Agriculture Reference
In-Depth Information
Respiration
and Mucus
+
+
Microbial
Turnover
Litter
Processing
+
CO 2
Loss
+
Soil Organic
Matter
Aggregate
Formation
FIGURE 9.1
Conceptual model to examine the mechanisms through which earthworms can affect system-
level flux of C. Note: + indicates an increase in CO
flux because of earthworms;
indicates a decrease in
2
output; and ° indicates that the effect of earthworms in CO
output can result in either an increase or a decrease.
2
Scheu (1987) observed a simultaneous decrease in microbial biomass and increase in microbial
respiration in earthworm casts after 2 weeks. Similarly, Wolters and Joergensen (1992) observed
a lower microbial biomass but a higher microbial activity per unit biomass in soils after 21 days
of earthworm activity. The simultaneous increase in respiration and decrease in microbial biomass
suggests that earthworm-worked soil may contain a smaller, but more metabolically active, micro-
bial community than soil without earthworms. Although the short-term dynamics of microbial
activity in casts are becoming better documented, there is very little information on the respiration
of older casts (Martin 1991). Investigations need to be initiated on how earthworms influence
microbial respiration and turnover under field conditions over longer periods of time.
It is well known that earthworms increase the rates of disappearance of surface litter, but less
is known about how this process affects soil respiration rates. Burial of litter increases its decom-
position rate because the litter becomes fragmented and is placed in a more favorable environment
for microbial activity (e.g., Beare et al. 1992). Mixing of litter with soil by earthworms may create
an even more favorable environment because of the high moisture content and availability of
nutrients of fresh casts. It is possible that earthworms may enhance microbial activity beyond that
due strictly to the input of litter C (Hendriksen 1997). Bohlen et al. (1997) reported that litter
associated with
middens had a higher microbial biomass and microbial activity than
litter not associated with middens. The movement of litter into soil organic matter pools and the
effects of earthworms on different fractions of soil organic matter are also of great interest. Whether
earthworms feed on labile or recalcitrant soil organic matter pools has important implications for
the long-term storage of C and N. The available evidence suggests that earthworms ingest different
pools of soil C selectively (Parmelee et al. 1990; Martin et al. 1992a,b; McCartney et al. 1997),
but the effects on ecosystem-level fluxes of C of this selective feeding are unknown.
A final component of the mechanistic model is the effect of earthworms on soil aggregate
formation. Although the overall effect of earthworms on other components of the model is to
increase system fluxes of C, the formation of soil aggregates by earthworms may decrease the
availability of C and increase the stabilization of soil organic matter (e.g., Gilot 1997; Scullion and
Malik 2000). However, few studies have examined this phenomenon. The results of Martin (1991)
were consistent with other research that reported increased short-term mineralization of organic
L. terrestris
 
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