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Reprmted from Crop Scien
ce
Vol. 39, No. 5
\"IO
Nitrogen Mineralization in a Pine Plantation Fifteen Years After Harvesting
and Site Preparation
Kathryn B. Piatek* and H. Lee Allen
ABSTRACT
with no apparent changes in total soil C (Johnson, 1992).
For example, following whole-tree harvesting, organic
matter was redistributed within the soil profile, but the
total organic matter pool had not changed 3 yr after
treatment (Johnson et al., 1991). The N pool in the
forest floor was 17% lower 8 yr after harvest, although
model-predicted values suggested a 25 to 40% N reduc­
tion (Johnson, 1995). Hendrickson et al. (1985) found
that microbial activity decreased in the forest floor fol­
lowing whole-tree harvest, probably because of the si­
multaneous decrease in the forest floor moisture and
water-holding capacity, whereas mineral soil in both
whole-tree and stem-only harvests had an increased mi­
crobial activity. Additionally, harvesting removes nutri­
ents accumulated in above ground biomass. In short­
rotation forestry, the proportion of N removed may
be higher due to higher N proportions in foliage and
branches, and this may over time deplete the site N
pool (Powers et al., 1990).
Mechanical preparation of harvested sites for tree
planting can further increase mineralization (Burger
and Kluender, 1982; Burger and Pritchett, 1984; Vi­
tousek and Matson, 1985; Fox et al., 1986; Vitousek et
al., 1992), although the effects of site preparation are
usually confounded by the effects of harvesting (John­
son, 1992). Mixing of organic matter with mineral soil
(Burger and Kluender, 1982) improved soil aeration
(Vitousek and Matson, 1985), and exposure of mineral
soil to direct solar radiation during site preparation has
been found to stimulate mineralization.
During the 1970s and 1980s in the southeastern USA,
harvested sites were commonly prepared for tree plant­
ing by shearing of residual stems, piling of slash into
windrows, then disking in the inter-windrow areas
(Haines et al., 1975). Operational shearing and piling,
done in large tracts, often led to the removal of topsoil
along with slash and debris, which decreased organic
matter and N content in the interwindrow areas com­
prising the planting surface (Pye and Vitousek, 1985).
Estimated displacement of N during this operation var­
ied but was as large as 300 to 650 kg N ha-1 (Morris et
al., 1983; Tew et al., 1986, respectively). Despite such
large N losses, N mineralization rates were elevated
for as long as five years following this method of site
preparation (Vitousek et al., 1992).
Limited information on longer-term effects of har­
vesting and site preparation on N mineralization is avail­
able. Several investigators have postulated that losses
of N during site preparation may result in N limitations
to forest productivity at mid-rotation when N demand
Intensive site preparation for forest tree planting may result in a
mid-rotation decline in soil N availability. Such decline has not been
fully documented. Tirls study was conducted in a loblolly pine (Pinus
taeda L.) plantation in the Piedmont of North Carolina to evaluate
the effects of nutrient removal during harvest and site preparation
on N availability at mid-rotation. Treatments, installed in 1981, con­
sisted of a combination of harvest (stem-only vs. whole-tree) and site
preparation (chop and bum vs. shear, pile, and disk), with a split-plot
of vegetation control (no herbicide vs. herbicide). In 1995 net N
mineralization was examined by monthly in situ soil incubations from
May through November (7 mo). Net N mineralization was approxi­
mately 3 times lower at mid-rotation than shortly after treatment. A
s•c drop in soil temperature at 10-cm depth helped explain "'50% of
this decline. At mid-rotation, harvest intensity, but not site preparation
intensity, affected N mineralization, with stem-only harvest plots min­
eralizing 11 kg N ha-1 more than whole-tree harvest plots during the
seven months. Chop-burn-no herbicide plots mineralized 34(±3) kg
N ha-t, chop-burn-herbicide: 30(±3) kg N ha-1, shear-pile-disk­
herbicide: 28(±3) kg N ha-1, and shear-pile-disk-no herbicide:
19(±3) kg N ha-1 in the seven months. Mid-rotation mineralization
was positively correlated with soil temperature and negatively corre­
lated with soil P and soil C:N ratio. The effect of harvest on N
mineralization was probably exerted through P nutrition, whereas the
lack of site preparation effects suggested that large nutrient removals
that occurred with shearing and piling did not have lasting and negative
effects on N availability in this plantation.
N net N mineralization following harvest and regen­
UMEROUS STUDIES HAVE DOCUMENTED
increases in
eration of forest stands (Burger and Kluender, 1982;
Burger and Pritchett, 1984; Vitousek and Matson, 1984,
1985; Waide et al., 1988). Mineralization usually in­
creases following forest harvest because canopy removal
can increase soil temperatures (Vitousek and Matson,
1985; Waide et al., 1988), while on drier sites, lower
evapotranspiration can increase soil water content
(Waide et al., 1988). In addition to exposing of the
forest floor to direct sunlight, harvesting equipment may
extensively mix litter with mineral soil, and compact
both the litter layer and soil (Boyle, 1976; Burger, 1983;
Gent et al., 1984; Turner and Gessel, 1990). Harvesting
may increase or decrease the mass of the forest floor,
depending on how much slash is left on the ground,
Kathryn B. Piatek, USDA-FS, Pacific Northwest Research Stn., Olym­
pia Forestry Sciences Lab., 3625 93rd Ave., Olympia, WA 98512-9193;
and H. Lee Allen, Dep. of Forestry, North Carolina State Univ., Box
8008, Raleigh, NC 27695. Sponsoring organization: Dep. of Forestry,
North Carolina State Univ. Received 18 May 1998. *Corresponding
author (kpiatek/r6pnw_olympia@fs.fed.us).
Published in Soil Sci. Soc. Am. J. 63:990-998 (1999).
990
PIATEK & ALLEN: NITROGEN MINERALIZATION YEARS AFTER SITE PREPARATION
is greatest (Burger and Kluender, 1982; Neary et al.,
1984; Morris and Lowery, 1988; Fox et al., 1989; Allen
et al., 1990; Thornley and Cannell, 1992). Net N mineral­
ization rates in clayey kaolinitic soils (0-15 em depth)
under loblolly pine in the Piedmont of North Carolina
were 80 to 90 kg N ha-1 yc1 at age 1 yr (Vitousek and
Matson, 1984; 1985), declining to 40 to 50 kg N ha-1
yr-1 at age 5 yr (Vitousek et al., 1992). Similarly, the
top 15 em of a podzolized sandy soil under Monterey
pine (Pinus radiata D. Don) aged 1, 2, and 3 yr mineral­
ized 73, 52, and 45 kg N ha-1 yc\ respectively (Smet­
hurst and Nambiar, 1995). Data on N mineralization
from soil organic matter at later stand ages are frag­
mented, but they indicate a further decline in mineral­
ization rates. For example, mineralization in fine loamy
soil under some mixed coniferous forests in the Sierra
Nevada at 0 to 14 em depth was 49 kg N ha-1 yc1 at
age 5 yr, 31 kg N ha-1 yc1 at age 17 yr, and 12 kg N
ha-1 yc1 at age 100 yr (Frazer et al., 1990).
The objectives of this study were to (i) determine the
extent of net N mineralization decline in a mid-rotation
loblolly pine plantation, (ii) assess the effect of nutrient
removal during harvesting and site preparation on net
N mineralization 15 yr after treatment, and (iii) correlate
mid-rotation mineralization rates with soil nutrient and
environmental conditions. By examining N mineraliza­
tion rates across a range of treatments, we tested the
hypothesis that N availability at mid-rotation is nega­
tively impacted by increasing intensity of N removal.
METHODS
Site Description
The study was conducted in the Piedmont of North Carolina
(Vance County), at 36° 25' N, 78° 30' W. Annual average air
temperature, based on a 64-yr record, is 14.8°C (59°F); the
summer mean is 24.6°C (76°F), and the winter mean is 4.7°C
(40°F). Average precipitation from January through May and
September through December is 87 mm (3.3 in.) per month,
increasing to 117 mm per month (4.6 in.) during the summer
months. Average total precipitation is 1133 mm (44.6 in.) per
year (Nat!. Climatic Data Center, 1997). Soils on the site
are fine, kaolinitic, thermic Typic Kanhapludults of the Cecil
series, commonly found in the Piedmont.
Previous Work
Net N mineralization was examined as part of a larger study
to determine impacts of forest management practices on long­
term site productivity. Previous work at this site has docu­
mented harvest and site preparation effects on nutrient bud­
gets (Pye and Vitousek, 1985; Tew et al., 1986), soil physical
conditions (Gent et al., 1984; Stewart, 1995), environmental
conditions and planted pine response to competition (Byrne et
a!., 1987; Nusser and Wentworth, 1987; Byrne and Wentworth,
1988; Fredericksen et al., 1991), pine plantation growth (Allen
et al., 1990), and N transformations (Vitousek and Matson,
1985; Vitousek et al., 1992). Detailed site descriptions are
provided in these earlier articles.
Treatments
In·1981, a 22-yr-old loblolly pine plantation of the previous
rotation was clearfelled. The study was installed as a split-
991
plot, randomized complete block design with three replica­
tions (blocks) within 1 km of each other. The treatments were
a combination of harvesting and site preparation assigned to
main plots and vegetation control assigned to subplots. Two
types of harvest were implemented: whole-tree and stemwood­
only removal. Whole-tree harvest resulted in an estimated
removal of 180 kg N ha-l, 19 kg P ha-1, 89 kg K ha-1, 178 kg
Ca ha-1, and 35 kg Mg ha-1• Stem-only harvest resulted in an
estimated removal of 57 kg N ha-l, 5 kg P ha-1, 35 kg K ha-',
51 kg Ca ha-1, and 14 kg Mg ha-1 (Tew et a!., 1986).
Following harvest, two types of site preparation were ap­
plied. In the lower intensity chop-burn, large pieces of logging
debris were fragmented with a roller drum chopper and
burned in a low-intensity, incomplete controlled burn. Chop­
ping and burning resulted in an estimated displacement of 46
kg N ha-1, and 0 kg ha-1 P, K, Ca, and Mg (Tew et al., 1986).
In the high intensity shear-pile--disk treatment, stumps were
sheared with a KG blade, and slash was piled away from the
planting site into windrows 47 m apart. Along with slash, piling
removed some topsoil. The area between the windrows was
disked to a depth of 7 to 12 em (Gent et a!., 1984). Shearing
and piling displaced an estimated 591 kg N ha-l, 34 kg P ha-1,
92 kg K ha-1, 363 kg Ca ha-1, and 64 kg Mg ha-1 (Tew et
a!., 1986). Vegetation control was implemented in subplots,
measuring 30 m by 15 m. In the herbicide treatments, an
application of hexazinone [3-cyclohexyl-6-(dimethylamino)-1­
methyl-1,3,5-Triazine-2,4(1H,3H)-dione] at the time of tree­
planting in 1982 was followed by a broadcast application of
glyphosate [N-(Phosphonomethyl)glycine] in late August,
once a year during the next 2 yr, and as-needed in the next
3 yr (Vitousek and Matson, 1985). In the no herbicide treat­
ments in 1995, pines were mixed with hardwood species. In
the chop-burn-no herbicide treatment, pine basal area was
11 m2 ha-1, while hardwood basal area reached 9 m2 ha-1•
Hardwood tree species in the chop-burn-no herbicide treat­
ment were present in the following order of abundance: oaks
(Quercus alba L., Q. rubra L., Q. falcata Michx.), red maple
(Acer rubrum L.), hickories (Carya glabra (Mill.) Sweet, C.
tomentosa Poir. Nutt.), dogwood (Comus florida L.), cherry
(Prunus serotina Ehrh.), sweetgum (Liquidambar styraciflua
L.), and small quantities of other southern hardwoods, such
as tulip poplar (Liriodendron tulipifera L.), sourwood (Oxy­
dendrum arboreum DC.), and white ash (Fraxinus americana
L.) (Mellin, 1995). Shear-pile--disk-no herbicide treatments
contained 23 m2 ha-1 pine basal area and 2 m2 ha-1 of hard­
wood basal area in the following order of abundance: tulip
poplar, oaks, red maple, black gum (Nyssa sylvatica Marsh.),
cherry, and small quantities of sweetgum, dogwood, hickories,
and white ash. There were 24 subplots in the main study.
Additionally, one unharvested control plot remained in each
block for a total of three plots. Pine trees on these plots were
36 yr old at the inception of the current study in 1995.
Field Sampling
Net N mineralization was assessed at plantation age 15 yr
by the sequential, in situ soil incubation method (Raison et
a!., 1987). Soil samples were collected monthly from early May
to early December. Each plot of the stem-only and whole­
tree harvest treatment (n = 24) was sampled, as were three
unharvested control plots. The forest floor layer was pushed
aside, and the A horizon was sampled to a depth of 0 to 15
em by taking four soil cores per plot. Samples were composited
per plot and transported to the laboratory for extraction. Addi­
tionally, four PVC tubes (S-cm in diam.) were inserted verti­
cally into the mineral soil to a depth of 15 em to incubate a
soil volume in the absence of plant uptake. Incubation lasted
992
SOIL SCI. SOC. AM. J., VOL. 63, JULY-AUGUST
an average of 31 d (minimum 26, maximum 35 d) in capped
tubes to prevent leaching with rain. After incubation, these
samples were also collected, composited by plot, and trans­
ported to the laboratory for extraction. Soil temperature was
measured with a temperature probe inserted to a depth of
10 em. Temperature readings were taken at the time of soil
sampling between 0900 and 1400 h, and the sequence of entry
into blocks and plots varied. Four temperature readings were
averaged per plot.
1999
on N mineralization rates were explored. For this purpose,
soil temperatures were summed across the measurement pe­
riod. Soil N, P, C, and N:P and C:N ratios, soil water, and the
sum of soil temperatures were correlated with the sum of
monthly N mineralization rates. Significance was accepted at
P :S 0.05 for all analyses.
RESULTS
Nitrogen Mineralization
Laboratory Analyses
Duplicate 10-g subsamples of soil from each plot were ex­
tracted with twenty-five mL of 2 M KC1 (Raison et a!., 1987).
The soil with solution was shaken mechanically for one hour,
and then centrifuged for 15 minutes. Supernatant was collected
with a pipette and unfiltered extracts were frozen for later
analysis. Extracts were analyzed colorimetrically for NHt and
N03 (Method 12-107-06-2-A and 12-107-04-1-B; Quickchem,
Lachat Instruments, Mequon, WI). Soil KCl-extractable N
was obtained by adding the values of KCl-extracted ammo­
nium and nitrate. Net N mineralization in the absence of plant
uptake or leaching was estimated by subtracting preincubation
soil N (initial) from incubated soil N (Raison et a!., 1987).
Monthly rates of net N mineralization were summed for the
May through December collection period.
Soil water content was determined gravimetrically for each
initial and end-of-incubation composite soil sample. Duplicate
10-g subsamples of soil from each plot were oven-dried for
24 h at 105°C. Gravimetric soil water content was converted to
volumetric water content by multiplying by treatment-specific
bulk densities from Stewart (1995).
Soil N, P, and C were determined for each plot on a soil
sample composited from three different collection dates. Each
sample was oven-dried at 70°C and ground in a Wiley mill to
pass through a 2-mm screen. Duplicate 0.5-g subsamples from
each plot were wet-digested in a sulfuric acid and hydrogen
peroxide mix (Parkinson and Allen, 1975), and total soil N
and P (total organic P and polyphosphates) were determined
colorimetrically (Method 13-107-06-2-D and 13-115-01-1-B for
N and P, respectively; Quickchem, Lachat Instruments). Addi­
tionally, soil total C and N concentrations were quantified with
a PE 2400 CHN Elemental Analyzer (Perkin-Elmer Corp.,
Norwalk, CT).
Statistical Analyses
Net N mineralization in the stem-only harvest plots
was, on average, 40 to 100 g N ha-1 d-1 greater than that
in the whole-tree harvest plots (Fig. 1). N mineralization
rates in the unharvested control plots were higher than
those of the treated plots during the spring and summer
months, with July differences being significant, but they
were lower than in the treated plots at the end of the
season (Fig. 1). N mineralization in the unharvested
control plots, chop-burn-no herbicide plots, and shear­
pile-disk-no herbicide plots peaked between July and
August. By contrast, N mineralization on shear-pile­
disk-herbicide and chop-burn-herbicide plots peaked
one month later (Fig. 1). Ammonium dominated the
extractable-N fractions in initial and incubated samples
(Fig. 2).
Harvest, and the site preparation X vegetation control
340
;:'22()
I
}' 180
I
ro 140 ..c
..9 100
c
0
.
Treatment effects on net N mineralization, soil tempera­
ture, soil moisture, total soil N, C, P, and ratios of N:P, C:N,
and C:P were tested by analysis of variance (ANOVA) for a
split-plot design (SAS, 1988) with whole plots of harvesting
and site preparation, and subplots of vegetation control (n =
24 plots). To determine whether net N mineralization in unhar­
vested control plots was statistically different from that in the
treatment plots, the following contrast statement was con­
structed in a randomized complete block design: T1 + T2 +
T3 + T4 + T5 + T6 + T7 + T8 - 8 (T9)
0, where T
treatment combination. Nine treatment combinations were
assigned as follows: T1 through T8 for each of the eight combi­
nations of harvesting, site preparation, and vegetation control;
the ninth treatment was the unharvested control.
Soil moisture content after the 31-d incubation was com­
pared to soil moisture content of bulk soil outside of the PVC
tube to test whether soil water content was affected inside the
capped incubators, thus changing N mineralization conditions
from those of the bulk soil. ANOVA for the average soil
water content inside and outside the incubators was conducted
every month.
The effects of soil nutrients and soil environmental factors
=
=
• stem On
•�'mole Tree
• unharvested Con ol
300
60
20
340
Q)
c
E
z
+"'
Q)
300
*crop, lli.KJv1-lertli
• OlojJ, lliJTvNo Helbicide
• Shear,
Pile, DiskMerbicide
+ Shear, Pile, DislqNo Herbicide
260
220
180
z 140
100
60
20 �----.-----.---�--�
May
Jun
Jul
.AJ.Jg
nme
Sep
Oct
Fig. 1. Effects of harvest intensity (upper panel) and site prep·
aration X vegetation control interaction (lower panel) on the sea­
sonal course of net N mineralization. Values shown reflect
extractable-N accumulated between two consecutive months.
Monthly rates were divided by the number of days in the incuba­
tion period.
993
PIATEK & ALLEN: NITROGEN MINERALIZATION YEARS AFTER SITE PREPARATION
22
6
I
20
Q....
"'0
18
:J
-ro
16
L..
Q)
0.. I
E
100
Q)
0
i'
z
.....
0 , ==== ==
, ==== ==
, ==== , ======
Jun
Jul
Aug
Sep
Oct
==== ,
·a
(f)
Nov
Time
effects were significant when monthly net N mineraliza­
tion rates were summed from May to November (Table
1). As noted for monthly values, summed N mineraliza­
tion was greater in stem-only harvest plots, exceeding
that of the whole-tree harvest plots by 11 kg N ha-1 for
the 7-mo period (Table 1). Nitrogen mineralization was
highest in the chop-burn-no herbicide treatment, fol­
lowed by equivalent levels in the chop-burn-herbicide
and shear-pile-disk-herbicide, and N mineralization
was lowest in the shear-pile-disk-no herbicide treat­
ments. The unharvested control plots mineralized 31 kg
N ha-1, a level similar to that in the stem-only harvested
plots and chop-burn-herbicide plots (Table 1).
Soil Temperature
Soil temperature differed significantly with vegeta­
tion control. The herbicide-treated plots were 0.6°C
warmer than non-herbicide-treated plots in July, Au­
gust, and September (Fig. 3). Accordingly, the sum of
soil temperatures was significantly higher in herbicide
treated plots than in non-treated plots (Table 2).
Throughout the growing season, soil temperatures in
Table 1. Net N mineralization rates (May to November) and
treatment effects in a mid-rotation loblolly pine stand 15 yr
after silvicultural treatments compared with Year 1 and 2
after treatment.
N mineralization (May to Nov.)
Vitousek &
Matson (1985)t
Year 1
Year 2
Current study:j:
t
,......
T""
I
220
200
Aug
Time
Jun
Fig. 3. Effects of harvest intensity on the seasonal course of soil tem­
perature at 10-cm depth (upper panel) and soil water conditions
(lower panel).
the unharvested control plots were generally lower than
those in the current-rotation plots. By November, how­
ever, soil temperature averaged 1.3°C higher in the un­
harvested control plots. In general, maximum soil tem­
peratures occurred in August and September.
Soil Water Content
In 1995, soils were driest in May and August and
wettest in June and October (Fig. 3). Vegetation control
had significant effects on soil water content in May and
June. In both months, plots not treated with herbicide
averaged 27 g kg-1 more soil water than those treated
with herbicide (Table 2). Soil water in the unharvested
plots averaged 30 g kg-1 more during the dry months
Year 15
--- kg ba-1 ---
Harvest
Stem-only
Whole-tree
SE§
Site prep X vegetation control
Chop-burn-herbicide
Chop-burn-no herbicide
Shear-pile-disk-herbicide
Shear-pile-disk-no herbicide
SE§
Unharvested control
8
240
Fig. 2. Ammonium and nitrate pools in incubated intact soil cores.
Average of all treatments.
Treatments
10
P>F
0.022
73.0
70.0
55.0
46.0
33.4
22.3
±2.6
66.0
71.0
100.0
79.0
85.0
50.0
74.0
57.0
18.0
19.0
29.8
34.3
28.1
19.1
±3.0
31.2
Table 2. Treatment means and treatment effects on soil water
and sum of soil temperatures 15 hr after silvicultural treatments.
Only significant treatment effects are shown.
Soil water
0.055
Data were adjusted to reflect the same collection period, i.e., May to No­
vember.
:j: Sample size was n = 24 for treated plots. Unharvested control n = 3.
§ Standard error of least square mean.
Treatments
May
June
--­g kg-I --Harvest
Stem-only
Whole-tree
Vegetation control
Herbicide
No herbicide
Sum soil T
•c
135"'
147"'
223"'
229"'
88.4"'
86.9"'
127**
155**
213*
239*
89.0***
86.3***
"',Not significant;*, **, ***,significant at the 0.05, 0.01, 0.001 levels, respec­
tively.
994
SOIL SCI. SOC. AM. J., VOL. 63, JULY-AUGUST
(May and August) than the herbicide treated plots. Soil
moisture inside the capped incubators 30 d after installa­
tion was not significantly different from that outside of
the incubators (data not shown).
Soil Nitrogen, Phosphorus, and Carbon
There were no treatment effects on soil N. Harvesting
effects on soil P were significant, with whole-tree har­
vested plots having 18% higher P content than stem­
only plots (Table 3). Carbon was affected by site prepa­
ration; where organic matter was removed 1S yr prior
in the shear-pile-disk treatments, soil C was 30% lower
than on the chop-burn plots, in which organic matter
was left in place or lightly burned. Ratios of soil N: P
and C: N were significantly lower on the shear-pile-disk
treatments than on the chop-burn treatments. None of
the treatment interactions were significant.
Relationships Between Net Nitrogen Mineralization and Soil Nutrient and Environmental Conditions Net N mineralization was negatively correlated with
soil P (r = -0.S3), but positively with N: P ( r = 0.62)
and C: N ratios (r = O.S3) and with the sum of soil
temperature ( r = 0.73) (Fig. 4). On a monthly basis, net
N mineralization was not correlated with any of the soil
nutrient or environmental variables.
DISCUSSION
Nitrogen mineralization can be assessed by a variety
of techniques, including ion exchange resin bags (Bink­
ley and Matson, 1983), buried bags (Vitousek et al.,
1992), and undisturbed soil core incubations (Raison et
al., 1987; Binkley et al., 1990; Hart et al., 1994). All
techniques have limitations. For this study, in situ soil
incubation in PVC tubes was used to estimate N miner­
alization, to exclude plant uptake, to minimize soil dis­
turbance, and to allow soil to incubate under site-specific
temperature and moisture conditions (Binkley et al.,
1990; Hart et al., 1994). This method measures net N
turnover, or the difference between microbial immobili­
zation and N mineralization (Hart et al., 1994), provid­
ing an estimate of plant-available N. In situ soil incuba­
tion does not indicate the intensity of microbial activity,
i.e., gross N mineralization rates. Thus, a low rate of
net N mineralization may be explained by either a high
rate of microbial N immobilization or a low rate of
microbial activity.
Table 3. Treatment means and treatment effects on soil N, P, C,
and nutrient ratios. Only significant treatment effects are
shown.
Treatments
SoiiN
Soil P
Soil C
N:P
C:N
g kg-1
Harvest
Stem-only
Whole-tree
Site preparation
Chop-burn
Shear-pile-disk
0.7"'
0.7"'
0.2*
0.3*
14.1"'
14.3"'
3.0"'
2.6"'
22.2"'
21.3"'
0.7"'
0.6"'
0.3"'
0.3"'
16.0*
12.3*
3.1*
2.5*
23.0*
20.5*
"', Not significant; * significant at the 0.05 level.
1999
Decline in Nitrogen Availability Since Plantation Establishment Net N mineralization in the A horizon (0-1S em
depth) was almost three times greater at plantation es­
tablishment than at mid-rotation. In Years 1 and 2 after
treatment, net N mineralization was 77 and 62 kg N
ha-t, respectively, measured from May to November
(Vitousek and Matson, 198S; Vitousek et al., 1992). At
mid-rotation, plantation age 1S yr, average net N miner­
alization was 28 kg N ha-1 during the same time period.
Mineralization in the unharvested control plots, 36-yr­
old in 199S, was 13 kg ha-1 higher than that reported
for these same plots at age 22 yr in 1982 (Vitousek and
Matson, 198S). The methods used in 199S differed from
those used by Vitousek and Matson (198S) in that we
incubated intact soil cores rather than mixed soil in
buried bags. The difference in methods notwithstanding,
it has been shown that a decline in N availability with age
of forests (Frazer et al., 1990; Smethurst and Nambiar,
199S), is a general phenomenon, so the decline observed
on our site could not be attributed exclusively to the
differences in methodology.
The observed reduction in net N mineralization be­
tween plantation establishment and mid-rotation cannot
be attributed to increased plant uptake in older stands
because the soil core method used here tested N miner­
alization in the absence of plant uptake (i.e., roots were
severed during insertion of PVC incubation tubes). Re­
duction of N mineralization rates may be due in part
to the decrease in growing-season soil temperatures.
Measured at 1S-cm depth between July and September,
average soil temperatures were 2S°C immediately after
treatment, and 20°C in 199S at 10-cm depth. Soil temper­
atures at 1S em can be expected to be lower than at 10
em, so that the actual drop in soil temperatures since
treatment was probably greater than soc. However, tak­
ing our conservative estimate of the difference in soil
temperature of soc and assuming a doubling of the
reaction rate for every 10-degree increase in tempera­
ture (Q1 0 of 2), we can account for 38 to 67% of N
mineralization rates observed directly after treatment.
A decrease in decomposition and N mineralization rates
has been observed upon lowering of soil temperatures
(Theodorou and Bowen, 1983; Bonan and Van Cleve,
1992; Kirschbaum, 199S). Other factors, such as changes
of moisture conditions and substrate quality since plan­
tation establishment, may contribute to the decrease in
N mineralization (Burger and Kluender, 1982; Theo­
dorou and Bowen, 1983; Waide et al., 1988). These fac­
tors were not assessed at this site in the earlier years.
Unharvested control plots exhibited higher N mineral­
ization rates in 199S than in 1982 or 1983, and that may
be due to higher soil temperatures at stand age 36 vs.
22 yr that result from taller canopy and greater solar
penetration to the forest floor. These differences could
not be explored for lack of sufficient data.
Nitrogen Availability at Mid-Rotation
At mid-rotation, plantation age 1S yr, effects of har­
vesting were thought to be no longer significant, given
the far greater nutrient removal and changes in soil bulk
995
PIATEK & ALLEN: NITROGEN MINERALIZATION YEARS AFTER SITE PREPARATION
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Fig. 4. Relationships between net N mineralization and sum of growing-season soil temperature, soil P, and N:P and C:N ratios.
density due to site preparation (Gent et al., 1984; Tew
et al., 1986; Stewart, 1995). Contrary to expectations,
net N mineralization was significantly reduced in plots
with higher harvest intensity. Reduction in N mineral­
ization in whole-tree harvest plots was also observed
during the first 2 yr after treatment establishment and
was attributed to the removal of N-rich substrate in
foliage and branches (Vitousek and Matson, 1985). At
plantation age 15 yr, however, there were no treatment
differences in total soil N in the surface soil, and N
mineralization rates and total N were not correlated.
Of the soil nutrients and their ratios examined at mid­
rotation, soil P was significantly affected by harvesting,
and net N mineralization and soil P were negatively
correlated. The influence of soil P on N mineralization
is uncertain; DiStefano and Gholz (1989) reported no
effects of P additions on net N mineralization in a slash
pine plantation in Florida, while Ryan et al. (1972) re­
ported that in anaerobic conditions, P decreased ammo­
nia formation through greater microbial activity and N
immobilization. Yanai (1991) observed that an increase
in P leaching from the forest floor 2 yr following forest
harvest was less than could be predicted by a reduced
plant P uptake. She concluded that mechanisms such as
a decrease in P mineralization rates or an increase in
microbial P immobilization probably help reduce P loss
from the forest floor after disturbance. In our study, the
differences in the amount and, possibly, character of
logging residue left on the forest floor in the stem-only
and whole-tree harvests warrant the creation of differ­
ent microbiology and chemistry, including P nutrition,
in the underlying A horizon 15 years later. The 15-yr
dynamics of such events are not possible to reconstruct
without actual data. Environmental variables did not
explain the difference in N mineralization between har­
vest treatments. In 1995, significantly higher soil temper­
atures were observed in the stem-only harvest plots in
July and November; however, N mineralization was not
significantly higher in these plots in those two months.
Thus, apart from a suspected P-involvement, variables
measured in this study could not explain the effect of
harvest intensity on net N mineralization at mid­
rotation.
A strong site preparation effect was expected because
piling of slash generally removes more nutrients from
the planting surface than either harvest treatment. In
fact, shear-pile-disk treatment did not result in a signifi­
cant mid-rotation decrease in net N mineralization, as
hypothesized (Burger and Kluender, 1982; Morris and
Lowery, 1988; Allen et al., 1990). Higher N mineraliza­
tion rates observed in the chop-burn-no herbicide plots
were expected because hardwood leaf litter generally
decomposes and mineralizes more readily than conifer
needle litter (Nadelhoffer et al., 1982; Zak et al., 1986;
Stump and Binkley, 1993; Piatek and Allen, unpublished
data, 1997). However, the shear-pile-disk-no herbicide
plots also contained hardwoods but exhibited the lowest
N mineralization rates. The hardwood component on
the two non-herbicide treatments differed in species
composition and in hardwood biomass relative to pines.
996
SOIL SCI. SOC. AM. J., VOL. 63, JULY-AUGUST
Oak species dominated the hardwoods in the chop­
burn-no herbicide plots, in contrast to a mix of species
including red maple, hickory, tulip poplar, sweetgum,
and oak in the shear-pile-disk-no herbicide plots (Mel­
lin, 1995). Hardwood species composition, in addition
to their presence, may have affected N mineralization
rates. Nadelhoffer and others (1982) reported that N
mineralization in predominantly oak sites was higher
than on conifer and other hardwood sites. On the other
hand, Zak and others (1986) observed lower rates of N
mineralization in oak-dominated as compared to sugar
maple-dominated ecosystems. Hardwood foliar biomass
production constituted 13% of the total dry foliar weight
collected on the shear-pile-disk-no herbicide plots, and
45% on the chop-burn-no herbicide (Piatek and Allen,
unpublished data, 1997). Such differing ratios of hard­
wood to pine foliar litter may have contributed to the
differences in N mineralization by affecting soil chemis­
try and microbial activity. Soil P availability, for exam­
ple, may modify the relationship between hardwood
litter chemistry and N mineralization (Pastor et al.,
1984). On the other hand, the small hardwood compo­
nent on the shear-pile-disk-no herbicide plots simply
may not be large enough to impart stimulating effects
of hardwood foliar litter on N mineralization.
Why did mid-rotation rates of N mineralization vary
with harvest intensity rather than site preparation inten­
sity, despite a far greater nutrient displacement and
changes in soil physical characteristics associated with
the shear-pile-disk site preparation? We found no clear
answer. Harvesting may have affected different ecosys­
tem components than site preparation and, as a result,
produced different conditions for N mineralization. Evi­
dence to support this hypothesis comes from soil nutri­
ent analyses and from studies of soil physical properties.
Harvest effects, but not site preparation effects, were
significant for soil P. Site preparation effects, but not
harvest effects were significant for soil C and for C: N
ratio. Harvesting, relative to site preparation, caused
little disturbance to the soil structure. In contrast, site
preparation either compacted the soil during chopping
or alleviated compaction by disking (Gent et al., 1984),
resulting early on in different aeration conditions which,
however, did not persist (Stewart, 1995). Harvesting also
affected the amount of nutrient-rich foliage that could
be incorporated into the soil as substrate for N mineral­
ization. Site preparation affected the amount of nutri­
ent-poor forest floor, as well as green foliage on stem­
only harvest plots. In terms of nutrient removals, whole­
tree harvesting removed 4% of total ecosystem N
(aboveground vegetation, forest floor, and mineral soil
from 0-60 em depth included; data from Tew et al.,
1986) and 29% of total P, while stem-only removed
1.3% N and 7.6% P. Site preparation, on the other hand,
removed 1% total ecosystem N and no P in chopping
and burning, and 13% N and 52% P in shearing, piling,
and disking.
Chopping and burning removed less nutrient capital
than even stem-only harvest. On the chop-burn plots
at plantation establishment, the forest floor most likely
immobilized site N resources for several years (Piatek
1999
and Allen, unpublished data, 1997) while microbial bio­
mass utilized the large C pool. At mid-rotation, that
forest floor was likely incorporated into the soil organic
matter and may have contributed to the observed N
mineralization. Under this scenario, the shear-pile-disk
plots, where large amounts of organic matter were re­
moved, should exhibit lower N mineralization rates due
to increasingly limiting amounts of easily decomposable
organic matter. We have not observed this effect statisti­
cally, but the much lower rate of N mineralization on
the shear-pile-disk-no herbicide plots may be an indica­
tion of future conditions, as was predicted by many
(Burger and Kluender, 1982; Neary et al., 1984; Morris
and Lowery, 1988; Fox et al., 1989; Allen et al., 1990;
Thornley and Cannell, 1992). In fact, a 16-yr-old loblolly
pine stand in the Piedmont of Durham County, North
Carolina, exhibited a tree-growth decline attributed to
a decrease in soil nutrients following windrowing (Fox
et al., 1989). Exactly when in a lifetime of a plantation
this potential N limitation occurs probably depends on
the chemistry and total amount of substrate for decom­
position.
Treatment effects alone explained 33% of the vari­
ability in the rate of net N mineralization. Addition of
block effects increased r2 of the model to 91%, sug­
gesting that variability in N mineralization is very depen­
dent on location and associated conditions. Even at a
microsite level, enrichment in C availability, such as
from a fallen log, seems to drive the rates of N transfor­
mations (Hart et al., 1994).
Higher sums of soil temperatures generally resulted
in higher overall N mineralization (r = 0.7). However,
within a given month, mineralization rates at mid-rota­
tion were not correlated with monthly soil temperatures
nor with monthly soil water content. In contrast, temper­
ature and moisture seemed to be the primary controls
in the seasonal trend of N mineralization in the first 2 yr
after treatment, in 1983 and 1984 (Vitousek and Matson,
1985). This might be expected since our temperature
had a narrower range across the season (8-21°C), while
Vitousek and Matson (1985) observed a much wider
range (0.4-36°C). After treatment effects, the seasonal
sum of soil temperature explained an additional 42%
of the variability in net N mineralization.
CONCLUSIONS
We observed that net N mineralization rates were
much lower at mid-rotation than at plantation establish­
ment, and that harvest intensity but not site preparation
intensity affected N mineralization at mid-rotation. Be­
cause mid-rotation N mineralization was correlated with
an inverse of soil P and C: N ratio and with the sum of
growing-season soil temperature, while harvest intensity
affected soil P and site preparation affected soil C:N, we
concluded that harvesting affected different ecosystem
components than site preparation and, as a result, cre­
ated different conditions for organic matter mineraliza­
tion. The hypothesis that shearing and piling will result
in lower net N mineralization at mid-rotation was not
supported, suggesting that the amount of nutrients re­
PIATEK & ALLEN: NITROGEN MINERALIZATION YEARS AFfER SITE PREPARATION
moved did not have lasting and negative effects on N
availability at plantation age 15 yr.
997
chemical properties. Soil Sci. Soc. Am. Book Ser. 5. SSSA, Madi­
son, WI.
Haines, L.W., T.E. Maki, and S.G. Sanderford. 1975. The effect of
mechanical site preparation treatments on soil productivity and
tree (Pinus taeda L. and Pinus e/liottii Engelm. Var. elliottii) growth.
In B. Bernier and C.H. Winget (ed.) Forest soils and forest land
ACKNOWLEDGMENTS
This study was supported by the Forest Nutrition Coopera­
tive at North Carolina State University and conducted on
land owned by Champion International. The reviews by M.
Barbercheck, L. Morris, D. Richter, S. Shafer, T. Wentworth,
and three anonymous reviewers were invaluable in the prepa­
ration of this manuscript. J. Robbins' help with figures is
greatly appreciated.
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