SoiV Biol. Biochem. Vol. 20, No. 5, pp. 693-701, 1988
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NITROGEN, SULFUR AND PHOSPHORUS DYNAMICS
IN DECOMPOSING DECIDUOUS LEAF LITTER IN
THE SOUTHERN APPALACHIANS
£
JOHN M. BLAIR*
Department of Entomology and Institute of Ecology, University of Georgia, Athens, GA 30602, U.S.A.
(Accepted 10 January 1988)
Summary—The decomposition rates and N, S and P dynamics of flowering dogwood (Comus florida),
red maple (Acer rubrum) and chestnut oak (Quercus prinus) litter were examined during 2 years in a mixed
deciduous forest in the southern Appalachians. Litter of the three species decomposed in the following
order (fastest to slowest): flowering dogwood > red maple > chestnut oak. Initial mass losses (first
6 months) were most highly positively correlated with concentrations of ethanol-soluble and total soluble
components. First-year annual decay rates were most highly negatively correlated with initial % lignin
and lignin-to-N ratios. Second-year decay rates were significantly slower than first-year rates for flowering
dogwood and red maple litter, but not for chestnut oak. This was apparently due to the greater proportion
of labile materials initially present in flowering dogwood and red maple litter. Relative concentrations of
N, S and P increased during the decomposition of each litter type, following any initial leaching losses.
In all cases, the increases in N, S and P concentrations exhibited negative linear relationships to % mass
remaining. For all three elements the slopes of these relationships were correlated with decay rates,
indicating a greater increase in N, S and P concentrations per unit mass lost in faster decomposing litter
types. Changes in the absolute amount of N (net immobilization or net release) followed a typical three
component curve (leaching, immobilization and release phases). Nitrogen release began when C-to-N
ratios decreased to between 25 and 34. Patterns of P and S fluxes varied more among litter types. Only
flowering dogwood litter, with a final C-to-P ratio of 305 appeared to release P by the end of the study.
Flowering dogwood litter also had a low initial C-to-S ratio (236) and displayed an immediate net release
of S which continued throughout the study. The other litter types, which had higher initial C-to-S ratios,
immobilized S throughout the study.
INTRODUCTION
Decomposition processes influence both structural
and functional aspects of terrestrial ecosystems (Swift
et al., 1979). The distribution and turnover of organic
matter contributes to the structural matrix of the
ecosystem, forming soil organic matter pools and
important nutrient exchange sites. The recycling of
nutrients contained in litter is an important aspect of
ecosystem dynamics, and the regulation of rates and
timing of nutrient release plays an integral role in
ecosystem functioning. The bulk of the above-ground
annual net primary productivity of most forest ecosystems is transferred directly to the decomposer
subsystem via litterfall (Swift et al, 1979; Seastedt
and Crossley, 1987). Therefore, the patterns of
decomposition of foliar litter and subsequent
nutrient release are important determinants of forest
ecosystem function.
Among the factors affecting litter decomposition
rates and'nutrient dynamics are litter quality (Fogel
and Cromack, 1977; Berg and Staaf, 1980,1981; Aber
and Melillo, 1980; Melillo et al, 1982), macro- and
microclimatic variables (Meentemeyer, 1978; Swift
et al, 1979), and biotic activity both microbial and
faunal (Reichle, 1977; Swift et al, 1979; Seastedt,
1984). Resource quality has been defined by various
authors to include, initial N concentration, C-to-N
ratio, initial lignin concentration, and the ratio of
, lignin-to-N. Resource quality affects not only rates of
mass loss, but also patterns and rates of nutrient
immobilization or release. In particular, C-to-element
ratios have been cited as important determinants of
whether an element will be immobilized or released as
litter decomposition proceeds (Gosz et al, 1973; Berg
and Staaf, 1981). Above a critical C-to-element ratio,
nutrients will be immobilized in microbial biomass
and byproducts as carbon is mineralized, thus lowering the C-to-element ratio. When the critical ratio
is attained element loss should become roughly proportional to mass loss (Berg and Staaf, 1981). In the
case of forest litter, which typically has high C-toelement ratios and slow decomposition rates, 1-year
studies are generally not long enough to characterize
adequately patterns of nutrient immobilization and
release.
The objectives of this study were (1) to quantify
rates of mass loss and patterns of nutrient (N, S and
P) flux in litter of three tree species, representing
different initial resource qualities, during a 2-year
study period and (2) to relate the observed patterns
to litter quality variables and C-to-element ratios.
MATERIALS AND METHODS
Site description
This study was conducted at the USFS Coweeta
Hydrologic Laboratory from January 1985-January
*Present address: Department of Entomology, 1735 Neil
Ave, Ohio State University, Columbus, OH 43210,
U.S.A.
693
~ ' -
694
JOHN M. BLAIR
1987. The Coweeta Laboratory, a 2185 ha forested
basic located in the southern Appalachian Mountains
of southwestern North Carolina (35°00'N latitude,
83°30'W longitude), consists of numerous smaller
watersheds (catchments) that serve as experimental
units. The watershed utilized in this study, WS 2, is
a 12.3 ha catchment located in the northeastern portion of the Coweeta basin. Elevations on WS 2 range
from 709 to 1004m and the average slope is 60%
(Swank and Crossley, 1986). The vegetation is an
uneven-aged mixed hardwood association dominated
by Quercus, Gary a and Acer spp (Berish and Ragsdale, 1985). Mean annual rainfall on WS 2 is
1770mmyr~' and is evenly distributed throughout
the year. Mean annual temperature is 13°C.
last collection date was 6 January 1987 (732 days in
the field).
Intact litterbags were oven-dried at 50°C, the litter
was reweighed, ground and subsamples were ashed at
500°C for 4h to determine % AFDM. After 6
months in the field soil contamination was evident in
at least some litterbags on each collection date.
Therefore, oven-dry litter masses were corrected for
soil infiltration before determining % mass loss using
the following soil correction equation:
Decomposition rates and nutrient dynamics
Litter decomposition rates and nutrient dynamics
in decomposing litter were quantified using litterbags
with an inside area of 10 x 10cm constructed of
fiberglass window screen material (1.6x1.8 mm
mesh). This mesh size allowed free access to most
microarthropods, which dominate the forest floor
fauna at Coweeta. Senescing leaves of flowering
dogwood (Cornus florida L.), red maple (Acer
rubrum L.) and chestnut oak (Quercus prinus L.) were
collected in October 1984 and air-dried at 22°C.
These three species were chosen to represent a range
of resource qualities and decomposition rates and
because of their abundance on WS 2. Litter of
these three species comprised approximately 46% of
total foliar litter inputs on WS 2 (L. S. Risley,
personal communication). Litter was collected from
several sites at similar elevations in the Coweeta
basin, pooled by species and mixed prior to filling
litterbags to assure uniform litterbag substrates. Approximately 2.5 g of air-dried leaves were placed in
tared bags. Five bags of each species were oven-dried
at 50°C to develop equations relating air-dry to
oven-dry mass. On 4 January 1985, 52 litterbags of
each species were placed in each of three plots
arranged in a transect on a mid-elevational area of
WS 2. Plots were spaced approximately 100-150m
apart. Five bags of each species were collected immediately to Determine mass loss due to handling. These
values were used to adjust initial masses of litterbags
prior to determining ; % mass lost. Subsamples of
initial litter were analyzed to determine initial -.%
ash-free; tiry mass-(AFDM), N, S and P concentrations ;and C-to-element:ratios using;the methods
indicated:^below. .Lignin and. (Cellulose contents. of
initial litter were, determined using the methods of
Goering'and van Soest (1970). Ada detergent lignin
(ADL) was determined following digestion of the
acid detergent fiber (ADF) fraction in 72% H2SO4.
Cellulose'was determined as the difference between
ADF and; AIJL, Water-soluble and ethanol-soluble
contents of initial litter were determined by sonicating ground samples of litter for 30 min, first in
water and then ethanol, and reweighing samples
following filtration and oven-drying at 50°C.
Litterbags were collected randomly from each of
the plots every 2 weeks from January to November
and in mid-December 1985 (n = 6 bags/species/date),
and every 2 months in 1986 (n = 3 bags/species/date)
for a total of 29 collection dates during 2 years. The
Fli is the fraction of litterbag content that is
actually litter;
SaAFDM is the % AFDM of the entire litterbag
sample;
S1AFDM is the average % AFDM of the soil at
the site;
LiAFDM is the initial % AFDM of the litter
substrate.
•The underlying assumption of this equation is that
organic matter content (% AFDM) of the litter and
the soil remain constant throughout the study. The
equation calculates the fraction of litterbag content
that is actually litter based on any reduction in %
AFDM of the sample caused by soil contamination.
The equation is preferable to simply calculating litter
mass loss as % AFDM remaining when soil moving
into the litterbags has a high organic matter content,
since organic matter from the soil would contribute
to the apparent AFDM of the litter. Decomposition
rates were calculated from % mass remaining data
using a single negative exponential decay model
X/X0 = exp(—kt), where X/X0 - percent mass remaining at time t, t — time elapsed in years, and
fc = the annual decay constant (Olson, 1963). The
single negative exponential model was fit to the data
by least squares regression of the natural logarithm
1
of mean % mass remaining over time. Annual decomposition rate constants (fc) were calculated from
first-year data, second-year data and total 2-year
data. In calculating second-year decay rates, initial
mass remaining at t = 0 was assumed to be the
predicted % mass remaining at t = 1 year, based on
first-year decay rate constants.
Nitrogen concentrations in the residual litter were
quantified for litterbags collected monthly in 1985
and bimonthly in 1986. Sulfur and P concentrations
were quantified for litterbags collected bimonthly
throughout the study. Nitrogen concentrations
were determined colorimetrically with a Technicon
Autoanalyzer following micro-Kjeldahl digestion
(Technicon, 1970). Sulfur and P concentrations were
determined following perchloric-nitric acid digestion
(Blanchar et al., 1965). Sulfur concentrations were
determined turbidometrically (Blanchar et al., 1965).
Phosphorus concentrations were determined using a
modification of the colorimetric technique of Murphy
and Riley (1962) as suggested by D. O. Wilson
(personal communication).
Nutrient (N, S and P) concentrations for litterbags
contaminated with soil (as indicated by reductions in
Fli = (SaAFDM—S1AFDM)
-=- (LiAFDM—SI AFDM)
where
Nutrient dynamics in decomposing leaf litter
% AFDM of the sample) were corrected using the
following equation:
LiNt = [SaNt - (FS1 x SINt)] -r FLi
where
LiNt is the nutrient concentration in the residual
litter;
SaNt is the nutrient concentration of the entire
sample;
SINt is the average nutrient concentration in the
soil;
FS1 is the fraction of the sample that is soil
(1 — FLi from the above soil correction equation);
FLi is the fraction of the sample that is litter from
the above soil correction equation.
Net nutrient immobilization-release at time t was
calculated as the product of % mass remaining and
nutrient concentration in the residual material at time
t divided by the initial nutrient concentration of that
litter type. Changes in C-to-element ratios were estimated assuming that C content equaled 50% of litter
AFDM (McBrayer and Cromack, 1980).
RESULTS
Annual decay rate constants (k) based on
first-year, second-year and total 2-year data for all
three spedes are presented in Table 1. First-year rates
are based on n = 24, second-year on n = 7 and 2-year
total on n = 30 dates. Coefficients of determination
(r2) and comparisons of predicted and observed %
mass remaining are included to indicate goodness-offit of the data to the single negative exponential
model.
Litter of the three species examined lost mass in the
following order in the first year (fastest to slowest):
flowering dogwood > red maple > chestnut oak. The
order of accumulated mass loss at the end of 2 years
was also flowering dogwood > red maple > chestnut
oak. For all three species the single negative exponential model adequately described patterns of
mass loss during the first year and during the entire
2-year study. However, coefficients of determination
indicated a poorer fit of second-year mass loss data
from flowering dogwood and red maple litter to a
single negative exponential model. Annual decomposition rates of both flowering dogwood and red
maple litter were significantly slower in year 2 than in
year 1. Flowering dogwood litter lost approx. 57% of
initial mass in year 1 and only 23% of the remaining
mass (10% of initial mass) in year 2. Red maple litter
lost approx. 47% of initial mass in year 1 and only
19% of remaining mass (10% of initial mass) in
year 2. Chestnut oak litter, which exhibited the
695
Table 1. Decomposition rate variables for flowering dogwood, red
maple and chestnut oak litter. Annual decomposition
rate constants
(k), coefficients of determination (r2), and predicted % mass
remaining are based on a single negative exponential model (Olson,
1963). See Materials and Methods for assumptions used in calculating second year decay rates
Flowering
dogwood Red maple Chestnut oak
First -year
Decay
rate (k)
-0.274
-0.850
-0.638 H
r2
0.952"
0.951
0.938
Predicted
% remaining* . 42.72
52.84
76.05
Observed
% remaining*
52.92
73.14
45.76
Second-year
Decay
rate (k)
-0,271 *»* -0.205'**
-0.351 ,
r1 . . . ..•,..... ,
. 0.573..;
.0.961
0.611
Predicted
::
% remaining*
32.57
43.06
53:44 "•'"•
Observed
% remaining*
37.20
42.29
53.53
Total (2 years)
Decay
rate (k)
-0.562
-0.393
-0.330
r2
0.899
0.869
0.962
Predicted
11
% remaining
32.52
45.52
51.73
Observed
% remaining'
42.29
53.53
37.20
'(t
=
1
yr).
b
(t = 2yr).
'"Indicates significant (P < 0.001) differences between first-year
and second-year decay rates based on /-test comparing slopes
(Zar, 1984).
slowest overall decomposition rate, had a slightly
higher, although not significantly different, decay rate
in the second year, relative to the first. Chestnut oak
litter lost approx. 24% of initial mass in year 1 and
30% of the remaining mass (23% of initial mass)
in year 2.
Several variables used to describe initial litter quality (% N, C-to-N ratio, % lignin, lignin-to-N ratio
and % solubles) are presented in Table 2 along with
first-year decay rate constants (fc) for litter of each of
the three species examined. The results of regression
analyses of each of these litter quality variables
(independent variable) with decay rates (dependent
variable) are presented in Table 3. Initial % lignin
(r2 = 0.987) and initial lignin-to-N ratio (r2 = 0.967)
were most highly correlated with first-year decay
rates, indicating the importance of lignin content in
influencing the decomposition rates of these litter
types.
Changes in relative concentrations of N, S and P
in the residual litter are i presented in Fig. 1. The
relative concentration of N in litter of all three species
increased throughout the study, although N concentrations of all three species on the last collection date
were slightly lower than on the previous date. Only
Table 2. Initial N concentrations, C-to-N ratios, lignin concentrations, lignin-to-N ratios, and water- and
ethanol-soluble concentrations of flowering dogwood, red maple and chestnut oak litter
C:N
Lignin :N % Water % Ethanol
First-year
decay rate % N ratio % Lignin
ratio
soluble
soluble
Species
Flowering
6.7
-0.850 0.75 62.9
6.1
35.1
dogwood
4.6
11.1
-0.638
0.56 85.7
17.3
Red maple
9.6
47.8
27.2
1.7
-0.274
0.87 55.7
25.2
Chestnut oak
23.6
JOHN M< BLAIR
696
Table 3. Initial litter quality variables as predictors 2of
first-year decay rates. Coefficients of determination (r ),
slopes and K-intercepts of regressions relating first-year
decay rate constants (k) to initial litter quality variables for
litter of the three species examined
Initial litter
quality variable
r2
Slope
K-intercept
% Nitrogen
0.271
-0.978
-1.29
0.138
0.007
-0.12
C:N ratio
% Lignin
0.987
0.029
-0.96
Lignin:N ratio
0.967
0.027
-1.05
% Water soluble
0.322
-0.015
-0.06
% Ethanol soluble 0.426
-0.040
-0.32
chestnut oak litter exhibited an initial decrease in N
concentration (days 27, 56 and 83), which probably
represents leaching of soluble N. Nitrogen concentrations in flowering dogwood, red maple and chestnut oak litter had increased by 157, 158 and 67%,
respectively, by the end of the study. Sulfur concentrations initially decreased in litter of all three species,
and then increased until the end of the study. The
initial decrease in S concentration was greatest in
flowering dogwood litter which had an initial S
concentration more than three times greater than
either red maple or chestnut oak. The final concentration of S in flowering dogwood litter remained 5%
lower than the initial concentration. Sulfur concentrations in red maple and chestnut oak litter had
increased by 105 and 84%, respectively, by the end of
the study. Phosphorus concentrations also decreased
initially in all three Utter types, presumably due to
leaching of soluble P-containing compounds. This
initial leaching loss was followed by a general increase in P concentrations in litter of all three species
throughout the study, although P concentrations
in residual litter were much more variable than S and
N concentrations. Phosphorus concentrations in
flowering dogwood, red maple and chestnut oak
litter had increased by 72, 181 and 76% by the end
of the study.
Change in the absolute amount of an element
during decomposition (net immobilization or net
release) is the net result of change in litter mass and
' change in the relative concentration of the element in
A. Mass
2.5 A. Nitrogen
2.0
1.5
1.0
0.5
0
O
B. Phosphorus
1
I''
•»— L
2 .10
C
O
u
40
140
05
fD. Sulfur
120L
I-
0
•iad < C. Sulfur
A
-•' '-
100,
k / v
80
/•-«•
_.^60
15
.10
40
.05
20
III
225 345
528
Days in the Field
732
Fig. 1. Changes in the relative concentration of (A) nitrogen,
(B) phosphorus and (C) sulfur in litter of flowering dogwood (DW), red maple (RM) and chestnut oak (CO)
decomposing over a 2-year period.
732
225 345
528
Days In the Field
Fig. 2. Changes in (A) mass and absolute amounts of (B)
nitrogen, (C) phosphorous and (D) sulfur in flowering
dogwood, red maple and chestnut oak litter decomposing
over a 2-year period.
Nutrient dynamics in decomposing leaf litter
the residual litter. Mass loss curves and changes in the
absolute amounts (net mineralization or net immobilization) of N, S and P in the residual litter are
presented in Fig. 2. Mass loss curves [Fig. 2(A)]
reflect the same patterns of decomposition discussed
previously. In particular, note the reduction in rates
of mass loss of flowering dogwood and red maple
litter in the second year of decomposition. Net fluxes
of N during the 2-year study are presented in
Fig. 2(B). Litter of all three species exhibited a net
immobilization of N above 100% of the initial
amount during decomposition. In the case of red
maple and chestnut oak litter net immobilization was
preceded by a short initial leaching (net loss) phase.
In litter of all three species it appeared that the net
mineralization phase had begun by the end of the
2-year study. In contrast to N, patterns of net P
immobilization-release varied among litter of the
three species [Fig. 2(C)]. All three species exhibited an
initial net loss of P followed by some degree of,
immobilization. Flowering dogwood litter, which had
the highest initial P concentration, displayed the least
net immobilization and the greatest net release. An
amount equivalent to approx 40% of initial P in
flowering dogwood litter was released by the end of
the study. Red maple litter exhibited a trend for
increasing P immobilization which continued
throughout the study. The maximum amount of P
immobilized in red maple litter was equivalent to
132% of initial P. The pattern of net P flux in
chestnut oak litter was irregular with peaks of P
immobilization occurring in the autumn of both years
1 and 2. Patterns of net S immobilization-release also
varied among litter of all three species [Fig. 2(D)].
Flowering dogwood litter, which had initial S concentrations 3.14 times higher than that of red maple or
chestnut oak litter, exhibited a rapid net mineralization of S which continued throughout the study. In
the first 111 days flowering dogwood litter lost the
equivalent of 60% of initial S and by the end of the
study had lost an additional 5%. Red maple litter
exhibited an initial net loss (leaching) phase, in which
approx 44% of initial S was lost, followed by a
gradual net immobilization in which the amount of S
increased to an amount equivalent to approx 90% of
initial S content. Chestnut oak also exhibited an
Table 4. Changes in mean C-to-element ratios during the decomposition of flowering dogwood, red
maple and chestnut oak litter
C:N
C:S
C:P
ratio
Day ratio
ratio
Species
234
63
Flowering
0
525
343
225 34
356
dogwood
285
345 29
346
320
528 27
458
248
732 25
305
758
0
86
1137
Red maple
549
225 54
590
473
543
345 42
459
528
35
446
370
405
732 34
769
894
56
Chestnut oak 0
588
225 51
846
345 45
558
710
481
643
528 33
446
508
732 34
697
initial net loss phase (equivalent to 22% of initial S)
followed by a rapid increase to an amount equivalent
to 141% of initial sulfur. The amount of S in chestnut
oak litter remained greater than 100% of the initial
amount throughout the study.
Carbon-to-element ratios on days 0, 225, 345, 528
and 732 for litter of all three species are presented in
Table 4. C-to-N ratios of all three species decreased
throughout the 2-year study. The C^o-N ratio of
flowering dogwood, which was initially 63, decreased
to 30 by the end of year 1 and to 25 by the end of
year 2. The C-to-N ratio of red maple litter, which
was initially 86, decreased to 42 by the end of year 1
and to 34 by the end of year 2. Chestnut oak litter,
which had an initial C-to-N ratio of 56, decreased to
45 by the end of year 1 and to 34 by the end of year
2. The C-to-S "ratios of red maple and chestnut oak
also steadily decreased throughout the study. The
initial C-to-S ratios of red maple and chestnut oak
were 758 and 769, respectively, and had decreased to
370 and 446, respectively, by the end of year 2. The
C-to-S ratios in flowering dogwood litter did not
exhibit this trend. C-to-S ratios in dogwood litter
were initially quite low (236) and tended to fluctuate
between 248 and 343 throughout the study. The
C-to-P ratios of red maple and chestnut oak litter
also decreased throughout the study. Initial C-to-P
ratios of red maple and chestnut oak litter were 1137
and 894, respectively, and had decreased to 405 and
508, respectively, by the end of the study. The C-to-P
ratio of flowering dogwood litter, which was initially
525, fluctuated somewhat during decomposition, but
had decreased to 305 by the end of the study.
Regressions of % mass remaining over N concentration in the residual material indicated a linear
relationship between mass loss and N retention in
the litter, measured as increased concentration
[Fig. 3(A)], as reported by Aber and Melillo (1980).
The slope of this relationship is a measure of the
increase in N concentration per unit C mineralized
(mass lost). This negative linear relationship also
exists between % mass remaining and increases in
S and P concentrations [Figs 3(B) and (C)]. It should
be noted that the regression equations relating
% mass remaining to nutrient concentration were
calculated after excluding any initial rapid loss in
nutrient concentration due to leaching of soluble
compounds (Aber and Melillo, 1980).
DISCUSSION
First- and second-year decay rates of chestnut oak
litter were not significantly different, although
second-year decay rates were slightly higher. These
results are similar to those reported by Seastedt et al.
(1983) in another study of chestnut oak litter decomposition. However, decay rates of flowering dogwood
and red maple litter were significantly lower in year
2. Differences between first-year and second-year
decay rates for litter of flowering dogwood and red
maple may be attributed to differences in climatic
variables or differences in substrate quality between
years. The southeastern U.S. experienced a severe
drought in 1986 which could account for a portion
of the observed decrease in decay rates in the
-;S
698
JOHN M. BLAIR
A. Nitrogen
100
Red Maple
= .823
b =-44.24
80
Chestnut Oak
.864
b =-48.84
60
40
2.0
1.0
3.0 0
1.0
Z.O
3.0 0
2.0
B. Sulfur
100
Dogwood
r2=.849
b =-439.91
o
§ 80
OL
Red Maple
r2=.945
b =-482.30
Chestnut Oak
* r2=.843
b =-634.93
O 60
40
0
100
0.2 0
0.1
0.2 0
0.1
0.2
C. Phosphorus
80
b =-452.38
Red Maple
r2=.927
b =-468.34
Chestnut Oak
r 2 =.6IO
b =-572.30
60
40
0.2 0
0.1
0.2 0
0.1
0.2
Nutrient Concentration (%)
Fig. 3. Regressions of % mass remaining over (A) nitrogen, (B) sulfur and (C) phosphorus concentrations
in the residual litter for flowering dogwood, red maple and chestnut oak.
0.1
second year. The 59-year mean for annual precipitation in the Coweeta basic is approximately
1810mm and is fairly evenly distributed throughout
the year. Precipitation in year 1 of this study (1985)
was 1370mm. Precipitation in year 2 (1986) was
only 1240 mm. Annual precipitation has been shown
to substantially influence terrestrial decomposition
rates (Meentemeyer, 1978). However, the lack of
significant differences between first- and second-year
decay \rates of chestnut oak litter, and reports of
lower second-year decay rates for other litter types in
years with more normal climatic regimes, indicates,
that factors o|her than climatic differences are important in lowering second-year decay rates. In particular, changes in substrate quality during the first
year of decomposition may be important. Analysis of
initial litter substrates indicated that cellulose and
lignin comprised about 47% and total solubles
(water + ethanbl extactable) about 27% of initial
chestnut oak litter mass. In flowering dogwood and
red maple litter, however, cellulose and lignin comprised only 25% and total solubles 42 and 58%,
respectively, of initial mass. Assuming that the soluble fraction represents a more labile litter component
which may be rapidly lost in the first year of decomposition, substrate quality should change to a greater
degree in the first year for Utter of flowering dogwood
and red maple than for chestnut oak. This would
account for the observed decrease in decay rates of
flowering dogwood and red maple litter in year 2.
These results suggest that extrapolation of first-year
decay rates to predict longer-term patterns of decay
or turnover times in forest floors may not be appropriate, particularly for litter types with a larger labile
component.
Regressions of various initial litter quality variables with observed decay rates indicated the relative
importance of initial lignin concentration in affecting
first-year decomposition rates. Other studies have
also reported a negative correlation between Utter
decay rates and initial lignin content (Fogel and
Cromack, 1977) or lignin-to-N ratios (MeUllo et al.,
1982). However, it has been suggested that other
factors may control decomposition rates in the earlier
Nutrient dynamics in decomposing leaf litter
stages of decomposition (i.e. Berg and Lundmark,
1987). In order to assess which litter quality variables
were correlated with initial mass loss during the early
stages of decomposition, I regressed the initial litter
quality variables with decay rates calculated from
only the first 6 months of decomposition.
Decay rates in the first 6 months had a lower
negative correlation with initial lignin concentrations
2
(r = 0.778) or lignin-to-N ratios (r2 = 0.492), but
were highly positively correlated with concentrations
of ethanol solubles (r2 = 0.938) and total solubles
(r2 = 0.896). These results suggest that the concentration of soluble compounds is more important in
affecting initial mass loss, while lignin concentration
becomes more important in the latter stages of decay.
Initial lignin concentration has also been reported
to affect the slope of the inverse linear relationship
between % mass remaining and N concentration in
the residual litter. Melillo et al. (1982) found that
higher initial lignin concentrations were correlated
with greater increases in N concentration per unit
mass lost. However, data from my study, although
limited to three species, indicate a negative correlation between initial lignin concentration and the
amount of N immobilized per unit mass lost
(r2 = 0.950). That is, the lower the initial lignin
concentration, the greater the increase in N concentration per unit mass lost. The slopes of the relationship between N concentration and mass loss for
litter of these three species were also highly positively
correlated with first-year (r2 = 0.989) and overall
2-year (r2 = 0.915) decay rates. That is, the higher the
decomposition rate, the greater the increase in N
concentration per unit mass lost. This is consistent
with the observation that accumulation of N in
decomposing litter is a microbially-mediated process
and, as such, is directly related to |the rate of energy
release, or mass loss^ from litter (Aber and Melillo,
1980; Berg and Staaf, 1981). Bosatta and Staaf (1982)
have also noted a positive relationship between decomposition rate and both N retention capacity and
specific immobilization rate and have suggested that,
given litter with similar initial N concentrations,
faster decomposing species will have higher rates of
N uptake during the net immobilization phase and
lower N mineralization rates per unit mass loss.
The same linear relationship between increase in N
concentration and mass loss applies to increases in S
and P concentrations as litter decomposes, if initial
losses due to leaching are excluded [Figs 3(B) and C].
The rates of increase in S and P concentration per
unit mass lost were also positively correlated with
first-year decay rates (r2 = 0.973 for S, r 2 = 0.939 for
P). This indicates that the increases in S and P
concentrations are also microbially mediated (i.e.
Swift et al., 1979). A linear relationship between P
and S concentrations and mass loss was also noted by
Staaf and Berg (1982) for decomposing Scots pine
;
litter.
Change in the absolute amount of an element
during decomposition (net immobilization or net
release) is a function of both mass loss and change in
the relative concentration of the element in the
residual litter. Patterns of net N immobilizationrelease were similar in litter of all three species and
followed the general three component curve proposed
699
by Berg and Staaf (1981) and others. Phase I is an
initial leaching phase characterized by a rapid release
of labile nitrogenous compounds. Phase II (the accumulation or immobilization phase) is characterized
by an increase in both the relative concentration and
absolute amounts of N in the litter. Phase III (the
release or mineralization phase) theoretically begins
following the attainment of a critical C-to-N ratio
and is characterized by a net loss of N, which should
eventually become roughly proportional to mass loss.
The critical C-to-N ratio at which mineralization
begins may vary with litter type and different soillitter systems (Berg and Staaf, 1981). Both red maple
and chestnut oak litter exhibited an initial leaching
phase. All three species exhibited a -net immobilization phase, which was expected based on their
high initial C-to-N ratios. The maximum amounts of
N immobilized in flowering dogwood, red maple and
chestnut oak litter were equivalent to 123, 129 and
125%, respectively, of initial N. Initial C-to-N ratios
of flowering dogwood, red maple and chestnut oak
litter were 63, 86 and 56, respectively. During the
immobilization phase C-to-N ratios of all three litter
types steadily decreased. It appeared that the onset of
net mineralization had occurred by the last collection
dates. The C-to-N ratios of flowering dogwood, red
maple and chestnut oak litter had decreased to 25, 34
and 34, respectively, which is within the range where
net N mineralization would be expected to occur
(Gosz et al., 1973, Upadhyay and Singh, 1985).
Patterns of P flux during decomposition were more
variable, but the same three component curves proposed for N dynamics appeared to apply to P. All
three species exhibited an initial leaching phase followed by some degree of P immobilization. The
maximum amount of P immobilized (88%) did not
exceed the initial amount in flowering dogwood litter.
However, the maximum amounts of P immobilized in
red maple and chestnut oak litter were equivalent to
132 and 128%, respectively, of initial P. Initial C-to-P
ratios of flowering dogwood, red maple and chestnut
oak litter were 525, 1137 and 894, respectively, and
had decreased to 305, 405 and 508, respectively, by
the end of the study. Only flowering dogwood litter
appeared to show any consistent net release of P by
the end of the study. This may indicate that a critical
C-to-P ratio had not yet been obtained in litter of red
maple and chestnut oak. Studies at other forest sites
have indicated critical C-to-P ratios between 360 and
480 (i.e. Gosz et al., 1973), but this value may be
lower at Coweeta.
Net fluxes of S varied among litter of all three
species examined. All three species exhibited an initial
leaching phase. In flowering dogwood litter there was
no net immobilization of S. Both red maple and
chestnut oak litter did exhibit a net immobilization
phase, although there was considerably more S immobilized in chestnut oak litter than in red maple
litter. The maximum amount of S immobilized in red
maple litter (91%) did not exceed the initial amount,
while the maximum amount of S immobilized in
chestnut oak litter was equivalent to 140% of initial
S. The lack of an immobilization phase in flowering
dogwood litter may be explained by its low initial
C-to-S ratio. The initial C-to-S ratio of flowering
dogwood litter was 236, while those of red maple
700
JOHN M. BLAIR
and chestnut oak were 758 and 769, respectively.
The C-to-S ratios of both red maple and chestnut
oak decreased throughout the study. In contrast,
the C-to-S ratio of flowering dogwood litter did
not decrease and tended to fluctuate between
235 and 343. These results indicate that the critical
C-to-S ratio at which net S mineralization takes
place is around 300. This is similar to the value
reported for decomposing litter at Hubbard Brook
(Gosz et al., 1973).
Increases in relative nutrient concentration during
the course of decomposition may be explained, in
part, by microbial incorporation of nutrients released
from litter as carbon is mineralized. However, increases in absolute amounts of nutrients require the
incorporation of nutrients from exogenous sources.
These exogenous sources may include particulate or
aerosol inputs. However, this does not appear to be
a major contributing factor since all litterbags were
presumably subject to the same inputs, yet patterns
of nutrient accumulation varied among litter of
different species. A more likely explanation is that
nutrients from exogenous sources (throughfall, leachate, fungal translocation) are being incorporated into
microbial biomass and stable microbial byproducts
as decomposition proceeds. The observation that
increases in nutrient concentration are linearly related to C respiration (mass loss) would seem to
support this idea. Potential sources of N inputs to
litter include throughfall, fungal translocation and
N-fixation. Free-living N-fixation in the litter layer on
WS 2 was estimated to be only 0.3 kg ha~' yr~'
(Waide et al., 1987) and does not appear to be an
important factor in the accumulation of N in decomposing litter. Changes in solution chemistry indicate
that inorganic N is removed as solution flows
through litter on the forest floor (Swank and Swank,
1984) suggesting that microbial incorporation of
throughfall inputs may contribute to N accumulation. Fungal translocation of N has also been
implicated in other studies of N dynamics in decom-'
posing litter (Fahey et al., 1985; Holland and
Coleman, 1987). Sources of exogenous S and P are
the same as for N with the exception of N2-fixation.
In conclusion, it appears that decomposition rates '
and N, S and P fluxes in these litter types during the
first 2 years can be explained largely on the basis of
substrate quality and its implied interactions with the
decomposer community. Although 2 years of data
were sufficient to establish some meaningful relationships between initial litter quality and decomposition .rates, longer studies will be required to
determine how further changes in substrate quality
affect later stages of decomposition. Two years
allowed for an adequate characterization of N, S and
P dynamics in the early net loss and immobilization
phases. However, longer-term studies are needed to
fully characterize patterns and rates of nutrient loss
during the release phase of decomposition.
Acknowledgements—I thank B. L. Haines and D. O. Wilson
for providing technical advice on nutrient analyses and
D. L. White for determining lignin and cellulose. I also
thank B. Berg, D. A. Crossley Jr, B. L. Haines and R. W.
Parmelee for helpful reviews of this manuscript. Thanks to
the U.S. Forest Service for their cooperation in the use of
the Coweeta facilities. This research was supported by a
National Science Foundation grant to the University of
Georgia Research Foundation.
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