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
3.1. Field study Overall relationships between observed and simu-
lated patterns of litter decay were reasonably consis- tent for the field study conducted in Wisconsin Aber
et al., 1984. Herein, we illustrate the results of simu- lations for maple Fig. 3 as representative of the en-
tire suite of simulations Table 3. Peak rates of mass loss corresponded to periods of favorable climate with
rates slowing during winter Figs. 2, 3a. For all lit- ter types, the extractives fraction of litter decreased
over time Fig. 3b, while the acid-insoluble fraction increased Fig. 3d. However, patterns of degradation
for the acid-soluble fraction of litter varied among litter types, and considerable differences existed be-
tween observations and simulations Fig. 3c. Even so, estimated values were within ±25 of observa-
tions throughout simulations. Simple correlations be- tween observations and simulations showed very high
agreement for mass loss, and reasonably high corre- lations for extractives and acid-insoluble fractions of
litter residues Table 3.
The GENDEC model does not explicitly estimate activities of extracellular enzymes. We assumed that
turnover rates of specific carbon fractions of litter
Table 3 Correlations r
2
between observed and simulated characteristics of litter residues during decomposition in field studies N = 11
Litter type Mass Extractives Acid solubles Acid insolubles
Maple 0.9661 0.7792
0.0416 0.7933
Aspen 0.9529 0.6851
0.7098 0.7758
White oak 0.9930 0.6159 0.0375
0.8806 Pine
0.9850 0.8900 0.0843
0.8548 Hemlock
0.9615 0.8968 0.2580
0.8817 Red oak
0.9914 0.4654 0.3204
0.9293
D.L. Moorhead, R.L. Sinsabaugh Applied Soil Ecology 14 2000 71–79 75
Fig. 3. Decomposition of sugar maple leaf litter in the Wiscon- sin field study Aber et al., 1984. A litter mass remaining,
B extractives fraction of litter, C acid-soluble fraction, D acid-insoluble fraction.
extractive, cellulose and lignin components would be proportional to the corresponding activity levels of
degradative enzymes. For example, the simulated pat- tern of enzyme activities in maple litter during the first
full year of the decomposition study Fig. 4, showed an early peak of activity associated with degradation of
extractives e.g., glucosidase or invertase, followed by a peak in activity of cellulase enzymes e.g., endocel-
lulase or exocellulase which, in turn, was followed by
Fig. 4. Simulated patterns of turnover rates surrogates of enzyme activity for chemical constituents of sugar maple leaf litter during
decomposition, in the Wisconsin field study Aber et al., 1984.
increasing activity of lignin-degrading enzymes e.g., phenoloxidase or peroxidase. Results for other litter
types revealed an overall similarity in general patterns of activity, with differences among litter types occur-
ring as a result of differences in litter quality Table 1. These temporal patterns in enzyme activities cor-
respond to general patterns of fungal colonization of decaying litter in that species present in early stages of
decay lack enzyme systems capable of degrading cel- lulose and lignin Frankland, 1966, 1969, 1976, while
species present in latter stages of decomposition have greater capacity to produce cellulolytic and lignolytic
enzymes.
Although few data exist to test our model predic- tions, a set of studies conducted in northeastern India
Kshattriya et al., 1992; Joshi et al., 1993 reported similar relationships between patterns of forest litter
decay and enzyme activities. In both studies, invertase activity hydrolyzes sucrose peaked early in litter de-
cay, correlated positively with soluble sugar content of litter, and declined throughout decomposition. A more
recent study by Dilly and Munch 1996 reported that activities of
b
-1,4-glucosidase hydrolyzes cellobiose peaked early and declined steadily during the decom-
position of alder litter in northern Germany. Because the extractives component of litter includes such com-
pounds as cellobiose and sucrose, the activities of glu- cosidase and invertase should be highest when these
compounds are being degraded most rapidly. The uti- lization of simple compounds by decomposers is more
energetically efficient than using complex polymers, so the degradation of simple compounds should pre-
cede that of polymers. Model results were consistent
76 D.L. Moorhead, R.L. Sinsabaugh Applied Soil Ecology 14 2000 71–79
with these expectations: turnover rates for extractives were greater and peaked earlier in simulations than
turnover rates for acid-soluble and acid-insoluble frac- tions Fig. 4.
In contrast to invertase activity, Kshattriya et al. 1992 and Joshi et al. 1993 noted that activity lev-
els of cellulase and amylase hydrolyzes starch in- creased more slowly and remained at higher rates
for longer periods of time. Enzyme activities also showed seasonal variations that correlated with favor-
able climatic conditions Kshattriya et al., 1992; Joshi et al., 1993. These patterns of cellulase activities were
consistent with simulations Fig. 4 in that turnover rates of acid-soluble fractions of litter peaked more
slowly than rates for extractives, and stayed at rela- tively higher levels for a longer period of time. Simula-
tions also showed a strong seasonal pattern of turnover for litter fractions.
Kshattriya et al. 1992 and Joshi et al. 1993 noted that cellulase activity was not correlated with cellulose
content of litter, which also was true for simulations. No clear trend in concentrations of the acid-soluble
fraction was apparent from these field and simula- tion studies Fig. 3c. Such inconsistencies in cellulose
degradation may result from the interactions of mul- tiple controls. For example, simpler compounds are
more readily utilized by microbiota than cellulose, and thus influence production of cellulase enzymes and
concomitant cellulose turnover. Also, nitrogen avail- ability has been shown to affect cellulose decay e.g.,
Berg et al., 1975 and should influence cellulase pro- duction, although we have no information on nitro-
gen dynamics in these systems. Finally, Linkins et al. 1984 have shown that the activity of endocellu-
lase sharply declines below a temperature threshold of about 10
◦
C, adding further complexity to interpreting seasonal responses of enzyme activities in the field
studies. Thus, it is difficult to determine the controls on cellulase activity and cellulose turnover.
Few other field data are available for comparison, but Sinsabaugh et al. 1992 reported patterns of en-
zyme activities associated with the decay of birch wood on a variety of sites in northern New York
Fig. 5. Interestingly, levels of enzyme activities were more closely related to stage of decay than time, and
generally were unaffected by site factors. Several en- zymes showed increased activity with progressive de-
cay glucosidase, exocellulase, xylosidase, phenoxi-
Fig. 5. Activity levels of enzymes units vary associated with birch wood decay in northern New York, USA Sinsabaugh et al.,
1992. A. glucosidase
m
mol gAFDM
− 1
h
− 1
, B xylosidase
m
mol gAFDM
− 1
h
− 1
, C endocellulase unit gAFDM
− 1
h
− 1
, D exocellulase
m
mol gAFDM
− 1
h
− 1
, E phenol oxidase
m
mol gAFDM
− 1
h
− 1
, F peroxidase
m
mol gAFDM
− 1
h
− 1
. Data presented are pooled from eight sites, including two stream
sites, two riparian zones, two hemlock stands, two deciduous stands.
dase while others showed no relationship to decom- position endocellulase, peroxidase. This lack of cor-
respondence between endocellulase and stage of decay is consistent both with simulations and results of stud-
ies by Kshattriya et al. 1992 and Joshi et al. 1993. In contrast to the field studies in India, Sinsabaugh et
al. 1992 found that glucosidase activities increased with time stage of decay, probably because suitable
substrate was unavailable in woody material before the action of other enzymes e.g., cellulases, xylosidase
released these compounds as products of cellulose and hemicellulose degradation.
It is unfortunate that none of these published stud- ies reported the chemical fractions of decaying litter
in conjunction with enzyme activities. Kshattriya et al. 1992 and Joshi et al. 1993 report results of
D.L. Moorhead, R.L. Sinsabaugh Applied Soil Ecology 14 2000 71–79 77
correlations between levels of enzyme activity and a number of environmental factors and chemical char-
acteristics of litter, but do not provide these data in more detail. Dilly and Munch 1996 only report
the activities of glucosidase, and Sinsabaugh et al. 1992 did not conduct chemical analyses of decaying
wood. Thus, published information is not adequate to support a more quantitative evaluation of our model
results.
3.2. Laboratory study The laboratory study of dogwood, maple and oak
leaf decay Linkins et al., 1990 provided more de- tailed measures of cellulase activities than the field
studies see above. Also, the chemical characteris- tics of these litter types are known well enough to
approximate in our simulation model. The constant moisture and temperature conditions of the laboratory
study suggest little limitation to decay imposed by a changing climate. Thus, we were reasonably confident
that close correspondence between simulated and ob- served patterns of mass loss would yield patterns of
litter chemistry dynamics that could be used to predict cellulose turnover acid-soluble materials.
Melillo et al. 1982 and others have shown that rates of litter decay are negatively correlated
with lignin-cellulose index LCI = lignin[lignin + cellulose] and that LCI increases during decomposi-
tion. We found that relationships between observed and simulated patterns of mass loss and LCI during
litter decay were highly correlated Table 4. More detailed comparisons of litter constituents were not
possible because carbon fractions of residues were not evaluated during the laboratory experiment. However,
patterns of exo- and endocellulase activities assayed during the experiment were compared to simulated
patterns of turnover for the acid-soluble fraction of
Table 4 Correlations r
2
between observed and simulated mass N = 11 and lignin-cellulose index LCI; N = 9 values of litter residues
during decomposition in laboratory incubations Litter type
N Mass
LCI Dogwood
14 0.8674
0.7109 Chestnut oak
15 0.7815
0.8875 Maple
13 0.9112
0.6747 Fig. 6. Patterns of simulated turnover rates for acid-soluble frac-
tions of litter and observations of exo- and endocellulase activities during a laboratory study Linkins et al., 1990. A chestnut oak,
B flowering dogwood, C red maple.
litter, as a surrogate for cellulase activity Fig. 6. Because units of activity for exo- and endocellulase
activity are not comparable to each other or to units of turnover estimated by simulations, all activities were
expressed as relative fractions of maximum activity levels. For example, the highest activity of endocellu-
lase on maple litter was observed on Day 100 of the experiment Fig. 6c. Thus, activities of endocellulase
at all other days were expressed as a proportion of this value.
A number of similarities existed between simulated patterns of turnover for acid-soluble fractions of litter
and observed patterns of cellulase enzyme activities. Decaying oak litter showed a continuous decline in
enzyme activities over time, similar to simulations Fig. 6a. Both dogwood and maple litter showed
an early peak of cellulase activities, followed by a
78 D.L. Moorhead, R.L. Sinsabaugh Applied Soil Ecology 14 2000 71–79
reduction in activity, a second peak of activity and then a gradual decline. This pattern also was pro-
duced by simulations, although timing of the initial peak in turnover rates was earlier than observed, and
the simulated decline following the initial peak was more severe than observed Fig. 6b and c. One of
the reasons why these discrepancies existed between simulations and observations is that the model shows
instantaneous responses to litter availability, without constraints imposed by limits in microbial growth
rates. Thus, the simulated peaks in early turnover may have been higher than was possible for micro-
biota to achieve during early colonization. A similar lag in initial decay of buried litter was observed by
Moorhead and Reynolds 1991.
Two reasons why simulations predicted such rapid shifts in enzyme activities result from model struc-
ture and underlying assumptions. First, the model is based on the assumption that microbiota degrade
extractives in preference to acid-solubles, and de- grade acid-solubles in preference to acid-insolubles,
given intrinsic rates of decay and availability of ni- trogen Moorhead and Reynolds, 1991. Hence, the
precipitous decline in cellulose turnover produced by the model occurred when turnover of extrac-
tives was sufficient to immobilize all available ni- trogen and none was left to support decay of the
acid-soluble pool. In reality, decomposer communi- ties are likely to be more diverse in their response
to nutrient-limited conditions. A second reason why the model produced such rapid shifts in predicted
levels of enzyme activities is that there are no pools of enzymes explicitly included in model structure –
enzyme activities were inferred from turnover rates of carbon fractions of litter. In fact, enzymes persist
in the environment and continue to catalyze chemical reactions for a period of time after being produced by
microbiota. For this reason, activity levels are likely to show more gradual changes over time than were
predicted.
4. Conclusions
