Applied Soil Ecology 14 2000 71–79
Simulated patterns of litter decay predict patterns of extracellular enzyme activities
Daryl L. Moorhead
∗
, Robert L. Sinsabaugh
Department of Biological Sciences, University of Toledo, Toledo, OH 43606, USA Received 5 March 1998; received in revised form 26 August 1999; accepted 20 September 1999
Abstract
Decomposition is a complex suite of processes that strongly affects the mineralization and immobilization of mineral nutrients. Thus, considerable research has focused on gaining a mechanistic understanding of litter decay. Models of decay
vary with respect to detail, but most utilize decay rate coefficients for particular chemical constituents of litter, derived from empirical observations of turnover. Recent studies have shown that the activities of extracellular enzymes are correlated with
decay, and represent instantaneous measures of biochemical processes responsible for the hydrolysis of particular chemical compounds. For these reasons, temporal patterns in turnover rates for particular litter constituents should correspond to activity
levels of particular degradative enzymes. To test this hypothesis, we modified a general model of litter decay GENDEC to predict activities of extracellular enzymes. This was accomplished by viewing turnover rates for carbon fractions of litter
extractive, acid-soluble and acid-insoluble compounds as surrogates for the activity levels of relevant extracellular enzymes e.g., glucosidase, cellulases, oxidases. The resulting temporal patterns of litter turnover rates generated by the model were
similar to observed patterns of enzyme activities. These results demonstrated that traditional modeling approaches may be used to predict patterns of enzyme activities, although existing data are not sufficient to conduct a rigorous quantitative test
of this approach. Conversely, assays of extracellular enzymes could be used to test models of litter decay with a high degree of chemical and temporal resolution, because enzymes catalyze specific reactions and measures of activity levels represent
instantaneous rates of degradation. ©2000 Published by Elsevier Science B.V. All rights reserved.
Keywords: Decomposition; Enzymes; Modeling
1. Introduction
Moorhead et al. 1996 recently discussed the strengths and limitations of common approaches used
to model litter decay. Most mathematical models relate decay rates to measures of climate and litter
quality, but these relationships often apply only to limited ranges of conditions Whitford et al., 1981.
∗
Corresponding author. Tel.: +1-419-530-2017; fax: +1-419-530-7737.
E-mail address: dmoorheuoft02.utoledo.edu D.L. Moorhead.
In theory, some of the limitations to models based on litter quality could be circumvented by simulating de-
composition as a product of the biological activities of decomposer organisms Bunnell et al., 1977; McGill
et al., 1981. However, decomposition emerges as a composite process at the community level, with
mechanistic controls varying among systems. While a major benefit of adding more mechanistic detail
to models is the confidence with which they can be applied to novel situations, a major drawback is
the added complexity of the resulting model. Com- plex models require more data to define parameters,
0929-139300 – see front matter ©2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 9 - 1 3 9 3 9 9 0 0 0 4 3 - 8
72 D.L. Moorhead, R.L. Sinsabaugh Applied Soil Ecology 14 2000 71–79
generate more uncertainties associated with parameter estimates, and are more difficult to interpret.
Recently, a new approach has been used to model litter decay, based on the activities of extracellular
enzymes Sinsabaugh and Moorhead, 1994. Because microorganisms produce enzymes that catalyze the
degradation of substrates in their immediate environ- ment, decomposition rates should be related to the ac-
tivities of enzymes associated with the degradation of key classes of compounds Sinsabaugh et al., 1991.
Measurements of particular enzyme activities provide a more precise insight to microbial activities than gen-
eral determinations of biomass or bulk respiration. Par- nas 1975 was among the first to present a model of
litter decay that was controlled by differential acqui- sition of macronutrients by decomposers. Sinsabaugh
and Moorhead 1994 extended this approach with the development of an explicit model of microbial allo-
cation of resources among community indicator en- zymes MARCIE, which estimates timing and levels
of activity for particular suites of enzymes, based on energy and nutrient availabilities.
The basic assumptions of the MARCIE model are that 1 enzymic degradation of complex molecules
is the rate-limiting step in both microbial production and litter decay, and 2 activities of key enzymes are
controlled by their rates of synthesis, determined by an allocation strategy that optimizes resource acqui-
sition by decomposers. Sinsabaugh et al. 1991 have shown that temporally-integrated rates of enzymic ac-
tivities correlate with mass loss patterns in litter, and Sinsabaugh and Moorhead 1994 used the MARCIE
model to simulate overall patterns of litter decay. In contrast, more traditional models of decomposition
utilize rate constants to calculate degradation of par- ticular litter constituents, derived from empirical ob-
servations of changes in litter chemistry during decay. Conceptually, these two approaches should be compat-
ible, but no study has determined if traditional models yield patterns of turnover for litter constituents that
correspond to patterns of enzyme activities.
The objective of the current study was to compare patterns of litter decay, based on traditional model-
ing approaches, to patterns of activities for extracel- lular enzymes responsible for the degradation of par-
ticular litter constituents. We used a general model of litter decay GENDEC; Moorhead and Reynolds,
1991 to estimate temporal patterns of degradation for three major categories of litter constituents: 1 extrac-
tives, 2 acid-solubles, and 3 acid-insolubles. These classes of chemical compounds frequently are moni-
tored during studies of litter decay and often are in- corporated in models of decomposition. The activities
of three groups of enzymes glucosidases, cellulases and oxidases are associated with the degradation of
these litter constituents, respectively, and have been monitored during litter decay in a limited number of
studies e.g., Sinsabaugh et al., 1991. Results of sim- ulations are compared to observed chemical changes
in decomposing litter and reported patterns of enzyme activities. This study serves as a novel evaluation of
traditional modeling approaches and suggests means by which enzymic data can be used to refine models to
more closely simulate patterns of microbial activities responsible for litter decay.
2. Experimental methods and modeling approach
