Agriculture Reference
In-Depth Information
The contributions of earthworms to soil fertility have been described in several hundreds of
articles and topics (Lee 1985; Edwards and Bohlen 1996; Lavelle and Spain 2001). This has led
to a growing expectation from soil users for provision of methods that protect soil fertility through
the enhancement of biological processes. Earthworms may be considered a biological resource for
farming systems, and the management of earthworm communities provides a promising field for
innovation in agricultural practices (Lavelle et al. 1999). Demand for techniques, making optimal
use of earthworms as a resource, is likely to increase, although basic research is still needed to
support such developments.
The relationships between earthworm activities and changes in soil properties are not thoroughly
understood, especially at large timescales of years to decades. Most research results have been
obtained in small-scale laboratory or field experiments that exaggerate the process under study and
can by no means be extrapolated readily to larger scales of time and space. However, recent research
on earthworm casts and other related biogenic structures have shown that these structures may
persist for rather long periods and provide the soil with specific properties that may survive the
death and elimination of earthworm populations that produced them. This property contributes
significantly to the resistance and resilience of soils to disturbances (Lavelle et al. 2004).
This chapter synthesizes information on the effects of earthworms on soil systems at scales
longer than 1 year, and earthworm behavior that may affect these processes is detailed.
EARTHWORMS AND SOIL FUNCTION:
THE DRILOSPHERE CONCEPT
For example, at a real scale of a small tropical farmerÔs plot, earthworm activities are only one
determinant of soil fertility, and their effects are likely determined by factors operating at larger
scales of time and space, such as climate, edaphic characteristics, and the quality and amount of
organic inputs (Lavelle et al. 1993). Earthworms participate in soil functions through the drilosphere
system, which involves earthworms, casts, and burrows and the whole microbial and invertebrate
community that inhabits these structures. As a result of earthworm digestion processes and creation
of soil structures, the composition, structure, and relative importance of the drilosphere system to
soils is clearly determined by climate, soil parameters, and the quality of organic inputs. Earthworms
in turn influence soil microbial communities and hence have effects on microbial processes related
to soil organic matter (SOM) status and nutrient dynamics. They also affect the activities of other
soil-inhabiting invertebrates, either by modifying their environment or through competition for
feeding resources (
Figure 8.1
)
.
Earthworms are not a homogeneous entity. They comprise several functional groups, each with
clearly distinct ecology and impacts on the environment (Bouch 1977). Current classifications based
on earthworm location in the soil profile and their feeding resources are still too general to describe
the large diversity in functions. Moreover, earthworms classified into a general category may not always
exhibit the behavioral traits expected to be associated to this category (Neilson et al. 2000; Mariani et
al. 2001). Classifications based more on impacts on soil parameters might be more useful.
The effects of earthworms on soil function thus depend on their interactions with a wide range
of identified abiotic and biotic factors that operate at rather different scales of time and space.
Furthermore, the effects produced will affect soil structures of different sizes and persist for highly
variable periods of time, depending on the factors with which they interact. For example, it is
expected that physical structures created by earthworms as a result of their interactions with other
soil components will last much longer than the flush of activity of dormant microorganisms that
they have activated in their guts (
Figure 8.2
).
Most studies have described processes at smaller scales of earthworm activities, typically in
microcosms
, small plots or small field enclosures. Results obtained under such conditions describe
existing processes but cannot be extrapolated directly to quantify and predict effects produced at the
