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Observations made in rubber plantations of different ages in Cte dÔIvoire seem to support this
hypothesis (Gilot et al. 1995). Observations of successional processes in an alpine spruce forest of
France showed that earthworm activities seem to have been reduced by an accumulation of low-
quality litter residues that they could not process rather than by the exhaustion of available organic
resources. In that case, spruce litter may have become palatable only after long periods of matu-
ration, during which fungal attacks progressively eliminated those toxic compounds present in fresh
litter. The effect of litter quality on earthworm activities has already been stressed in studies on
Finnish spruce forests, in which the
input of high-quality litter allowed earthworm populations to
increase significantly even in an acid environment (Huhta 1979).
However, earthworms may participate in the accumulation of organic matter through (1) an
overall increase of amounts of organic matter produced in an ecosystem and (2) the protection of
SOM in structures of the earthworm drilosphere (burrow system) (Martin 1991). In a 3-year
experiment at Yurimaguas and Lamto, the combination of C consumption by inoculated earthworms,
the increased capture of C by plants, and protection of SOM in compact casts did not result in
significant changes in the abundance of C. Nevertheless, there were clear indications that the quality
of organic matter, estimated by either physical fractionation or respirometry, was modified. Long-
term consequences of such modifications are not predictable yet.
Physical effects on soils resulting from earthworm activities seem to persist for long periods.
Blanchart et al. (1993) demonstrated that casts deposited by large earthworms (of the compacting
category) had largely kept their structures 30 months after the earthworms had disappeared. This
resistance of casts, which seems to be because of an outstanding stability of aggregates, seems
largely dependent on soil type (Zhang and Schrader 1997). Stabilized earthworm casts tend to
conserve organic matter because little microbial activity is possible in these compact structures
(McInerney and Bolger 2000; Zhang et al. 2003).
Nevertheless, in natural conditions, such macroaggregates cannot comprise more than 40 to
60% of the total soil of the Lamto savannah (Blanchart et al. 1993). Despite their continuous
production, these aggregates do not accumulate beyond that limit, probably because earthworm
populations that decompact soil also regulate the extent of aggregation. These may be the small
species earthworms of Eudrilidae at Lamto. They may also be species of Enchytraeidae, ants,
termites, myriapoda, or microarthropods. Therefore, the long-term efficiency of these processes of
protection of aggregates in earthworm casts depends largely on (1) the maximum percentage of
large aggregates present at a given site and (2) the lifetime of these aggregates.
Spatial patterns of earthworm populations may be understood best in terms of their overall
impact on soils. At Lamto, there was a significant negative correlation in some seasons among
populations of compacting earthworm species that accumulate large compact casts in the upper
20 cm of soil, decompacting species that produce fine granular-shaped casts from soil macroag-
gregates, and the large casts produced by the former species (Rossi 1997). This may indicate
that earthworm subpopulations have successions inside a patch. However, in the Colombian
savannah studied by Decans and Rossi (2001), the spatial distribution of earthworm populations
did not show much change over a 3-year monitoring period, indicating that possible changes in
population distribution patterns occur only at rather long scales of time. Populations of earthworm
species that compact soils can develop once those that decompact soils have produced small
aggregates rich in the organic matter that populations of the compacting species may ingest.
Such distribution patterns may also be determined by local availability of assimilable organic
matter. Studies in both hevea plantations and spruce forests showed clearly that the impacts of
earthworms become important when organic matter has been sufficiently prepared by digestion
by a succession of termite and arthropods digestions in the first case and the development of
fungal colonization in the second.
These results and our hypotheses open several avenues for future research:
