(0.47)
(1.00)
(1.12)
0.90
0.37
---
-------
1.32
1.35
0.48
(0.67)
(0.58)
---
0.90
0.68
---
pH
March 3, 2000
May 4, 2000
June 26, 2000
5.84
5.44
5.72
(0.31)
(0.19)
(0.44)
5.57
5.61
---
-------
5.68
5.44
5.68
(0.74)
(0.58)
(0.78)
0.93
0.64
0.54
a
DON=dissolved organic nitrogen
+ denotes significance at p < 0.10 level
* denotes significance at p < 0.05 level
** denotes significance at p < 0.01 level
Values in each row followed by different letters are significantly different (p < 0.10) using ANOVA and Tukey's HSD
test.
130
Appendix B. N concentrations (mg L-1) and pH of soil solution collected at 1.0-m depth with respect to time.
Significance levels are from one-way ANOVA results for each treatment.
Treatment
Date
NO3-N
NH4-N
-------------------------------------
total
inorganic N
mg L-1
DONa
total N
pH
-----------------------------------
Bole-only
March 3, 2000
May 4, 2000
June 26, 2000
sig. level
0.95
1.49
1.37
0.31
0.09
0.07
0.04
0.44
1.04
1.56
1.41
0.37
0.15
0.31
0.23
0.54
1.19
1.88
1.63
0.12
5.84
5.44
5.72
0.18
Total-tree
March 3, 2000
May 4, 2000
June 26, 2000
0.57
0.46
---
0.08
0.07
---
0.64
0.52
---
0.34
0.33
---
0.90
0.37
---
5.57
5.61
---
Total-tree plus
March 3, 2000
May 4, 2000
June 26, 2000
sig. level
0.83
1.12
1.52
0.86
0.07
0.03
0.03
0.72
0.90
1.15
1.96
0.13
0.23
0.20
0.09
0.86
1.32
1.35
0.48
0.35
5.68
5.44
5.68
0.18
Forest
March 3, 2000
May 4, 2000
June 26, 2000
sig. level
0.05
0.04
0.06
0.89
0.11
0.09
0.05
0.77
0.16
0.13
0.11
0.90
0.82
1.16
0.43
0.20
0.98
1.29
0.54
0.15
5.60ab
5.30b
5.88a
0.02*
a
DON=dissolved organic nitrogen
+ denotes significance at p < 0.10 level
* denotes significance at p < 0.05 level
** denotes significance at p < 0.01 level
For each treatment, values in each column followed by different letters are significantly different (p < 0.10) using ANOVA and Tukey's HSD
test.
131
Appendix C. Moisture content (g g-1) of surface soil (0-10 cm) during field
incubation period, March 28 to June 19, 2000. Values are means followed by
standard deviation in parenthesis below. Harvested treatments were compared by
ANOVA and were compared separately to nonharvested forest with a t-test.
Date, 2000
April 25
(4 weeks)
May 26
(8 weeks)
June 19
(12 weeks)
Boleonly
Total- Total-tree
tree
plus
0.94
0.93
0.85
(0.09)
(0.13)
(0.10)
0.92
0.79
0.82
(0.16)
(0.13)
(0.10)
0.79
0.73
0.78
(0.14)
(0.07)
(0.17)
+ denotes significance at p < 0.10 level
* denotes significance at p < 0.05 level
** denotes significance at p < 0.01 level
ANOVA
sig.
level
Forest
0.46
0.93
t-test
significance
level
BoleTotal- Total-tree
only
tree
plus
0.95
0.38
0.42
0.96
0.73
0.76
0.30
0.98
0.57
(0.11)
0.40
0.82
(0.12)
0.84
0.72
(0.08)