Agriculture Reference
In-Depth Information
In vermicomposting, complex interactions occurring between the organic matter, microorgan-
isms, earthworms, and other soil invertebrates result in a rapid biooxidation and stabilization of
the organic matter. Most vermicomposting systems sustain complex food webs and at the same
time modify the chemical forms of several nutrient elements into longer-lived organic compounds
important for nutrient dynamics (Domnguez et al. 1997).
Although populations of some sensitive organisms may be reduced drastically or eliminated
during vermicomposting, the substrate maintains an overall increased active community of decom-
poser organisms, which in addition to earthworms, includes enchytraeids, nematodes, springtails,
mites, protozoa, and very large populations of microorganisms.
The complex food webs in the vermicomposting systems can be represented as a pyramid with
primary-, secondary-, and tertiary-level consumers. The base of the pyramid, the source of energy,
is composed of decaying organic matter, including plant and animal residues. In the same way as
in soil, the spatial scales at which soil organisms act in a vermicomposting system are determined
mainly by their size, number, and modes of operation. At the microbial microscale, there are
basically bacteria (unable to move long distances except if transported by water or larger soil
organisms), fungi (in which hyphal growth provides the capacity to colonize new zones), and
actinomycetes. Concomitantly, still at a microscale level but gradually increasing in size and spatial
influence, the microÏfood web includes microinvertebrates, such as nematodes, protozoa, and
rotifers, which feed primarily on microorganisms. At the mesoscale level, there are larger organisms,
such as enchytraeids and mesoarthropods, that feed on decaying organic matter, microorganisms,
and microinvertebrates and are important in facilitating nutrient cycling and the small-scale dispersal
of microorganisms. Finally, at the macroscale level, there is the main component of the vermicom-
posting system, the earthworms, which feed on and disperse microorganisms. As they feed on
decaying organic matter, earthworm burrowing and tunneling activities aerate the substrate and
enable water, nutrients, and oxygen to filter through it; their feeding activities increase the surface
area of organic matter for microorganisms to act on. As some decomposers die, more food is added
to the food web for other decomposers.
As organic matter passes through the earthwormÔs gizzard, it is finely ground before digestion.
Then, digestive microorganisms, and possibly enzymes and other fermenting substances, continue
the breakdown process. The organic matter passes out of the earthwormÔs body in the form of casts,
or
vermicomposts
, which are rich in nutrients and microorganisms and are of fine quality and
structure.
Earthworms can exert various influences on soil microorganisms and invertebrate populations
directly or indirectly via comminution, burrowing, casting, grazing, and dispersal. Not only does
the physicochemical and biological status of the organic matter and soil change for the better during
the course of these activities, but the characteristics of the drilosphere may also be altered dramat-
ically (see reviews by
Brown
1995 and
Doube and Brown
1
998; see
Chapter 12
, this volume). The
drilosphere is the soil system influenced directly or indirectly by earthworm activities (Lavelle
1988), whether in the gut of the earthworm (internal processes) or in its burrows and casts (external
processes).
As a consequence, the entire soil invertebrate community plays an important role in organic
matter degradation through its interactions with soil microorganisms.
Because vermicomposting systems teem with an enormous biodiversity of microorganisms and
invertebrates, they provide ideal sites for complete and effective inoculation of the organic wastes
with complex communities of beneficial soil organisms. This may be especially important for
producing bedding plant container media and for soils that have been intensively chemically
managed or have become impoverished. As the understanding of soil ecology increases, the deter-
mination and analysis of the structure of decomposer food webs in organic amendments may
become an important predictive tool in evaluating their potential qualities and value.
An interesting example of such interactions in the food web is the effect of earthworms (
E.
andrei
) on nematode populations during vermicomposting (Domnguez et al. 2003) (
Figure 20.4
)
.
