Agriculture Reference
In-Depth Information
appears to be a decreasing interest in the effects of agricultural management practices on earthworms
and on earthworm introductions into land uses and nutrient cycling processes. By contrast, there
seems to be increasing interest in the effects of earthworms on soil physical properties and on plant
growth. The two most significant changes over the four symposia are the increasing interests in
earthworm ecotoxicology and in organic waste management with earthworms. The increased
number of presentations on earthworm ecotoxicology is particularly surprising because there have
been three international conferences on earthworm ecotoxicology: in Sheffield, U.K., 1991 (Greig-
Smith et al. 1992); in Amsterdam, the Netherlands, 1997 (Shepard et al. 1998); and in Arhus,
Denmark, 2001. Each was attended by more than 100 researchers. This suggests that there is a
great increase in the use of earthworms as ecotoxicological tools; in laboratory tests; in artificial
soils, microcosms, and terrestrial model ecosystems; and in field experiments (see
Chapter 16
and
Chapter 17
, this volume). There also seems to be much interest in the use of earthworm physiology
and other associated processes as biomarkers to assess the impact of pollutants on soil ecosystems
(see Chapter 16, this volume).
The increased interest in the use of earthworms in waste management and vermicomposting
seems a real and significant trend, particularly in developing countries such as India. The presen-
tations at the four symposia cover a wide range, including the biology and ecology of vermicom-
posting species of earthworms, methods of vermicomposting, nutrient transformations, and field
and greenhouse studies of the effects of vermicomposts on plant growth (see
Chapter 18
to
Chapter
20
, this volume). The greatest interest in this topic seems to be in the United States, U.K., Spain,
Mexico, and Australia.
The strong microbial connection with earthworm feeding and activity is emerging as a central
theme in soil nutrient process studies. For example, it is clear that earthworm activity greatly
accelerates crop residue decomposition and nutrient mineralization, and there is growing evidence
that microorganisms serve as a primary source of nutrition for earthworms (Edwards and Fletcher
1988). In an ecological sense, this relationship might be considered a ÑkeystoneÒ association. With
respect to organic residue quality, earthworms may selectively ingest high-quality residues (Bohlen
et al. 1997), but their relative effects on decomposition may be greater on low-quality residues,
which earthworms fragment, inoculate with microbes, and incorporate into soil. High-quality
substrates tend to decompose rapidly even in the absence of earthworms. This idea has also been
suggested for other soil invertebrates (e.g., microarthropods, millipedes, woodlice, etc.) involved
in forest litter decomposition (Seastedt et al. 1983).
Lavelle et al. (
Chapter 8
, this volume) make the interesting point that the cost of earthworm
activity, in terms of the status of organic carbon in agroecosystems, must be considered because
earthworms tend to stimulate microbial activity and accelerate litter turnover and possibly cause
losses of soil carbon if adequate organic inputs are not maintained. The agricultural management
implication of this possibility is that some ÑbaseÒ amounts of organic substrates must be added to
agricultural soils periodically to feed microorganisms and earthworms, with additional amounts
needed to transform organic matter into soils if there have been soil losses.
As mentioned, substrate quality has a strong influence on interactions between earthworms and
microorganisms (see
Chapter 2
and
Chapter 12
, this volume). These observations also have impli-
cations for mixed residue management in agroecosystems. Although the mineralization of rapidly
decomposing residues may contribute to short-term nutrient availability, more slowly decomposing
materials (i.e., those with higher C:N ratios) may immobilize nutrients and add to nutrient pools
with longer turnover times and more sustained nutrient availability (Tian et al. 1995). Further, very
recalcitrant materials, especially those left on the soil surface, contribute to a Ñmulch effect,Ò which
protects soil surfaces from erosive forces (e.g., rainfall impact, wind) and protects and ameliorates
the soil microclimate (Lal 1991; Hendrix et al. 1992).
With respect to effects of earthworms on plant growth, a number of interesting observations
have been reported. Several papers have indicated that the effects of earthworms on plants differ
with different earthworm species, different plant species, and different soil types (Chapter 8, this
