Agriculture Reference
In-Depth Information
Excretion
and Mucus
+
Microbial
Turnover
+
Leaching
Uptake
Runoff
Denitrification
Available
Nitrogen
Litter
Processing
+
Soil Organic
Matter
+
Aggregate
Formation
Conceptual model to examine the mechanisms through which earthworms can affect system-
level availability and fate of N. Note: + indicates that earthworms increase the availability of N;
FIGURE 9.2
indicates
a decrease in availability; and ° indicates that the effect of earthworms can result in either an increase or a
decrease [in N availability].
−
that determine the overall role of earthworms in ecosystem fluxes of C and N. Experiments that
address more than one component will provide greater understanding of the mechanisms by which
earthworms affect the cycling of C and N than will experiments or studies addressing a single
component. The greatest insights into the effects of earthworms on storage or loss of C and N at
the ecosystem scale will probably come from studies that operate on large temporal or spatial
scales. Furthermore, investigating earthworm invasions and their effects on nutrient cycling and
soil organic matter dynamics can provide insights on the integrated effects of earthworms at these
larger spatial scales (Alban and Berry 1994; Bohlen et al. 2004a,b).
E
B
M
COSYSTEM
UDGET
ODELS
We have incorporated concepts from the conceptual mechanistic models into conceptual ecosystem
models that compare nutrient fluxes in the presence (
Figure 9.3
)
and the absence
(
Figure 9.4
)
of
earthworms in an agroecosystem. These models emphasize the major pathways by which earth-
worms change the retention and loss of C and N, incorporating the effects of earthworms on soil
biological, chemical, and physical processes. To determine the roles of earthworms in agroecosys-
tem sustainability, it may be necessary to focus on processes by which earthworms increase or
decrease the storage or loss of nutrients and how they influence productivity and nutrient uptake
by crops. Quantification of these nutrient budget models could provide data essential for determining
whether earthworms are always beneficial organisms in the context of agroecosystem sustainability
or whether they may sometimes have deleterious effects.
The presence of earthworms can change the sizes of various nutrient pools and fluxes of C and
N significantly (Figure 9.3). Earthworms reduce pool sizes of surface litter (Bohlen et al. 1997),
coarse and particulate soil organic matter (Parmelee et al. 1990; McCartney et al. 1997), and
microbial biomass (Blair et al. 1997). Through interactions of earthworms with the microbial
community and by processing organic matter, earthworms can increase the system flux of CO
2
(gaseous C loss), as discussed in the section on mechanistic models (
Figure 9.1
).
These same interactions, coupled with earthworm excretion, can also lead to increased avail-
ability of N. In a long-term earthworm manipulation study in Ohio, the increases in available N
corresponded to increased concentrations of nitrate in some years (Blair et al. 1997). Available N
can be retained within an ecosystem by microbial or plant uptake, although we did not observe
increased plant N uptake associated with increased earthworm populations in our field experiments
