Agriculture Reference
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FIGURE 12.5 Grazing by A. trapezoides on rhizosphere soil surrounding the roots of wheat seedlings: (left)
controls showing ungrazed rhizosphere soil attached to the root; (right) bare roots exposed following removal
of rhizosphere soil by earthworm feeding. (Photograph by J. Coppi.)
plants (Binet et al. 1997), and sugar cane (Spain et al. 1990) than in adjacent soils. Doube
(unpublished data) found a population of about 100 earthworms m −2 (a mixture of A. trapezoides ,
A. rosea, and M. dubius ) among the roots of Salvia verbenaca, whereas none were recovered
from Salvia -free soil 1 m away.
Root herbivory by the anecic earthworm L. terrestris has been inferred from pulse-labeling
experiments (Cortez and Bouch 1992), from direct observation (Carpenter 1985; Shumway and
Koide 1994), and indirectly by the assessment of root fragments in earthworm guts or casts. For
example, Reddell and Spain (1991a) observed root fragments in the fresh casts of P. corethrurus .
Similarly, Baylis et al. (1986) suggested that earthworms graze on living clover roots.
In contrast, studies on endogeic earthworm species in a rhizotron (Gunn and Cherret 1993) and
on A. trapezoides (Figure 12.5) suggest that endogeic species can consume the rhizosphere soil
(possibly containing root hairs) but do not eat the roots. In both cases, the rhizosphere microor-
ganisms will form at least part of the earthwormÔs diet, and so earthworm activity has the potential
to influence rhizosphere function. Endogeic earthworm species also tend to accumulate in soil
patches with higher levels of OM (Hughes et al. 1994). Because plant roots and organic residues
are both localities of relatively high levels of microbial activity, and earthworms recognize and
consume some microorganisms selectively, it seems probable that endogeic earthworm species
aggregate in such areas in response to microbial activity. Whether such behavior enhances the
earthworm reproductive performance (and corresponding abundance) has not been demonstrated,
but the positive links between organic residues, microbial activity, and earthworm food make such
an association very likely.
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