Agriculture Reference
In-Depth Information
contents imply site- (spatially) or temporally specific effects arising from different soil types,
management practices (OM or fertilizer additions, cropping systems), sampling dates, and compo-
sition of the earthworm communities.
Effects of earthworms on denitrification are also important. For instance, Knight et al. (1992)
reported three to five times greater denitrification rates in earthworm casts than in surrounding soil,
contributing to as much as 29% of the total denitrification losses in a heavily N-fertilized (200 kg
N ha −1 ) pasture in Devon, U.K.
Although earthworms have very little direct effect on P cycling in soils (only trace quantities of
P are excreted in liquid wastes) (Bahl 1947), they may have substantial indirect effects because of
enhanced phosphatase (acid and alkaline) activity in earthworm casts, burrows, and guts; increased
availability of P (organic and inorganic) in casts; reduction in Al-binding of P; and increased incor-
poration of surface-applied fertilizers and plant litter in the soil (Satchell and Martin 1984; Syers and
Springett 1984; Alter and Mitchell 1992). Thus, as much as 9 to 13 kg ha −1 of organic plus inorganic
P were accumulated in earthworm casts in 1 year in New Zealand pastures (Syers et al. 1979b). These
casts, containing more P than does the surrounding soil, form microsites rich in P that can enhance
microbial activity and root growth (Mouat and Keogh 1987), particularly in soils poor in phosphorus.
The size and composition of earthworm populations, even within one locality, vary widely with
soil type and management practices (e.g., cultivation or liming) (Lee 1985; Robinson et al. 1992),
and it is possible that such population differences may reflect differences in the composition and
abundance of the microbial communities that form the basis of the decomposer food web in soil.
For example, it is widely recognized that microbial populations are influenced strongly by soil pH
(Gupta 1994), and some species of earthworms are sensitive to soil pH and are more abundant in
neutral to slightly acid soil (Lofs-Holmin 1986). However, the relationships between such changes
in microbial populations and the reproductive success of earthworms are still not known.
The type of plant community and the corresponding litter-soil-rihzosphere conditions can also
have large-scale effects on earthworm populations. For instance, Boettcher and Kalisz (1991)
reported changes in the earthworm community structure (abundance and species composition) in
a vegetation sequence involving hemlock, rhododendron, and yellow poplar, with one earthworm
species ( Bimastus parvus ) largely replacing another ( Komarekiona eatoni ) along the sequence.
Similarly, in agroecosystems, earthworms appear to be more abundant in soils under certain crops
(e.g., clover) (Weternacher and Graff 1987) or under certain grasses in meadows (Babel et al. 1992)
or cereals (Edwards and Bohlen 1996).
The possibility that earthworms may be used to introduce and disperse beneficial microorgan-
isms through soil was reviewed by Doube et al. (1994e). They examined the possibility of using
pellets of a mixture of earthworm food and beneficial microorganisms (rhizobia for root nodulation,
pseudomonads for biological control of take-all, and Metarhizium sp. for the biological control of
root-feeding scarab larvae). These microorganisms could be applied to the field and dispersed
through soil as a consequence of the feeding activities of earthworms. The success of this process
relies on developing a food that is attractive to earthworms, on survival of the microorganism in
the food pellet and during passage through the earthworm gut, and on effective earthworm dispersal
through the soil.
Although only in the early stages of development, a number of these constraints have been
examined in laboratory experiments, and Doube et al. (1994e) considered that this novel mechanism
showed considerable promise. Success would require that earthworm activity alter the composition
and functioning of microbial communities in soils on a broad scale.
SUMMARY AND CONCLUSIONS
The microbial decomposition of organic residues in soil provides the energy and nutrients that promote
and sustain the biological fertility of soils. Surface crop residue inputs into agricultural systems in
temperate and subtropical regions are commonly on the order of 2 to 10 tons ha −1 year −1 , and similar
 
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