Agriculture Reference
In-Depth Information
matter in earthworm casts. In contrast, long-term results from the same study revealed that respi-
ration rates were lower in earthworm casts than in the surrounding soil, indicating increased
protection of organic matter in the stable soil aggregates created by earthworm casting. The effects
of casts on C mineralization or stabilization are complicated further by the factors that influence
stability of the casts, such as the timing of wetting and drying cycles, soil texture, and cast age.
An example of the complexity of these dynamics was shown in a laboratory study, which indicated
that earthworm casts could lose more nutrients than uningested soil when there was an increasing
intensity of wetting-drying cycles (McInerney and Bolger 2000). The extent and timing of increased
protection or loss of C from earthworm casts needs further study.
Even less well studied is the influence of earthworm burrows, or the drilosphere, on C mineral-
ization. The drilosphere is enriched in soluble organic C, which stimulates microbial activity (Parkin
and Berry 1999). Grres et al. (2001) showed that C mineralization rates were significantly greater
in earthworm burrows (41
H
g CO
-C g
d
) than in earthworm casts (31
H
g CO
-C g
d
). Specific
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1
−
1
−
1
−
1
2
2
C mineralization rates (
) were also greater in earthworm burrows (2.56) than
in casts (1.86), indicating that the greater respiration rates in burrows were not simply because of
enrichment of burrow walls with organic matter, but because of a greater turnover rate of the C present.
Thus, much of the stimulatory effect of earthworms on soil microbial respiration seems to be because
of enhancement of microbial activity at specific microsites within the soil, and the net effects of
earthworms on total soil C mineralization is the integrated sum of these microsite effects. In the longer
term, C flux is influenced by the substantial changes that take place in these microenvironments over
time as casts and burrows age and overall soil structure is modified by earthworm activity (Brown et
al. 2000).
One factor not considered in the conceptual model of C flux is the potential effect of earthworms
on the loss of dissolved and particulate forms of soil C. Earthworm casts are susceptible to erosion,
especially in cultivated soils and other erosion-prone systems. In a temperate maize agroecosystem,
the erosion of earthworm casts contributed to annual sediment losses that could amount to up to
32.6 g C m
H
g CO2-C mg C
d
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1
−
1
of organic C, which compares with the amounts of C lost from earthworm
respiration in some systems (Binet and Le Bayon 1999; Le Bayon et al. 2002). Few researchers
have examined the influence of earthworms on losses of dissolved C, but earthworms can increase
the leaching losses of DON, which presumably would be linked to losses of dissolved organic C
(Subler et al. 1997; Domnguez et al. 2004). Losses of dissolved C can represent a substantial
proportion of total C loss in some ecosystems, and the effects of earthworms on such losses are
not well known.
A mechanistic model that is similar to the one presented for C can be developed for the effects
year
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2
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1
of earthworms on the availability or loss of N (
Figure 9.2
)
. Generally, the same components
incorporated into the carbon model are also included in the nitrogen model. An important distinction
is an earthworm excretion compartment. Although the contribution of earthworms to total soil
respiration is often considered small, estimates of earthworm excretion of N can be quite large (18
to 50 kg N ha
; Lee 1983). The state of knowledge and need for further research to improve the
understanding of the influence of earthworms on N cycling, microbial turnover, burial of litter, and
aggregate formation are the same as for carbon. However, the potential fate of N in the N model
differs from that of C in the C model. Although C is eventually lost from the system, mainly as
CO
−
l
, increased concentrations of available N can lead to a loss of N from the system through
leaching, overland flow, and gaseous flux or retention in the system by microbial or plant uptake.
In addition, because N availability can affect both plant productivity and decomposition, there are
important feedbacks between changes in N availability and net C exchange that need to be explored
in future research.
Each component of the models, with the exception of earthworm respiration, interacts strongly
with the other components. It is the interactions among microbial activity, excretion of mucus,
processing of organic matter, and protection of C and N from microbial attack in stable aggregates
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