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
Although few studies of decomposition have exam- ined simultaneous changes in litter mass, chemistry
and enzyme activities, different aspects of decay can be compared between studies. Herein, we use a gen-
eral model of litter decay GENDEC; Moorhead and Reynolds, 1991 to simulate the decomposition of leaf
litter under field and laboratory conditions. The first set of simulations are compared to observed changes
in litter chemistry during a field study of litter decay Aber et al., 1984. In addition, the general pattern of
simulated turnover in litter constituents is compared to temporal patterns of enzyme activities obtained from
other field studies Kshattriya et al., 1992; Sinsabaugh et al., 1992; Joshi et al., 1993, which do not report lit-
ter chemistry. The second set of simulations are com- pared to results of a laboratory experiment, in which
litter mass loss and activities of cellulase enzymes were monitored Linkins et al., 1990.
2.1. Modeling approach GENDEC is representative of semi-mechanistic de-
composition models that include the effects of climate and litter quality on patterns of decay Moorhead and
Reynolds, 1991, 1993, 1996. It simulates the dy- namics of six pools of carbon and nitrogen Fig. 1,
including: 1 labile plant compounds extractives, 2
D.L. Moorhead, R.L. Sinsabaugh Applied Soil Ecology 14 2000 71–79 73
Fig. 1. Carbon flow diagram for GENDEC.
holocellulose acid-soluble, 3 resistant plant com- pounds acid-insoluble, 4 live microbial biomass,
5 dead microbial cell walls acid-solubles and acid-insolubles; see below, and 6 dead microbial
cytoplasm extractives. These categories of chemical compounds often are determined by chemical analy-
ses for decaying litter see review of analytical meth- ods by Ryan et al., 1990. Nitrogen flows are assumed
to balance calculated carbon flows, given the N : C ratios of decomposing materials. The loss of carbon
from each dead organic matter pool is a function of moisture and temperature conditions, and nitrogen
limitations. The details of model structure and opera- tion are provided elsewhere Moorhead and Reynolds,
1991, 1993, 1996, but one modification of GENDEC was made for the current study; dead microbial cell
walls were assumed to consist of approximately equal fractions of acid-soluble and acid-insoluble materials.
The decay rates for these pools of materials were set equal to those of litter pools for acid-soluble and
acid-insoluble compounds. Climate drivers used for simulations are explained in the following descrip-
tions of field and laboratory studies.
2.2. Field studies The first step in our efforts to estimate activities of
extracellular enzymes associated with litter decay was to ensure that behavior of the decomposition model
GENDEC was consistent with expected patterns of decay, including the dynamics of particular chemical
fractions of litter that could be related to enzyme ac- tivities. This required observations of mass loss and
carbon fractions in decaying litter, in conjunction with some measure of climate moisture and temperature
to drive simulations. Fortunately, Aber et al. 1984
Table 1 Initial chemical fractions and total Kjeldahl nitrogen TKN content
of litter dry mass used in simulations of decomposition in field studies Aber et al., 1984
Litter type Extractives Acid solubles Acid insolubles TKN
Sugar maple 44.8 43.1
12.1 0.83
Aspen 31.1
47.5 21.4
0.83 White oak
32.4 47.4
20.2 0.84
White pine 32.8
44.7 22.5
0.44 Hemlock
35.8 39.6
20.6 0.83
Red oak 30.0
45.2 24.8
0.82
reported mass loss and carbon fractions extractives, acid-solubles, acid-insolubles of six litter types that
were incubated in fine-mesh litterbags, over a period of 732 days, on the floor of a sugar maple stand in
Wisconsin, USA. This suite of litters provided a rea- sonably broad range of initial litter quality character-
istics for simulations Table 1. The effects of climate on decomposition were estimated as a monthly scalar
DEFAC; Fig. 2, according to Parton et al. 1987, given local climate records Parton, unpublished.
When simulated patterns of decay for particular categories of litter constituents are similar to obser-
vations see above, we may interpret turnover rates as surrogates for activity levels of various groups of
enzymes. This is because Sinsabaugh et al. 1991 have shown that patterns of litter decay and enzyme
activities are highly correlated, so the activity levels of enzymes responsible for the degradation of par-
ticular litter factions might be predicted by assuming that they are proportional to the simulated turnover
rates of these constituents. For example, the extrac- tives component of litter contains compounds that are
Fig. 2. Microclimate scalar used to drive model simulations for the Wisconsin field study Parton, pers. commun..
74 D.L. Moorhead, R.L. Sinsabaugh Applied Soil Ecology 14 2000 71–79
degraded by
b
-1,4-glucosidase and invertase cel- lobiose and sucrose, cellulose acid-soluble is hy-
drolyzed by
b
-1,4-endoglucanase and
b
-1,4-exoglucanase, and polyphenolic compounds acid-insolubles are
degraded by phenol oxidase and peroxidase enzymes. Unfortunately, such relationships have seldom been
examined in an integrated manner for decomposi- tion of bulk litter because few experiments have
investigated both the patterns of litter chemistry and activities of extracellular enzymes. However,
studies conducted in northeastern India Kshattriya et al., 1992; Joshi et al., 1993 and northern New
York, USA, Sinsabaugh et al., 1992 can be used to evaluate model estimates of temporal patterns for
some enzyme activities. Kshattriya et al. 1992 and Joshi et al. 1993 reported activities of invertase,
cellulase and amylase during the decomposition of tree leaves, and Sinsabaugh et al. 1992 reported
mass loss of birch wood in conjunction with the ac- tivities of
b
-1,4-glucosidase,
b
-1,4-endoglucanase,
b
-1,4-exoglucanase, phenol oxidase and peroxidase. 2.3. Laboratory studies
In addition to field investigations, the enzyme activi- ties associated with litter decay have been examined in
laboratory experiments. One of the few studies that si- multaneously measured both chemical characteristics
of decaying litter and enzyme activities was performed by Linkins et al. 1990. In this investigation, litter
bags containing senescent leaves of flowering dog- wood Cornus florida, red maple Acer rubrum and
chestnut oak Quercus prinus were placed in plastic basins containing forest floor material collected from a
mixed deciduous forest site in southwestern Virginia, USA. These microcosms were maintained under con-
stant temperature ca. 20
◦
C and moisture holding capacity conditions, over a 9 month period and litter
was analyzed periodically for mass loss, fiber com- position and the activities of endocellulase and exo-
cellulase. Simulations were conducted for these litter types using initial chemical characteristics Table 2
and assuming no temperature or moisture limitations. As in comparisons with field studies, turnover rates of
the acid-soluble fraction of litter were considered to be proportional to activities of cellulase enzymes, and
compared to laboratory results.
Table 2 Initial chemical fractions and total Kjeldahl nitrogen TKN content
of litter dry mass used in simulations of decomposition in laboratory incubations cf. LIDET, 1995
Litter type Extractives Acid solubles Acid insolubles TKN
Dogwood 62.0
37.7 0.4
0.99 Chestnut oak 30.6
44.1 25.4
1.14 Red maple
54.9 27.5
16.6 0.87
3. Results and discussion
