Agriculture Reference
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The type of land-use also greatly affects earthworm communities. The main factors
that severely deplete populations are intensive cultivation, pesticides and soil denudation
(see, e.g., Edwards, 1983; Lofs-Holmin, 1983; Lavelle and Pashanasi, 1989; Lavelle
et al ., 1994b; Edwards et al ., 1995; Edwards and Bohlen, 1996). In the humid tropics,
cropping of soils recently cleared from their native vegetation depletes the stocks
of assimilable soil organic matter and reduces the carrying capacity for earthworm
communities. In terms of assimilable C, the cost of earthworm communities has been
estimated at in the humid savannas of Lamto (Côte d'Ivoire) (Lavelle, 1978);
this C is mostly, but not exclusively, derived from fresh residues largely concentrated in
the coarse organic fractions of soil organic matter (Martin et al., 1991; 1992). In tropical
agroecosystems, the cost of maintaining active earthworm communities of ca. 400 kg
fresh weight has been estimated at (Charpentier, 1996; Gilot et al ., in press).
Chemicals also affect earthworms in diverse ways: fertilisers and a wide range of
pesticides do not affect them, provided concentrations are not unduly high. However,
when N fertilisation leads to soil acidification, communities may be significantly
depressed (Potter et al., 1985) and a few pesticides, notably the organochlorine com-
pounds, are extremely toxic (see, e.g., Lee, 1985; Edwards and Bohlen, 1996).
Neutralisation of acid pH following liming always enhances earthworm communities.
In a Finnish spruce forest, lime and the litter of deciduous species were applied and
earthworms ( Allolobophora caliginosa ) were introduced (Huhta, 1979). After two years,
earthworm biomass had significantly increased in all treatments. The application of
litter alone had a similar effect although it only resulted in a slight increase in pH.
After three years, earthworm biomass significantly decreased in the litter treatment,
probably because the resource was exhausted (Figure IV.44).
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