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and split them into smaller aggregates (Derouard et al. 1997; Blanchart et al. 1999). The feeding
behavior that allows such a selection has never been described in detail. The long-term consequences
of that behavior on the dynamics of soil processes and SOM dynamics have not been addressed
directly. Endogeic species of earthworms may deposit 20 to 200 t dry soil ha
surface casts that
contain a significant proportion of SOM yearly. A much larger quantity of casts is deposited inside
the soil, and a volume of soil equivalent to the whole soil of the upper horizons may be passed
through earthworm guts in a few years (Darwin 1881; Lavelle 1978). Nevertheless, the higher
concentration of fine particles in casts than in the surrounding bulk soil suggests that earthworms
may possibly reingest the same soil several times, whereas microsites with a relatively coarser
texture may be avoided by earthworms.
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1
S
P
H
D
E
PATIAL
ATTERNS
OF
ORIZONTAL
ISTRIBUTION
OF
ARTHWORMS
Several authors have pointed out the aggregated distribution of earthworm populations that occur
in such diverse ecosystems as an arable soil from Germany (Poier and Richter 1992), a deciduous
forest of England (Phillipson et al. 1976), humid African (Lavelle 1978) and Colombian savannahs
(Decans and Rossi 2001; Jimenez et al. 2001), and an artificial pasture in southern Martinique
(Rossi et al. 1997). In some places, the earthworm distribution was independent of basic soil
parameters such as depth or clay or carbon contents. Furthermore, different earthworm species
tended to have different horizontal distributions (Poier and Richter 1992); in an almost monospecific
earthworm community of
in Martinique, Rossi (1997) observed different
distribution patterns for adults and juvenile earthworms. These observations suggest that some
earthworm species have patchy population distributions with an average patch diameter of 20 to
40 m. Such patches seem to be independent of soil parameters, at least to some extent. The dynamics
of earthworm populations in such a patch are not synchronized with the populations of other patches.
The occurrence of such distribution patterns suggests that earthworm activities concentrate on
patches probably only for limited periods of time before becoming locally extinct. In the moist
savanna at Lamto (Cte dÔIvoire), complementary patterns have been observed between large
earthworm species that produce large casts and tend to compact the soil and smaller species that
produce fine granular casts after ingesting the larger casts of the first type of species with root litter
(Blanchart et al. 1999). In that case, the observed patterns suggest a succession of patches made
of ÑcompactingÒ and ÑdecompactingÒ species with complementary effects on soils.
Polypheretima elongata
C
. D
S
OMPACTING
VS
ECOMPACTING
PECIES
The physical structure of earthworm casts is very relevant to the dynamics of SOM at intermediate
scales of months to years. Two different types of casts may be distinguished in this respect: the
globular casts of compacting species and the granular casts of decompacting species. Casts of the
first category may be surrounded by a thin 10- to 20-m
cortex made of clay minerals and organic
particles, which seem to reduce aeration and hence inhibit microbial activity at the scale of weeks
to months (Martin 1991; Blanchart et al. 1993). Soils colonized by monospecific communities of
such earthworm species are prone to compaction.
In an experiment in which the earthworm
H
had been introduced into soils
under a yam and a maize culture, the proportion of large aggregates bigger than 2 mm increased
significantly in soils that had been sieved previously, but no significant effect was observed in a
Millsonia anomala
situations.
Similar effects have been observed after inoculation of the peregrine, pantropical, endogeic species
P.
into soils under a traditional cropping system of Peruvian Amazonia. After six succes-
sive crops, earthworms had increased the proportion of macroaggregates (>2 mm) significantly from
25.4 to 31.2% at the expense of smaller (<0.5 mm) aggregates with proportions that had decreased
corethrurus
