Reprinted from the Soil Science Society of America Journal Volume 63, no. 1, Jan.-Feb. 1999
677 South Segoe Rd., Madison, WI 53711 USA
Soil Carbon and Nutrients in a Coastal Oregon Douglas-Fir Plantation with Red Alder
Kermit Cromack, Jr.,* Richard E. Miller, Ole T. Helgerson, Robert B. Smith, and Harry W. Anderson
ABSTRACT
Carbon and nutrients in the forest floor and mineral soil
were measured to determine amounts and variation among
eighteen 0.081-ha plots in a Douglas-fir [Pseudotsuga menziesii
(Mirb.) Franco] plantation growing with volunteer red alder
(Alnus rubra Bong.). Ten years earlier, the preceding mature
conifer stand was clearcut and nearly all logging slash and
forest floor were consumed by slash fire. Forest floor mass in
the 9-yr-old plantation averaged 9.86 Mg ha–1, with 3.71 Mg
C, 98.0 kg N, 10.6 kg P, and 8.4 kg S ha–1. Mineral soil to 1-m
depth averaged 176 Mg C, 8330 kg N, 3340 kg P, and 605 kg
S h a – 1 in the <2-mm fraction. The 2- to <6-mm soil fraction
averaged an additional 100 Mg C, 4480 kg N, and 1700 kg P ha–1.
Net mineralizable N as NH4 (anaerobic N mineralization index)
totaled 99 kg N ha–1 in the top 45 cm of the mineral soil and
62 kg N ha–1 in the 45- to 100-cm depth. Density fractionation
showed that the light fraction (<1.65 Mg m –3 ) was only 13.4%
of the fine soil mass of the 0- to 15-cm depth, yet contained
about 40% of the total C and N capital in the <2-mm size
fraction. The substantial amounts of C and nutrients in this
low-bulk density soil (<2-mm fraction, 0.30 Mg m – 3 ) indicate
a fertile soil despite large previous losses of organic matter
and N from the site.
C
F ORESTS in the Douglas-fir region of the Pacific
Northwest grow in a climate and on soils generally
favorable for tree growth (Waring and Running, 1998).
Accumulations of soil organic matter and N are substantial,
ranging from 44.1 to 429 Mg C ha–1 and from 2878 to 38
000 kg N ha–1 (Meurisse, 1976; Binkley, 1983; Edmonds
and Chappell, 1994). Coastal forests have an especially large
potential for accumulating N from symbiotic N-fixing trees,
particularly red alder and Sitka alder [Alnus sinuata (Regel)
Rydb.] (Binkley, 1983; Binkley et al., 1984; Bormann et al.,
1994).
A long-term study was established in 1979 to assess
OASTAL
K. Cromack, Jr., Dep. of Forest Science, 020 Forestry Sciences Lab., Oregon
State Univ., Corvallis, OR 97331; R.E. Miller and H.W. Anderson, USDA
Forest Service, Pacific Northwest Research Station, Forestry Sciences Lab.,
3625-93rd Ave. SW, Olympia, WA 98512; O.T. Helgerson, Washington State
Univ. Cooperative Extension, P.O. Box 790, Stevenson, WA 98648; and R.B.
Smith, U.S. Forestry Sciences Lab., P.O. Box 640, Durham, NH 03824.
Paper 3137 of the Forest Research Lab., Oregon State Univ., Corvallis.
Received 12 Sept. 1997. *Corresponding author (cromackk@fsl.orst.edu).
Published in Soil Sci. Soc. Am. J. 63:232-239 (1999).
changes of C and nutrient capital in the forest floor and soil and
in productivity of a coastal Oregon Douglas-fir plantation with
admixed volunteer red alder. Growth plots were established
and initial tree measurements were taken. Red alder, which
had colonized the site naturally, was thinned to residual densities
ranging from 0 to 186 trees ha–1 (Miller andObermeyer, 1996).
The objectives of this soil research on a typical Oregon
Coast Range site were (i) to determine the amounts and
variation of C and nutrients in the forest floor and soil and
(ii) to relate the variation in amount and distribution of N to
stocking of red alder. An adjacent uncut portion of the former
mature stand afforded an opportunity for comparison of C and
nutrient capital in the forest floor with that in the plantation
site.
MATERIALS AND METHODS
Study Site
The study site is a 22-ha reforested clearcut (≈123°55’ W,
44°27’ N) ≈10 km east of Waldport, OR and the Pacific Ocean.
Elevation averages ≈100 m above sea level. Soils are Andic
Haplumbrepts (loamy sands, mixed acid family), ≈1 m deep,
over saprolite of weakly cemented marine sandstone from
the Flournoy formation (Baldwin, 1981). Soil series include
Slickrock (medial over loamy, ferrihydritic over isotic,
mesic Alic Fulvudand) on the steeper slopes and Bohannon
(Fine-loamy, isotic, mesic Andic Haplumbrept) on the ridges
(Corliss, 1973). The understory plant community in the study
area is salmonberry (Rubus spectabilis Pursh.) and swordfern
[P o l ystichum munitum (Kaulf.) Presl]. The site previously
supported a stand of 130-yr-old Douglas-fir and western
hemlock [Tsuga heterophylla (Raf.) Sarg.] that originated
after wildfire. That stand was harvested by clearcutting in 1969;
stump diameters exceeded 100 cm.
After two applications of herbicide to kill and desiccate
vegetation, the logging slash was broadcast-burned in August
1970. The intense fire removed the existing forest floor from
almost the entire study area. The site was planted in January
1971 with Douglas-fir seedlings that were grown for 2 yr in
a nursery and for 1 yr as a nursery transplant before being
planted on the site. Four years later, volunteer red alder in
some portions of the plantation were all or partially controlled
by spraying with 2-4, 5 trichlorophenoxy acetic acid.
To sample plantation growth, eighteen 0.081-ha plots were
established after the 1979 growing season on ridgetop and
sideslope positions. Plot aspects ranged from east through
CROMACK ET AL.: SOIL CARBON AND NUTRIENTS IN A DOUGLAS-FIR PLANTATION WITH RED ALDER
north to west, and slopes from 15 to 75%. Six plots each were in areas
that had received full, partial, or no herbicide application.
Forest Floor and Mineral Soil Sampling
In June 1980, about 9.5 growing seasons after planting, the forest
floor and soil were sampled at nine randomly located points in each
plot. Sample locations on stumps or on remnants of rotten logs were
rejected and alternative locations randomly selected. The forest floor
above the mineral soil was removed within a 25 by 25 cm square
sampling frame. The nine subsamples for each plot were combined
into a composite sample. For a measure of within-plot variation, Plot
17 was sampled twice.
Soil samples to 1-m depth were collected at the same sampling
points for chemical analysis and determination of soil bulk density. A
double-cylinder sliding-hammer core sampler was used for the depths
0 to 15, 15 to 30, and 30 to 45 cm (Blake and Hartge, 1986). The
inner cylinder removed a soil core 7.5 cm in diameter and 7.5 cm long.
To sample the lower half of each 15-cm layer, the 7.5-cm-long corer
was offset horizontally a distance of one to two core diameters. The
two cores from each of the nine sampling points provided a composite
of 18 cores for each 15-cm-thick layer. For the 45- to 100-cm depth,
samples were taken with a soil sampling tube (2.5-cm i.d.). The
nine samples representing this layer also were composited. At some
sampling points (36 of 162), coarse sandstone fragments stopped the
tube before 100 cm was reached. Cores from interrupted samples
were composited with others from the given plot, but these composites
were used only for chemical analyses. In the three upper layers, the
composited samples were used both for determining soil bulk density
and for chemical analyses.
Some gravel and all cobble-sized (>7.62 cm) rock fragments were
excluded by these 7.5- or 2.5-cm diameter samplers. Since the volume
occupied by these coarse fragments was not sampled, this volume
was estimated and subtracted from the total volume of each sampling
depth. Based on modal profile descriptions (Corliss, 1973) for the
series at this study site, the volume of coarse fragments was estimated
to be 7.5% in the 0- to 45-cm depth and 25% in the 45- to 100-cm
depth. We applied these same corrections to the data from each plot.
Several years later, two uniform sampling locations ≈25 m from the
plantation were subjectively chosen to represent apparent extremes of
forest floor depth in an undisturbed portion of the residual 150-yr-old
stand. At both locations, the same sampling frame was used for three
forest floor sub-samples ≈5 m apart.
Sample Processing
Forest floor and soil samples were stored indoors at 16 to 21°C
during each 3-d period of sampling. Thereafter, samples were spread
evenly in trays and dried in a forced-air oven at approximately 40°C
for 72 h. Dried soil samples were weighed, placed in a rotary tap device
to break down the softer aggregates, then sieved for 2 min on a motordriven sieve to separate samples into three size fractions: fine, <2 mm;
intermediate, 2 to <6 mm; and coarse, ≥6 mm. Each fraction was
weighed to obtain its contribution proportional to the total soil mass in
the respective depth layer. The three fractions were stored separately in
paper bags at room temperature before chemical analysis.
Bulk density for the upper three 15-cm soil layers was
computed from subsamples dried at 70°C. Bulk density (whole
soil) included all three size fractions collected in the sliding
hammer core sampler (Blake and Range, 1986) because most
233
coarse particles were hard soil aggregates or soft, weathered gravel,
which we assumed were nutrient-rich and should be included in bulk
density and nutrient estimates. We used linear regression and the bulk
densities of the three upper layers in each plot to extrapolate whole soil
bulk density for the 45- to 100-cm depth as follows:
Whole soil bulk density (Mg m–3) = 0.50 + 0.0044X
[1]
where X = midpoint for each soil depth, r = 0.65, P < 0.001, and n
= 54. For the 45- to 100-cm depth, the midpoint is 72.5 cm and the
regression estimate for whole soil bulk density is 0.819 Mg m–3.
Before chemical analyses, forest floor samples were redried at
70°C, weighed, and a subsample ground in a Wiley mill to pass a
0.064-mm mesh. Subsamples of the fine soil fraction (<2 mm) were
ground with a mortar and pestle to pass a 0.042-mm mesh. Subsamples
of the intermediate fraction (2 to <6 mm) were reduced, to pass a
0.042-mm mesh, in a heavy duty metal shatterbox that pulverized the
admixed hard aggregates, gravel, and rock fragments. Subsamples of
the fine fraction used for incubation were not ground. These anaerobic
incubations for mineralizable N were made after an 8-wk storage of
samples dried at 40°C. Previous work has shown Oregon forest soils
to be relatively stable in mineralizable N after similar storage periods
(McNabb et al., 1986).
Chemical Analyses
Total N in forest floor and soil samples was determined from
NH4-N concentration (Technicon Autoanalyzer, Technicon Industrial
Systems, Tarrytown, NY) in samples digested by the micro-Kjeldahl
procedure (Nelson and Sommers, 1980). Nitrate was not measured,
however. Net N-mineralization potential was determined by anaerobic
incubation (Keeney and Bremner, 1966). The only modification to
this procedure was that specified quantities of soil and solutions were
increased fourfold. Samples were incubated for 7 d at 40°C, and initial
concentrations of KC1-extractable NH4-N in another subsample were
subtracted from post incubation values. Ammonium-N concentrations
were determined using automated colorimetric procedures on a
Technicon Autoanalyzer (Keeney and Nelson, 1982). No initial or post
incubation KC1-extractable NO3-N was measured, since denitrification
was assumed to occur under anaerobic incubation conditions.
Total C in both forest floor and soil was determined by dry
combustion in a LECO WR-12 C analyzer (LECO Corp., St. Joseph,
MI) (Nelson and Sommers, 1982). Soil from the <2-mm fraction
of the 0- to 15- and 15- to 30-cm depths was separated by specific
gravity in an aqueous solution of NaI (Spycher et al., 1983) into two
density fractions: <1.65 Mg m–3 (light fraction) and ≥1.65 Mg m–3
(heavy fraction). This fractionation process is based on the rationale
that detrital material is separated from organo-mineral and mineral
material at a density of 1.65 Mg m–3 (Spycher et al., 1983). Total N
in both the light and heavy density fractions was determined from
NH4-N concentrations (Technicon Autoanalyzer) in samples digested
by the micro-Kjeldahl procedure (Nelson and Sommers, 1980). We
lacked sufficient material in the light fraction samples after processing
to determine both C and N for this density fraction. Consequently, total
C in the light density fraction was calculated as the difference between
C concentration in the whole soil and that in the dense fraction.
Total P and total S in forest floor samples and total S in
fine soil samples were analyzed after digestion in nitric and
perchloric acid (Olson and Sommers, 1982; Tabatabai, 1982).
After digestion, P was analyzed with the molybdate blue
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SOIL SCI. SOC. AM. J., VOL. 63, JANUARY-FEBRUARY 1999
method (Olson and Sommers, 1982) and S was analyzed
with the BaSO4 gravimetric procedure (Association of
Official Agricultural Chemists, 1955); however, in both
fine and intermediate soil P was analyzed on a Technicon
Autoanalyzer in samples digested by micro-Kjeldahl.
No C or nutrient analyses were made of the coarse soil
fraction ( ≥ 6 mm), most of which was weathered rock
fragments.
Nutrient element concentrations in the fine (<2 mm) and
intermediate (2 to <6 mm) size-fractions were extended
from bulk density of the whole soil (Mg m–3) to kilograms
per hectare for each soil layer with the equation:
Nutrient mass (kg h a – 1 ) =
∑ {bulk density of the whole soil (Mg m–3)
× g kg–1 of fine (or intermediate) size fraction
× g kg–1 element concentrations
of fine (or intermediate) size fraction
× cm thickness of soil layer sampled
× (1 – rock volume [%]/100%)
× 10–1}
[2]
Carbon mass (kg h a – 1 ) was multiplied by 10–3 to pre­
sent results for C mass as megagrams per cubic meter.
RESULTS Forest Floor
Forest floor accumulation averaged 9.86 Mg ha–1 in
the 9-yr-old plantation and contained an average of 3.71
Mg C, 98.0 kg N, 10.6 kg P, and 8.4 kg S ha –1 (Table 1).
Forest floor in the adjacent mature stand ranged from
21.8 to 91.7 Mg ha–1 (Table 1). Concentrations of both C
and N were greater in the forest floor of the mature stand,
but P concentrations were similar. Carbon and nutrient
mass in the forest floor of the mature stand ranged from
approximately twofold to more than 10-fold greater than
in the plantation.
Bulk Density and Size Fractions
Mean gross bulk density (total soil) in the plantation
increased from 0.535 Mg m –3 at the 0- to 15-cm depth
to 0.667 Mg m –3 at the 30- to 45-cm depth (Table 2).
Mean bulk density extrapolated for the 45- to 100-cm
depth averaged 0.819 Mg m–3. The proportion of the fine
fraction in the soil decreased from 561 g kg–1 (56.1%) at
0- to 15-cm depth to 363 g kg –1 at 45- to 100-cm depth.
The intermediate fraction increased slightly with depth,
while the coarse fraction more than doubled, from 122 g
kg –1 at 0- to 15-cm depth, to 261 g kg–1 at 45- to 100-cm
depth. Among the 18 plots, the proportion of the ≥6-mm
fraction was the most variable of the size fractions. The
standard error was 12 to 13% of the means in the three
upper layers, compared with 7% in the 45-to 100-cm
depth.
Soil Nutrient Concentrations and Capital
Nutrient concentrations to the 100-cm depth were
greater in the fine soil than in the intermediate soil
(Table 3), averaging 33% more C, 47% more N, and
73% more P. Element ratios in the fine soil for C/N, C/
P, N/P, and N/S averaged 21:1, 57:1, 2.7:1, and 14.6:1,
respectively. In the intermediate fraction, element ratios
for C/N, C/P, and N/P averaged 23:1, 74:1, and 3.1:1,
respectively. Net mineralizable N (potentially available
NH4) in the fine soil ranged from 86.9 mg N kg–1 in the
CROMACK ET AL.: SOIL CARBON AND NUTRIENTS IN A DOUGLAS-FIR PLANTATION WITH RED ALDER
top 15 cm to 49.8 mg N kg –1 in the 45- to 100-cm soil
depth (Table 3).
The light fraction (<1.65 Mg m –3) in the fine soil
fraction (<2 mm) averaged 134 g kg –1 (13.4%) at the
0- to 15-cm depth and declined to 73 g kg –1 at the 15­
to 30-cm depth (Table 4). Carbon concentrations in the
light fraction were 433 g kg –1 at the 0- to 15-cm soil
depth and 286 g kg –1 at the 15- to 30-cm depth. At the
0- to 15-cm depth, 40.0% of total soil C in the <2-mm
size fraction was in the light fraction, even though that
fraction represented only 13.4% of fine soil weight. At
the 15- to 30-cm depth, the proportion of soil C in the
light fraction was 23.1%, indicating that at lower depths,
most soil C is associated with the heavier organo­
mineral fraction. Nitrogen concentrations in the light
fraction were 18.4 and 15.5 g kg –1 for the 0- to 15- and
15-to 30-cm depths, respectively. The proportion of
total soil N in the light fraction (<2-mm size fraction)
was 41.9 and 26.2% for the 0- to 15- and 15- to 30-cm
depths, respectively.
The heavy fraction (≥1.65 Mg m –3) mass means (fine
soil fraction <2 mm), with standard errors, were 866
(14) and 927 (9) g k g – 1 for the 0- to 15- and 15- to 30­
235
cm depths, respectively. Mean heavy fraction C and N
concentrations (0-15 cm depth) were 99 (4) and 4.2 (1.1)
g kg –1, respectively. For the 15- to 30-cm depth, mean
heavy fraction C and N concentrations were 75 (9) and
3.2 (0.4) g kg –1, respectively.
Total soil capital to the 1-m depth was large, averaging
276 Mg C, 12 800 kg N, and 5030 kg P ha –1 (Table 5).
The fine soil fraction had 64, 65, and 66% of the total C,
N, and P, respectively. Hence, at least one-third of soil
C, N, and P was in coarser soil fractions that increased
with increasing soil depth, with less accumulation at 45
to 100 cm (Fig. 1). Total S capital in the fine fraction
to the 1-m depth averaged 605 kg ha–1 (Table 6). Net
mineralizable N totaled 161 kg ha –1 in the soil profile;
99 kg N ha –1 (61%) occurred in the top 45 cm (Table
6). Net mineralizable N in the fine fraction represented
≈2% of total N in the fine soil fraction. Among the 18
plots, soil at the 45- to 100-cm depth generally was
more variable in nutrient content than overlying soil
(Table 6). Contents of P and S generally were more
variable than content of C and N.
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SOIL SCI. SOC. AM. J., VOL. 63, JANUARY-FEBRUARY 1999
Nitrogen Capital and Red Alder Stocking
The amount of N in the forest floor and soil of the
plantation was not correlated with the amount of red
alder basal area, which ranged from 0 to 40% of the total
basal area per plot (Fig. 2). The correlation coefficient
for the 0- to 45-cm depth was -0.21 (P = 0.400) and for
the 0- to 100-cm depth was -0.11 (P = 0.670). Amount of
N in the soil was more strongly correlated with average
slope percentage, which ranged from 15 to 70% (Fig.
3). The correlation coefficient was -0.39 (P = 0.108) for
the 0- to 45-cm depth and -0.19 (P = 0.446) for the 0to 100-cm depth. The obvious outlier in both figures is
Plot 18, which was above average in both mass of fine
soil and N concentration.
DISCUSSION
Forest Floor Accumulation
In the 18 yr since the earlier harvest of the adjacent
plantation site, the canopy above the soil sampling
locations in the residual stand remained undisturbed.
We therefore assumed a steady-state condition where
annual decomposition balanced additions of litter. The
two sampling locations in the mature stand were pur­
posefully chosen to represent extremes in forest floor depth.
Our sampling indicated that the preharvest stand probably
had 22 to 92 Mg ha–1 of forest floor containing 307 to 1660
kg N ha–1 that was vulnerable to loss to the atmosphere
as gases and particulates when slash was burned (Table
1). These ranges span averages of 86 Mg ha–1 forest floor
dry weight and 1300 kg N ha–1 for salmonberry-swordfern
communities in the Oregon Coast Range (Youngberg, 1966).
Photos taken at tree planting, and the apparent paucity of
charred woody debris in the plantation, indicate that most of
the original forest floor was consumed by the intense slash
fire used to remove logging debris and to retard development
of competing vegetation. Such consumption by slash fires
after harvest of mature timber has been reported frequently
(Youngberg, 1953; Little and Ohmann, 1988). Losses of
N, S, and soil organic matter, which are more temperature
sensitive, could impact ecosystem productivity, particularly
where fires are more severe (De Bano et al., 1998).
In the 9 litter-years since planting, total net forest floor
accumulation averaged 3.71 Mg C and 98.0 kg N ha–1
(Table 1), already exceeding the N capital reported for the
forest floor in a 23-yr-old Douglas-fir stand on Vancouver
Island (Binkley, 1983). Annual rates of net accumulation
in our study averaged 0.412 Mg C ha–1 and 10.9 kg N ha–1,
corresponding with a C/N ratio of 37.8. Annual rates of forest
floor accumulations in three Douglas-fir stands that averaged
25 yr of age and that were located in riparian areas of the
Oregon Coast Range averaged only 0.147 Mg C ha–1 and
3.44 kg N ha–1 (corrected Table 3, Entry and Emmingham,
1995; J.A. Entry, 1997, personal communication). More
rapid forest floor accumulation in our coastal plantation
might be explained by its better soil quality and alder
component. Mean forest floor C concentrations in the
plantation (Table 1) were 367 g kg –1, indicating a wellestablished O horizon (Federer, 1982).
Presence of volunteer red alder during the 9 yr before
establishment of the experimental plots should have
resulted in some accretion of N in both the forest floor
and mineral soil, because alders can nodulate and fix N
early in their life cycle (Binkley et al., 1994). Lack of
a significant correlation of N content with basal area of
red alder initially present (Fig. 2) and with slope per­
CROMACK ET AL.: SOIL CARBON AND NUTRIENTS IN A DOUGLAS-FIR PLANTATION WITH RED ALDER
tentage (Fig. 3) could be attributed to the plot-to-plot variation
in N content of the soil, approximately twofold for the 0- to
45-cm depth and 2.8-fold for the 0- to 100-cm depth. Our
use of an average estimate of coarse fragment volume for all
plots, instead of a plot-specific estimate, undoubtedly reduced
the accuracy and precision of our estimates of N content.
Moreover, the herbicide treatment applied 4 yr after planting
to 12 of the 18 plots would have reduced basal area of red
alder at 9 yr after planting (our x-variable), but not affected
the amount of N fixed before the herbicide treatment (our
y-variable for both figures). Future resampling of the forest
floor and soil for changes in C or nutrients (N, P, S) after two
to three decades remains an option, especially on six of 18
experimental plots where remaining red alder will constitute
a substantial component. Plot-specific estimates of gravel and
cobble content could increase accuracy of those changes and
our initial estimates of C and N content.
Accumulations of N in the forest floor and mineral soil
have been shown to be greater in intermixed alder and
Douglas-fir stands than in pure Douglas-fir stands (Tarrant
and Miller, 1963; Binkley, 1983; Binkley et al., 1984). For
example, a 23-yr-old mixed stand of Douglas-fir and red alder
on Vancouver Island had 33.6 Mg biomass ha–1 and 502 kg
N ha–1 in the forest floor, while an adjacent equal-aged pure
stand of Douglas-fir had only 7.0 Mg biomass ha–1 and 36 kg
N ha–1 (Binkley, 1983). A 55-yr-old western Washington red
alder stand accumulated ≈55 kg ha-1 N yr-1 (Cole et al., 1990).
A recent study by Busse et al. (1996) demonstrated long-term
increases in soil fertility (total N), soil quality (total C), and
forest productivity due to silvicultural retention of understory
vegetation with N2-fixing species in an experimental ponderosa
pine (Pinus ponderosa Douglas ex P. Lawson & Lawson)
plantation in central Oregon. Mechanisms of soil N and C
incorporation and retention are being investigated in current
forest ecosystems research (Johnson, 1992, 1995).
Soil Bulk Density
Bulk densities of the fine soil fraction in the top 45
cm averaged 0.302 Mg m–3, if we assume that this <2-mm
fraction occupied ≈50% of the total core volume, that is in
direct proportion to its weight. Such low bulk densities are
typical for coastal soils in Oregon and Washington, which are
rich in organic matter and have ashy (Andic) properties. Gross
bulk densities for the whole mineral soil (Table 2) are similar
to values reported for Andic soil pedons by Homann et al.
(1995), who found Andic soil bulk densities to be significantly
correlated with organic C concentrations in both A and B soil
horizons.
Volunteer red alder in this Douglas-fir plantation may have
contributed to low bulk density. In a 30-yr-old Douglas-fir
plantation in the Cascade Range of southwest Washington, soil
bulk density was lower and organic matter higher where red
alder was admixed (Tarrant and Miller, 1963). Similar effects on
soil organic matter and bulk density were measured by Bormann
237
and DeBell (1981) near coastal Washington. Mean bulk
density in the top 15 cm at our study area was ≈22% lower
than the mean bulk density in the top 20 cm of soil in the 13
pure red alder stands investigated by Bormann and DeBell
(1981). The nearly 2.5-fold greater concentrations of soil
organic matter found in our study area probably contributed to
the lower bulk density. Soils with Andic properties have low
bulk densities related to soil mineralogy and shear strength
(McNabb and Boersma, 1993), as well as to soil organic C
concentration (Homann et al., 1995) and are highly porous
(Dyrness, 1969).
Nutrient Concentrations and Amounts
Soil C and N content to the 100-cm depth in this 9-yr-old
plantation (Table 5) exceed those in older alder-conifer stands
in coastal (Bormann and DeBell, 1981; Binkley, 1983) and
noncoastal sites (Tarrant and Miller, 1963) in Washington and
British Columbia, but are similar to those in a 55-yr-old alderconifer stand at Cascade Head on the Oregon coast (Franklin
et al., 1968; Binkley et al., 1992). In 14 Douglas-fir stands in
coastal Oregon, Edmonds and Chappell (1994) found soil C to
average 164.3 Mg ha-1, and soil N to average 8500 kg N ha-1.
Amounts they reported for coastal hemlock stands were similar
to those in our 9-yr-old plantation, when reduced by one-third
because we included both the fine and intermediate fractions in
our analyses.
Phosphorus concentrations in the fine soil fraction (Table 3)
averaged more than twice the mean for soils in the United States
and are similar to other P concentrations reported for the Pacific
Northwest (Stevenson, 1986). Mean total P concentration in
the 0- to 15-cm fine soil fraction was 1.66 g kg-1, about twice
the mean available P found from a sequential P fractionation
in the three plots having the highest density of red alder in our
plantation (Giardina et al., 1995). Sulfur concentrations in the
fine soil fraction (Table 3) were in the middle of the range (0.1­
0.5 g kg-1) reported for many soils (Tabatabai, 1982).
Soil in this plantation is fertile in potentially mineralizable N
(86.9 mg kg-1 at 0-15 cm depth). Net mineralizable N as NH4
was about 1.7-fold greater than in the mixed alder-conifer stand
at Wind River, and ≈80% of that at Cascade Head under pure
red alder (Sollins et al., 1984). Our mean value of 86.9 mg kg-1
for the 0-to 15-cm depth was close to the value (82.6 mg kg-1)
reported by Myrold (1987) for the top 20 cm of the mineral soil
in a coastal Oregon western hemlock stand. Net mineralizable N
at our site averaged ≈2% of total N, which is in the middle of the
1 to 3% range of total annual available N reported for a variety of
soils (Wild, 1988). The mineralizable-N content is sufficiently
large that addition of N as fertilizer would not be recommended
(Shumway and Atkinson, 1978). Data from this plantation
showed no measurable response 14 yr after fertilization with
225 kg N ha-1 as urea (R.E. Miller, 1998, unpublished data
on file at the U.S. Forestry Sciences Laboratory, Olympia,
WA). The 9-yr-old stand averages ≈12 900 kg N ha-1, of
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SOIL SCI. SOC. AM. J., VOL. 63, JANUARY-FEBRUARY 1999
which 98 kg is in the forest floor and 8180 kg N ha-1 is
in the 0- to 45-cm depth. In contrast, the mature stand
averages ≈982 kg N ha-1 in the forest floor. Our estimates
of soil N are slightly underestimated because we did not
analyze for NO 3, but are slightly overestimated because
we oven-dried our soil at 70°C instead of the conventional
105°C. Although future harvesting and burning on this
site will cause some losses of elements such as C, N,
P, and S, maintenance of a substantial capital of soil
organic matter and a favorable nutrient balance should
be possible if forestry practices minimize soil erosion
and nutrient losses. Research on coastal Oregon forest
productivity also affords opportunities to study effects of
soil nutrient availability, including trace elements, on tree
growth and nutrition (Waring and Running, 1998).
Our study also demonstrates the value of keeping the
intermediate size fraction (2 to <6 mm) for chemical
analyses. By including it, we increased our estimate of
soil content of C, N, and P by at least 50%. For soils
with a high proportion in that fraction, such as strongly
aggregated and shotty soils, including soils in this
region with Andic properties, the chemical analyses
of soil fractions >2 mm could be important. During
sample preparation, these >2-mm fractions either could
be pulverized and added to the original <2-mm fraction
or chemically analyzed separately, as we did. A recent
study also kept the <2-mm and the >2-mm soil fractions
separate for chemical analyses, then calculated C and
nutrient concentrations on a whole soil basis (Carlyle,
1995). To improve our understanding of nutrient cycling
and soil weathering processes, soil nutrient analyses
could be extended to include all size fractions.
Carbon and Nitrogen Fractionation
Fractionation of the <2-mm soil at 0- to 15-cm depth
showed that the 13.4% concentration of light fraction
(specific gravity <1.65 Mg m–3) at our site was similar to
that reported for a mixed alder-conifer stand near Wind
River in the Washington Cascade Range and for a 55-yr-old
red alder stand at Cascade Head in coastal Oregon (Sollins
et al., 1984). Estimates of C and N concentrations in light
fractions are conservative, because density fractionation
probably results in some solution losses of these elements
(Swanston and Myrold, 1997). Carbon concentration in the
<2-mm fraction from our site averaged ≈6% less than that
from Cascade Head (also a fertile coastal site), but ≈25%
more than in the mixed alder-conifer Wind River stand
(Sollins et al., 1984). In both our study and in previous work
by Spycher et al. (1983), the light fraction was a substantial
reservoir for soil C and N.
Current research indicates that the light fraction separated
by NaI at a specific density of ≈1.7 Mg m–3 is useful for
showing management effects on soil C and N (Bremer et
al., 1994; Boone, 1994), especially where N-fixing species
are used as a source of soil residues
(Barrios et al., 1996; Jones et al., 1997). Swanston and
Myrold (1997) report that 15N from labeled red alder litter
is incorporated into both the light and the heavy fractions;
however, recovery of 15N after 21 mo was greater in the light
fraction than in the heavy fraction. The high N concentrations
(15.5-18.4 g kg-1) found in the light fraction in our study, and
low C/N ratios (18.4-23.5) suggest that this soil component
could be an important contributor to the mineralizable-N
pool through decomposition and humification processes, as
suggested originally by Ford and Greenland (1968). Isolation
of the light fraction from the soil provides opportunities
for better chemical characterization of particulate organic
matter by using solid state 13C nuclear magnetic resonance
spectroscopy (Skjemstad et al., 1997). Currently, 13C isotopic
analysis of the light fraction is being used to characterize
effects of long-term vegetation changes on soil C (Connin
et al., 1997).
CONCLUSIONS
Microsites in the original 130-yr-old stand averaged ≈22 to
92 Mg ha-1 of forest floor that contained 307 to 1650 kg N
ha-1. Postharvest slash burning probably released most of this
to the atmosphere as gases and particulates. In the 9 yr after
planting the new forest, net accumulation of forest floor in the
plantation averaged 1.1 Mg ha-1 yr-1. The content of C and N in
the forest floor and mineral soil of the 18 plots was more
closely related to slope percentage than to the basal area of
volunteer red alder in the plantation. About one-third of the
C and nutrients in the soil were in the 2- to <6-mm fraction,
which is frequently discarded before chemical analysis.
These coarse fragments should either be ground to pass a
2-mm sieve or, at least, a subsample should be analyzed to
assess its significance to total C and nutrient content. Density
fractionation of the fine soil (<2 mm) from the 0- to 15-cm
depth showed that 13.4% was in the light fraction (<1.65 Mg
m–3). About 40% of soil C and N, however, was in this light
fraction, which may be more sensitive to future changes in
forest management practices.
ACKNOWLEDGMENTS
This research was supported by USFS Co-op Aid Grant no. 80-87
and by NSF grant DEB-9632122. This report is a joint contribution
of the Department of Forest Science, Oregon State University, and
the USDA Forest Service Pacific Northwest Research Station. The
authors thank C. Glassman for chemical analyses; G. Spycher for
help with density fractionation; S.G. Stafford for statistical advice;
C.T. Dyrness and W.O. Russell, III for information about geology
and the soil series; P. Homann and A.R. Tiedemann for review of
earlier manuscript drafts; J. Lattin for editing; and G. Bracher for
figure illustrations.
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