SoiV Biol. Biochem. Vol. 20, No. 5, pp , 1988 Printed in Great Britain. All rights reserved

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1 SoiV Biol. Biochem. Vol. 20, No. 5, pp , 1988 Printed in Great Britain. All rights reserved /88 $ Copyright 1988 Pergamon Press pic 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 *Present address: Department of Entomology, 1735 Neil Ave, Ohio State University, Columbus, OH 43210, U.S.A. 693 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

2 694 JOHN M. BLAIR 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. 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 m 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% H 2 SO 4. 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 1 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: Fli = (SaAFDM S1AFDM) where -=- (LiAFDM SI AFDM) 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/X 0 = exp( kt), where X/X 0 - 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 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 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

3 % 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 (r 2 ) 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 Nutrient dynamics in decomposing leaf litter 695 Table 1. Decomposition rate variables for flowering dogwood, red maple and chestnut oak litter. Annual decomposition rate constants (k), coefficients of determination (r 2 ), 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 First -year Decay rate (k) r 2 Predicted % remaining* Observed % remaining* Second-year Decay rate (k) r ,..., Predicted % remaining* Observed % remaining* Total (2 years) Decay rate (k) r 2 Predicted % remaining 11 Observed % remaining' Flowering dogwood ,271 *»* Red maple H 0.952" '** ; : : Chestnut oak , :44 " '" '(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 (r 2 = 0.987) and initial lignin-to-n ratio (r 2 = 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 First-year C:N Lignin :N % Water % Ethanol decay rate % N ratio % Lignin ratio soluble soluble Species Flowering dogwood Red maple Chestnut oak

4 696 JOHN M< BLAIR Table 3. Initial litter quality variables as predictors of first-year decay rates. Coefficients of determination (r 2 ), 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 % Nitrogen C:N ratio % Lignin Lignin:N ratio % Water soluble % Ethanol soluble r Slope K-intercept 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 O A. Nitrogen B. Phosphorus I''» L C O u I iad < C. Sulfur III 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. 1 fd. Sulfur A 120 L - ' '- 100, k / v 80 / -«_.^ 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.

5 Nutrient dynamics in decomposing leaf litter 697 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 % 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 Species Flowering dogwood Red maple Chestnut oak Day C:N ratio C:S ratio C:P ratio 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 5 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

6 -;S 698 JOHN M. BLAIR A. Nitrogen Red Maple =.823 b = Chestnut Oak.864 b = Z.O o 80 OL B. Sulfur Dogwood r2=.849 b = Red Maple r 2 =.945 b = Chestnut Oak * r 2 =.843 b = O C. Phosphorus b = Red Maple r 2 =.927 b = Chestnut Oak r 2 =.6IO b = 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. 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 12 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

7 Nutrient dynamics in decomposing leaf litter 699 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 (r 2 = 0.778) or lignin-to-n ratios (r 2 = 0.492), but were highly positively correlated with concentrations of ethanol solubles (r 2 = 0.938) and total solubles (r 2 = 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 (r 2 = 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 (r 2 = 0.989) and overall 2-year (r 2 = 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 (r 2 = for S, r 2 = 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 ; 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, 5 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 1% 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

8 700 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 N 2 -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. 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