Agriculture Reference
In-Depth Information
IV.4
THE DRILOSPHERE
Earthworms are ecosystem engineers
sensu
Jones
et al.
(1994) in that they effectively
modify their environments. Their large size and their capacity to move substantial
quantities of soil mean that their activities dramatically change the distribution patterns
of organic resources and the fluxes of water and gases in soil. Both the distributions
and activities of micro-organisms are modified by earthworms and through this, the
dynamics of soil organic matter and nutrient release.
Bouché (1975) introduced the concept of the drilosphere (from the Greek 'drilos'
= earthworm) to describe the 2 mm thick zone surrounding the walls of earthworm
galleries. This term will be used here in the broader sense of describing those components
of the soil (including microbial and invertebrate populations) that are affected by
earthworm activities. The drilosphere therefore includes galleries and burrows, but also
extends to surface and subterranean casts, 'middens' (accumulations of litter surrounding
the openings of burrows) and the peculiar micro-environment represented by the gut and
its contents. Earthworms accumulate organic matter within their structures as a conse-
quence of their burrowing and feeding behaviours. Active communities can therefore
incorporate a large part of litter fall and root litter into the drilosphere and transform
a substantial proportion of the upper soil horizons into casts of different ages and origins
(Section II.3.3.2).
The drilosphere may be considered the functional equivalent of the rhizosphere and
litter systems both of which are micro-environments with a localised and intense micro-
biological activity. This activity is stimulated by the fine subdivision and thorough
mixing of litter debris with soil inorganic materials, and by the production of cutaneous
and intestinal mucus. The latter materials have analogous effects to the root exudates
of the rhizosphere and the leaf leachates of the litter system.
4.1
Classification of drilospheres
The effects of earthworms on soil properties differ markedly, depending on the ecological
categories of the species involved (see Chapter III.4.3.1.2) :
(
i
)
Epigeic species are part of the litter-system and their roles are similar to those
of other epigeic invertebrates. They have little or no impact on the physical properties of
the mineral soil horizons and their effects are thus marginally 'drilospheric';
Anecic species strongly influence soil properties by translocating leaf-litter and
mixing it with the soil, and through the construction of their subvertical burrow systems;
Endogeic species affect soil organic matter dynamics and physical structure by
building and maintaining a macro-aggregate structure, and a dense network of macropores.
(ii)
(iii)
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