Mass loss and nitrogen dynamics of decaying litter of

Mass loss and nitrogen dynamics of decaying litter of grasslands: the apparent low nitrogen immobilization potential of root detritus T. R. SEASTEDT EPO Biology arid Institute of Arctic arzri Alpine Research, University of Colorndo, Boulder, CO 80309, U.S.A. AND

W. J. PARTONA N D D. S. OJIMA Natural Resource Ecology Lnboratory, Colorado Stare Utziversitv, Fort Collit~s,CO 80523, U.S.A.

Can. J. Bot. Downloaded from www.nrcresearchpress.com by Renmin University of China on 06/07/13 For personal use only.

Received July 30, 1991 SEASTEDT, T. R., PARTON,W. J., and OJIMA,D. S. 1992. Mass loss and nitrogen dynamics of decaying litter of grasslands: the apparent low nitrogen immobilization potential of root detritus. Can. J . Bot. 70: 384-391. Litter-bag studies and simulation modeling were used to examine the relationship between mass loss and nitrogen content of decaying prairie foliage and root litter. In contrast with forest studies. grassland roots were low in lignin and nitrogen, decayed more rapidly than foliage, and demonstrated very low nitrogen immobilization potentials. Our findings agree with reports indicating that buried substrates with high C:N ratios do not immobilize substantial amounts of nitrogen and that nitrogen-limited environments induce steeper slopes in the mass loss - nitrogen concentration relationship. However, results suggesting rapid nitrogen mineralization contradict our own studies demonstrating reduced inorganic nitrogen availability in soils of frequently burned prairie. Simulation of observed patterns using the CENTURY grassland model indicated that these results could not occur without creating soil organic matter with unrealistically high C:N ratios. Litter-bag studies of buried substrates therefore may provide an incomplete perspective on the mass loss and nitrogen dynamics of buried litter in grassland and agroecosystem soils. Key words: Andropogon gerardii, C:N ratio, decomposition, immobilization, mineralization, nitrogen. SEASTEDT, T. R., PARTON,W. J., et OJIMA,D. S. 1992. Mass loss and nitrogen dynamics of decaying litter of grasslands: the apparent low nitrogen immobilization potential of root detritus. Can. J. Bot. 70 : 384-391. Les auteur ont utilisC la technique des sacs de lititre et des modtles de simulation pour examiner les relations entre la perte en masse et le contenu en azote des litikres foliaires et racinaires en dCcomposition dans une prairie. Contrairement aux Ctudes conduites en milieu forestier, les racines des espkces de prairie contiennent peu de lignine et d'azote, se dCcomposent plus rapidement que le feuillage et ne montrent que de trks faibles potentiels d'immobilisation de I'azote. Les rCsultats obtenus s'accordant avec les rapports qui indiquent que les substrats enterres posCdant des rapports C:N ClevCs n'immobilisent pas d'importantes quantitCs d'azote et que les milieux oh I'azote est le facteur limite, se caracterisent par des pentes plus prononcCes pour la relation perte en masse et teneurs en azote. Cependant, les rCsultats suggbrant une niinCralisation rapide de I'azote sont contredits par les Ctudes des auteurs, lesquelles montrent une disponibiliti reduite en azote inorganique disponible dans les sols de prairie frtquemment brQICs. La simulation des patrons observes h I'aide du modtle de prairie CENTURY indique que ces rCsultats ne pourraient &treobtenus sans la crCation d'une matikre organique du sol possedant des rapports C:N irrCalistes. Les etudes h l'aide de sacs de litikre peuvent donc conduire h une perception imcomplkte de la perte en masse et de la dynamique de l'azote des lititres enterrees dans les sols de prairie et d'agroecosysttmes. Mots c l b : Andropogon gerardii, rapport C:N, dCcomposition, immobilisation, mineralisation, azote. [Traduit par la rCdaction]

I

Introduction The amount of organic matter and nitrogen found in tallgrass prairie soils tends to exceed amounts measured from forest soils from similar regions (Jenny 1941, 1980; Brady 1974). Foliage litter likely played a very modest role in the development of the prairie soil organic matter, as frequent fires removed most aboveground litter (Ojima et al. 1990). Inputs from leachates, estimated from soil dissolved organic carbon concentrations, are small (Fedynich 1989). Soil organic matter profiles in grasslands parallel profiles observed for root mass (Weaver 1958). While the relatively high belowground production of prairie grasses is one explanation for the size of this humus reservoir, fundamental differences in the ratio of nitrogen and carbon losses (gaseous and leaching) to nitrogen and carbon humified during the decay process may also affect humus amounts (Broadbent and Nakamura 1974). Certainly, substrate chemistry, especially lignin content, and soil texture affect patterns in soil organic matter amounts observed over regional gradients (Anderson and Coleman 1985; Parton et al. 1987). The amount of carbon respired by microbes per unit of

nitrogen required by these microbes, as mediated by the physiochemical regime of the environment, can be evaluated by mass loss - nitrogen concentration relationships of decaying litter (Aber and Melillo 1980, 1982; Melillo et al. 1982; McClaugherty et al. 1985). Under conditions of environmental nitrogen limitation, decaying litter is more efficient, i.e., immobilizes less nitrogen per unit of carbon lost, but differences in environmental nitrogen may not affect decay rates, which instead are under partial control by substrate quality (Pastor et al. 1987) and soil texture (Sorensen 1981). Frequently burned tallgrass prairie produces substantial quantities of root detritus low in nitrogen content. Empirical evidence and simulations using the CENTURY grassland model have indicated that nitrogen limitation, caused by immobilization of nitrogen on root detritus, limits plant productivity on frequently burned sites (Ojima et al. 1990; Seastedt et al. 1991). Thus, while we hypothesized high nitrogen efficiency in the decay of root detritus (i.e., relatively large mass loss per unit of nitrogen retained, or a steep slope for the mass loss nitrogen concentration curve), we also predicted a strong

SEASTEDT ET

immobilization potential for this element by microbes on decaying substrates. These potentially conflicting hypotheses are compatible if decay rates of the substrate are relatively slow.


Study site and methods Research was conducted on the Konza Prairie, a tallgrass research site located about 15 km south of Manhattan in the Flint Hills region of northeastern Kansas. The area is on the western edge of the tallgrass biome (see Anderson 1990) and receives an average of 83 cm of precipitation per year (Bark 1987). The vegetation is dominated by tallgrass, C, plant species such as big bluestem (Andropogotz gerardii Vit.), little bluestem (At~dropogotlscoparius Michx.), Indiangrass (Sorgastrutn ruttans (L.) Nash), and switchgrass (Panicrttn virgntum L.). Soil temperatures during the growing season may peak near 30°C and are a function of burning and the size of the grass canopy (Hulbert 1969; Seastedt and Briggs 1991). Sites used in this study were not burned during the course of the experiments. Big bluestem foliage and flowering stem decomposition was measured in large (10 x 10 cm or larger) litter bags with 3-mm mesh openings, placed vertically within litter or hung from a suspended line in the canopy of tallgrass prairie. This litter was therefore not in direct contact with the soil. Forty litter bags were placed on each of four upland and four lowland sites. A subset of the bags was harvested over three dates in year I and twice in year 2. Location did not affect decay rates (Seastedt 1988), and these data are treated as a simple random sample. Only a subset of samples that were analyzed for nitrogen content are reported here. A second set of experiments measured the mass loss and nitrogen concentrations of senescent foliage litter either tethered or placed in small (ca. 5 x 5 cm) litter bags with 1- or 3-mm mesh openings placed horizontally on the soil surface. Ten samples of tethered litter or bags of each mesh size were placed on 12 plots in a bottomland prairie dominated by Brot71us inertnis. A portion of this litter was in direct contact with the soil surface. As the type of containment (tethering vs. litter bags) or litter-bag mesh size did not affect foliage decay rates (Seastedt 1988), results have been pooled for analyses presented here. Nitrogen analyses were made o n 10 replicates of the senescent foliage harvested immediately after being placed in the field and on samples harvested after 6, 12, 24 months in the field. These small litter bags with two different mesh sizes were also used with root litter buried 5 - 10 cm deep in soil. Ten bags of each mesh type were inserted vertically into the soil on each of the 12 plots. Initial nitrogen determinations and harvesting of root samples were made in conjunction with foliage collections. Mesh size in the root studies did have modest effects on mass loss (Seastedt et al. 1988) but did not statistically affect the relationship between mass loss and nitrogen content. Results were again pooled for analyses presented here. Seastedt (1988) reported on the decomposition of individually tagged flowering stems placed on the soil surface, and those results are compared with data reported here. The present study includes an additional year of data for the experiments using small litter bags that was not reported in the 1988 studies. The inverse relationship between mass loss and nitrogen concentrations for these decomposition experiments was measured using two procedures. Individual point data (percentage of initial mass remaining and nitrogen concentrations from individual litter-bag or tethered samples) were used, as were data aggregated to represent means of mass loss and nitrogen concentrations observed through time. Both data sets were subjected to linear regression analysis. Initial mass and nitrogen concentration data were not used in those regressions. The aggregated data are commonly reported in the literature, and these types of data was used by Aber and Melillo (1980) to develop concepts involving the inverse relationship between mass loss and nitrogen concentrations. Since we had accesss to the raw data, however, we chose to evaluate the two procedures to see if a within-time relationship between mass loss and nitrogen content (information that is lost in the aggregation process) could potentially alter the outcome of

AL.

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the analysis. A simple, contrived data set was constructed and manipulated to assist with interpretations. A question to address with the field data, therefore, was whether or not the aggregation process affected interpretations of differences among substrate types, or between the same substrate in different environments. Lignin values of foliage and roots obtained from sites dominated by big bluestem were obtained from published and unpublished sources. General agreement was found on values for roots; foliage lignin values exhibited greater variability. Foliage lignin values range from a low of 5.1 % of mass (Wedin and Tilman 1990) to a high of 12% reported for Flint Hills bluestem foliage, obtained near the present research site (Launchbaugh and Owensby 1978). Despite this range in lignin values, all substrates had relatively low lignin concentrations compared with most values reported from temperate forests (e.g., Melillo et a/. 1982; Aber et a/. 1990; M. Harmon, Oregon State University, unpublished data). The C E N , ~ U R Ymodel was developed to study carbon and nutrient dynamics of grasslands (Parton et a/. 1987, 1988). We used a microcosm version of this model (i) to test simulated mass loss - nitrogen relationships with observed values and (ii) to evaluate the consequences of substrate C:N ratios, lignin content, presence of inorganic nitrogen, and variation in soil texture on the patterns of the inverse relationships. Details of model construction have been reported previously (Parton eta/. 1987). Here, we started with a given amount of litter with known C:N ratio and lignin characteristics and ran the model using optimal temperature and moisture conditions. Carbon and nitrogen in litter were divided into structural and metabolic components based on the lignin to nitrogen ratio, and each component decayed at a specified rate or was transferred into an active soil carbon pool (microbial carbon) or a slow soil carbon pool (the decaying substrate carbon). The model contains a third passive soil carbon pool, but this pool is not involved in the short-term litter decay processes reported here. Both the active and slow pools have adjustable C:N ratios that were manipulated as part of this study. The model assumes that all decomposition occurs with the simulated litter bag, i.e., fragmentation losses of litter or export of carbon and nitrogen away from the substrate by fungi are not represented.

Results Mass loss and nitrogen corzcentrntions Patterns of decay and nitrogen amounts for the various substrates, as observed through time, are shown in Fig. 1. Litter chemistry, decay rates, slopes of the mass loss - nitrogen concentrations and an estimate of the maximum amount of nitrogen immobilized per gram of initial litter mass are reported in Table 1. The initial nitrogen concentrations, C:N ratios, o r lignin values did not appear as important to decay o r nitrogen dynamics a s did the location of-the litter. Litter bags containing foliage and stems that were not in contact with the soil surface decomposed more slowly than similar subs relatively low strates placed in contact-with the soil. ~ o o t had nitrogen concentrations and approximately equal lignin content compared with foliage but exhibited decay rates equal to o r greater than other substrates. Analysis of conjectural data demonstrated that the slope of the inverse relationship between mass and nitrogen concentration differs dependingupon the type of data used in the regression (Fig. 2). The use of individual litter-bag estimates of mass loss and nitrogen concentrations (Fig. 2A) has a smaller slope than the same data aggregated by date (Fig. 2B). This particular comparison represents one possible scenario where within-date relationships between percent mass remaining and nitrogen content are positive (a result that occurs if fungal colonization prevents fragmentation o r leaching losses). T h e removal of within-date variation in nitrogen concentrations prior to the

CAN. J . BOT. VOL. 70, 1992

386

STEMS ABOVE SlRFACE


FOLIAGE ABOVE S U F A C E

TIME (YEARS)

TIME (YEARS)

FOLIAGE ON SURFACE

ROOTS

TIME (YEARS)

TIME (YEARS)

FIG. 1. Percentages of initial amounts of mass (solid lines) and nitrogen (broken lines) for foliage, stems, and roots. Symbols represent means, bars are standard errors. Note the different scale required for the stem results.

TABLE1. Litter characteristics and slopes of the inverse mass loss

Substrate Foliage Above surface On surface Stems Above surface On surface" Roots

-

nitrogen curves

Mass loss to N concn. slope

Initial N (%)

Initial C:N

Lignin

(%I

Decay rate (k, per year)

0.38 0.60

1 10 43

9.5" No data

-0.28 -0.59

-39.3 (r -54.8 (r

0.18 0.18 0.49

244 244 90

7.9" 7.9" 10"-13.7"

-0.13 -0.25 -0.66

-81.1 (r = 0.71) No data -98.6 (r = 0.66)

All points = =

0.70) 0.90)

Avg. by dates -44.1 (r -67.9 (r

N immobilized (mg/g litter)

0.78) 0.99)

4.13 0.47

-160.3 (r = 0.85) - 144.2 (r = 0.81) -198.9 (r = 0.99)

1.98 2.33 0.13

= =

"Average of Griffin and Jung (1983). Cochran er 01. (1988).Wedin and Tilman (1990). and Launchbaugh and Owensby (1978). "~ochraner 01. (1988). 'From Seasredl el al. (1988). Raw data werc unavailablc for comparison. "M. Harmon, Orcgon Statc Univcrsity, unpublished data. "Wedin and Tilman (1990).

analysis in Fig. 2B) reduces the spread in x-axis values and increases the slope of the relationship. By comparing the regressions from a number of these contrived comparisons, we concluded that any factor that increases variation in withindate nitrogen concentrations will result in larger differences between point and aggregated analyses. This effect will be

most evident for those substrates exhibiting slow decay and high nitrogen content variability. The relationship between mass remaining and nitrogen concentrations differed among substrates (Fig. 3). Aggregation of the data shown in Fig. 3 by collection date consistently increased the slope of the inverse relationship and also reduced


SEASTEDT ET

residual sources of variance (Table 1). Aggregation doubles the slope of the relationship for stems and roots but has a less dramatic effect for foliage. Regardless of the form of analysis, roots exhibited the greatest amount of mass loss per unit increase in nitrogen concentrations. Estimates of the amount of additional nitrogen immobilized by these litter types was extracted using the formula for the nitrogen factor developed by Aber and Melillo (1982). The decaying roots and surface foliage, net mineralization began early in or midway through the study. The amount of additional nitrogen immobilized per gram of roots or surface foliage was therefore obtained from the data (Fig. 1) rather than the formula. These numbers may be low if peak nitrogen concentrations were missed by infrequent sampling. The combination of relatively rapid decomposition and a large negative slope for the mass loss-nitrogen concentration curve results in a very low immobilization potential for roots (Table 1). The foliage in contact with the soil also immobilized relatively little nitrogen. However, this foliage was of unknown age at the initiation of the study and as suggested by its initial nitrogen concentration, may have already immobilized substantial nitrogen prior to the initiation of the experiment. model simulations The slope of the inverse relationship between mass loss and nitrogen content was predicted reasonably well for foliage used observed C:N ratios and an initial lignin concentration of 10% (Table 2). However, using nominal conditions, the model failed to mimic results for roots (Fig. 4). Only when the C:N ratios of the active and (or) slow carbon pools were adjusted to unrealistic levels could patterns generated from litter-bag results be duplicated (Table 2). The linearity of the relationship could be maintained if C:N ratios were adjusted; however, allowing the ratio to exhibit any substantial changes other than a decrease from raw litter to soil organic substrates would result in substantial nonlinearity. The nonlinear appearance of the raw root data (Fig. 3) could be mimicked if the C:N ratio of the active soil component was held at nominal conditions, while the slow component's C:N ratio was set at 60, thereby allowing mineralization to occur. Doubling of the ll&nin contents of foliage, stems, or roots had almost no effect on the inverse relationship (Table 2). High initial nitrogen concentrations affected the linearity of the relationship but had only a modest effect on slopes. Pastor et al. (1987) reported inverse linear relationship slopes averaging -90.5 and -33.2 for unfertilized and fertilized litter, respectively, in their study of little bluestem foliage in Minnesota. The nominal conditions of CENTURY accurately predicted the slopes of little bluestem mass loss and nitrogen concentrations under conditions of nitrogen enrichment, but similar to results for stems and roots, the model required substantial modifications in C:N ratios of soil organic matter to duplicate results observed in the unfertilized treatment (Table 2). Direct manipulation of the C:N ratio of the slow component of decaying organic material had modest effects on the slope of the inverse relationship, whereas manipulation of soil texture had relatively larger effects (Fig. 5). As described by Parton et al. (1987), texture controls the efficiency of the conversion of carbon into stable organic matter, i.e., sandy soils have relatively higher losses of carbon to respiration or~leaching than do clay soils, and the steeper slopes observed by Pastor et al. (1987) are explained, in part, by the high sand content of their study site. The effects of external inorganic nitrogen availability on the

AL.

387

0

CT a

LU

40 0.80

0.90

1.00

1.10

1.20

1.30

NITROGEN CONCENTRATION

1.40

1.50

(Oh)

CENTURY

NITROGEN CONCENTRATION (%) FIG.2. Analysis of the invcrsc relationship bctwccn mass loss and nitrogen concentrations using individual samplc estimates ( A ) vcrsus data aggregatcd by collection date (B). Contrived individual data points (c.g., individual litter bags) are shown in (A). Diffcrcnt symbols represent diffcrcnt datcs. The samc data. using only thc average nitrogen concentration for each datc, are shown in (B). A fit of thc linear model. percentage of initial mass (M) = cr 17 x nitrogen concentration (N), results in thc equations M = 110 - 30.4 x N. r' = 0.09, in ( A ) and M = 190 - 100 x N, r' = 0.65. in (B).

+

slope of the mass loss -nitrogen relationship were investigated by manipulating the amount of available inorganic nitrogen in the soil. High nitrogen availability resulted in relatively low C:N ratios of decaying substrates, whereas low N availability increased this ratio. If, for example, C:N ratios of the active soil organic matter pool were allowed to float between 3 and 15 while C:N ratios for the slow pool were allowed to float between 12 and 20, then slopes for roots would vary between -25 and -40, with nitrogen-pool environments exhibiting steeper slopes (Table 2). Thus, when the C:N ratios of the decomposing substrates remain restricted to realistic values (Parton et al. 1987), variation in the nitrogen availability of the environment cannot explain the empirical results for roots.

Discussion The inverse relationship between mass loss and nitrogen concentrations of decaying litter is an ecosystem characteristic. The relationship does not consider the spatial variation in decay and nitrogen relationships observed at a single time interval. Aber and Melillo (1980) recognized that the inverse relationship, as measured using aggregated data, did not provide particularly strong correlations in mull-type soils, where

CAN. I. BOT. VOL. 70. 1992


388 FOLIAGE ABOVE SURFACE

STEMS ABOVE SURFACE

NITROGEN CONCENTRA-TION (%)

NITROGEN CONCENTRATION (%)

FOLIAGE ON SLRFACE

ROOTS


NITROGEN CONCEN'TRATION (%)

FIG.3. Thc relationship bctwcen mass loss and nitrogcn concentration for individual samples of foliage, stcms, or roots summarized in Fig. The lines reprcscnt the best linear fit to thcsc data. TABLE 2. Simulated linear, inverse relationships between mass and nitrogen concentrations of litter with variable C:N and variable lignin relationships generated by the C E N T U R Y grassland model -

Substrate

Initial C:N

Lignin (%)

SOMI C:N

-

Linear regression

SOM2 C:N

Intercept

Slope

I'

Foliage Foliage Foliage Stems Stems Stems Roots Roots Roots Roots Foliage" Foliage" Rootsb Rootsc NOTE: C:N and lignin valucs taken from Tablc I. SOM I and SOM2 arc thc activc and slow soil organic mattcr conipartnlcnts, rcspcctivcly. Linear rcgrcssion is % mass = intcrccpt slopc X N concn. "Attcmpr to siniulatc ficld rcsults from Pastor cr 01. (1987). " ~ o o t sin nirrogcn-limitcd cnvironrncnt (high C:N ratio orgzanic rnnttcr crcatcd). 'Roots in unliniitcd niincral nitrogcn environment (low C:N ratio organic mattcr crcatcd).

+

389

SEASTEDT ET AL. (I) (I)

a

H


u

20

1 0

0.4 0.8 1.2 1.6 NITROGEN CONCENTRATION (%)

2.0

FIG. 4. Simulated mass loss - nitrogen relationships using the C E N T U R Y model. C:N ratio and lignin contents of roots were used. The linear relationship results from C:N ratios of 12 and 15 for the active and slow soil organic components, respectively, and the curvilinear relationships from C:N ratios of 12 and 60 for these two components, respectively.

mixing and fragmenting of detritus by fauna could greatly alter nitrogen content. Similarly, the difference between the slopes of the inverse relationship for point and aggregated data may also be related to microsite variability. Individual pieces of detritus probably do not exhibit the linear relationship between mass loss and nitrogen concentrations through time, but instead exhibit pulses of nitrogen increases and decreases based on changes in microflora and (or) grazing by microbivores and detritivores (Seastedt and Crossley 1981). The steeper slopes for the inverse relationship with aggregated versus point data are a consequence of reducing the impact that individual bags containing unusually low or high nitrogen concentrations have on the slope of the regression. We found by simulating various patterns that this difference between point and aggregated data became small when nitrogen concentrations increased substantially through time or mass loss was exceptionally rapid. Thus, since different litter types vary in their abilities to accumulate nitrogen, the effect of aggregation on the slope of the inverse relationship will be substrate as well as site specific. Comparison of the two slopes, however, does provide an index of variability in the decay process. Our empirical results suggested that root detritus began to lose nitrogen when the average C:N ratio of this substrate was still over 50. Although soil incubation studies have shown that mineralization of buried substrates can begin very rapidly (Broadbent and Nakashima 1974; Jenkinson 1977; Berg et al. 1987), those studies used materials with much lower C:N ratios than found in tallgrass prairie litter. Holland and Coleman (1987), however, reported essentially no immobilization in buried wheat straw with an initial nitrogen content of 0.576, a value very similar to that observed in the roots used here, and Parker et al. (1984) found no nitrogen immobilization in decaying roots of a desert annual with an initial C:N ratio of 64. In another agroecosystem study, Andren et al. (1990) reported net mineralization from four of five root litter studies. Those researchers also reported that substrates buried in soil immobilized less nitrogen than identical material on the surface, perhaps because of plant root competition for available nitrogen. Our finding that the immobilization potential for buried litter is substantially less than that of surface litter contrasts


(I) (I)

0 LT

w

a

-30% SAND

---- 70%

---

SAND

100% SAND

0 ' 0

0.5

1.O

2 .O

1.5

NITROGEN CONCENTRATION

('lo)

FIG. 5. (A) Simulated effects of various C:N ratios of the slow component of decaying litter. (B) The influence of soil texture on the mass loss - nitrogen relationship of litter with C:N ratios of 12 and 16 for the active and slow components, respectively.

sharply with similar data for fine roots in a deciduous forest. Aber et al. (1990) report equal or slower decay rates for fine roots than for surface foliage, high initial lignin and nitrogen concentrations of roots, and high immobilization potentials for fine root detritus. Buried roots at their site, a mor soil forest in Wisconsin, clearly do not behave like the grassland or agroecosystem studies discussed here. Some of the differences between forest and nonforested sites are clearly environmental, i.e., the water limitation of the prairie surface explains the more rapid decomposition of buried litter. The tallgrass prairie is also dominated by species that produce vegetation with high C:N ratios, which contributes to the low nitrogen status of the litter and perhaps to the low nitrogen availability in the soil environment in general (Wedin and Tilman 1990). Finally, the fine mesh (0.1 mm) bags used in the forest root study would minimize what little mixing does occur in mor soils. Simulations conducted here assume no losses of either carbon or nitrogen other than by microbial respiration or mineralization processes. Given that assumption, we were forced to use unrealistic C:N ratios for the soil carbon pools to mimic results obtained for stems and the buried litter. Yet the tallgrass prairie soil environment contains an impressive array of microbes, microbivores, and detritivores. The hypothesis that


390

CAN. 1. BOT. VOL. 70, 1992

buried litter nitrogen is being constantly pumped into the soil by fungal hyphae and (or) removed by grazing from the surface of buried substrates, thereby preventing in situ immobilization, is consistent with d a t a indicating absolute nitrogen losses from high C:N substrates (Anderson and Ineson 1983; Anderson et al. 1985). This hypothesis would also be consistent with the idea that net mineralization in freauentlv burned prairie is reduced owing to enhanced immobilization (Ojima et al. 1990). The immobilization of nitrogen by microbes, however, must occur in soil rather than on the~litteritself. This interpretation is necessary to reconcile the litter-bag findings and detritus nitrogen concentration values with the volume of empirical results and model simulations indicating the general absence of inorganic nitrogen in prairie soils (Seastedt et al. 1991). Prairie litter decaying above the soil surface exhibits the mor-type soil pattern (i.e., little mixing), whereas root litter experiences a mull-type environment. Since most roots decay in a mull environment, however, models developed from litter-bag studies may not adequately describe decomposition and mineralization patterns. In similar fashion, litter-bag procedures used to estimate decay and mineralization rates in soil may also not be totally appropriate for research on buried substrates in mull soils. Litter disappearance may be adequately measured, but nitrogen dynamics are incompletely portrayed with this method. Surprisingly, tallgrass prairie roots do not exhibit those characteristics (high lignin, high nitrogen) often assumed to be required for the generation of substantial quantities of humic substances (Aber and Melillo 1982: Melillo et al. 1982). However, as mentioned in the introduction, the standing crdp of soil carbon and nitrogen of the tallgrass prairie rivals or exceeds the amounts found in other temDerate biomes. The amount of soil detritus inputs, along with an environment conducive to the retention and adsorption of organic carbon particulates and leachates from the c a n o p y and upper soil horizons (Hayes and Seastedt 1989), is ultimately more important to the development of the soil organic matter pool than is the doubling of plant lignin or nitrogen content.

Acknowledgments Field experiments were supported by a National Science Foundation (NSF) - Long-term Ecological Research grant to Kansas State University. Rosemary Ramundo provided substantial field and laboratory assistance. The microcosm version of the CENTURY model used here was developed with funding from NSF grants BSR-8406628 and BSR-8105281 and a USDA -ARS cooperative research project to Colorado State University. Several anonymous reviewers provided important observations and clarifications that were incorporated into the manuscript. Aber, J. D., and Melillo, J. M. 1980. Litter decomposition: measuring relative contributions of organic matter and nitrogen to forest soils. Can. J. Bot. 58: 416-421. Aber, J . D., and Melillo, J . M. 1982. Nitrogen immobilization in decaying hardwood leaf litter as a function of initial nitrogen and lignin content. Can. J. Bot. 60: 2263-2269. Aber, J . D., and Melillo, J . M., and McClaugherty, C. A. 1990. Predicting long-term patterns of mass loss, nitrogen dynamics, and soil organic matter formation from initial fine litter chemistry in temperate forest ecosystems. Can. J. Bot. 68: 2201-2208.

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