Agriculture Reference
In-Depth Information
Two major kinds of casts may be distinguished:
(i)
(ii)
globular casts comprising coalescent round or flattened units (Figure I.17);
granular casts formed from an accumulation of small, fragile and fine-textured pellets.
Production of either type of casts appears to be related to soil texture and the size of
the worm. Specific anatomic features have been identified that allow the worm either to
release large units at regular intervals that will form the globular cast, or a thin constant
flux of cast material that will form the poorly structured granular casts (Lapied and
Rossi, unpublished data).
4.3.3
ASSOCIATED MICROFLORA
The microbial communities of earthworm guts and casts differ little from those of
uningested soils, at least in terms of their species composition. Under favourable
conditions, the endogeic drilosphere may comprise a large proportion or even the entire
volume of the soil (Section II.3.3.2). This may result from the deposition of casts over
long periods, by different earthworm populations.
Soil micro-organism populations tend to decline slightly in the anterior part of the
gut of the tropical earthworm
Pontoscolex corethrurus.
In this preliminary phase of
the reorganisation of microbial communities, some micro-organisms may be killed and
digested by the worm. In the median and posterior parts of the gut, and in the fresh
casts, microbial activity is highly enhanced, although no significant changes are
observed in the density and composition of microbial communities (Barois, 1987).
However, the overall microbial biomass is significantly greater in fresh casts (Figure
IV.51). These results indicate that, broadly, the microflora of the earthworm gut differs
little from that of the non-ingested soil. Detailed microscopic observation of gut walls
has demonstrated the existence of specific micro-organisms although it is possible
that the environmental conditions that prevail in the gut differentially stimulate micro-
bial populations of distinct species or strains. The gut contents of endogeic earthworms
and their casts must be considered as specific microhabitats, fully continuous with
the soil. In the first two days following deposition microbial biomass decreases steadily
and stabilises at values close to those of the control soil (Figure IV.51).
4.3.4
FUNCTIONING OF THE ENDOGEIC DRILOSPHERE
Effect on soil physical properties
Endogeic earthworms in particular, may favourably affect soil structure by promoting
the formation of large stable macro-aggregates greater than 2 mm in diameter (see
Chapter I). Their effects on water infiltration and bulk density may also be important
although both positive and negative effects have been reported. In Western Africa,
Casenave and Valentin (1989) found a clear relationship between the presence of earth-
worms, mainly endogeics, and rates of water infiltration. However, endogeic subsystems
may not be as important as those of the anecic drilosphere due to the even distribution
of macropores and the lack of real galleries (Figure IV.52).
