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the decomposition rates of surface-applied litter (e.g., Tian et al. 1997), little is known about the
fate of this material once it is incorporated into the soil. Much of the incorporated surface litter
resides in belowground earthworm casts, and a significant portion lines earthworm burrows, indi-
cating the importance of these microsites for subsequent C transformations (Jgou et al. 1998).
McCartney et al. (1997) observed strong seasonal effects of earthworms on the coarse and inter-
mediate-size classes of soil organic matter, with amounts of these size classes greater in plots with
decreased earthworm plots relative to control plots. The overall question of whether earthworms
increase or decrease net C storage is still unresolved and may depend on the temporal and spatial
scale at which the question is asked.
CONCEPTUAL MODELS
We present a series of conceptual models as an initial step toward developing more sophisticated
simulation and system-level models for predicting the influences of earthworms on nutrient cycling
processes. Our conceptual models emphasize the kinds of information that need to be incorporated
into such models, provide a basis for exploring fundamental questions about the influences of
earthworms on biogeochemical cycles, and may help direct the development of mechanistic sim-
ulation and ecosystem-level nutrient budget models. Such simulation models may be used to assess
how earthworms, by their respiration and excretion of C and N and through their effects on microbial
turnover, processing of organic matter, and aggregate formation, affect the production of CO
and
the availability of N in the soil. Nutrient budget models can help determine whether earthworms
contribute to sustainability or degradation of ecosystems by examining those processes that affect
storage of C and N within the system or loss of C and N from the system. Both types of models
need to incorporate spatial and temporal patterns and abiotic constraints (Lavelle et al. 1998).
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M
M
ECHANISTIC
ODELS
Our first conceptual model explores mechanisms by which earthworms affect system-level C flux
(
Figure 9.1
). There are five major components by which earthworm activity can affect soil respi-
ration: (1) earthworm respiration, (2) mucus production, (3) microbial turnover, (4) processing of
litter and soil organic matter, and (5) changed soil structure (aggregate formation, burrows). These
processes are not mutually exclusive, and most involve interrelated processes that influence organic
matter mineralization and heterotrophic microbial activity.
Although the direct contribution of earthworm respiration to the system flux of C is generally
a small proportion of the total heterotrophic respiration (e.g., 5 to 6%; Lee 1985), when earthworm
populations are large, their direct contribution to system respiration can be as high as 30% (Hendrix
et al. 1987). There are insufficient data available to generalize about the contribution of earthworm
respiration to total soil respiration, and more field-based studies are needed under a variety of
environmental conditions. Advances in this area could also be made by modeling C flux based on
known earthworm respiration rates in relation to temperature combined with detailed analysis of
earthworm population dynamics in the field. There is very little information on rates of C excretion
in earthworm mucus, which can be a particularly labile form of C. Using
C-labeled earthworms,
Scheu (1991) demonstrated that the production of mucus was a significant pathway of C loss from
earthworms and exceeded the C lost by their respiration.
Much of the available data demonstrating the importance of earthworm-microbial interactions
in soil CO
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production come from microbiological studies of fresh casts and laboratory studies.
Earthworm casts typically have higher microbial biomass and respiration rates than surrounding
soil (Scheu 1987; Lavelle et al. 1992). This increase of microbial activity in fresh casts is greater
than would be predicted by enrichment of soil with added organic matter and is probably stimulated
by the readily available C sources in the casts (Tiunov and Scheu 2000). However, the situation is
more complex because earthworms also seem to increase microbial turnover.
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