Agriculture Reference
In-Depth Information
an earthworm biomass equivalent to
ca.
800 kg fresh weight the densities of
bacteria and protists were 100 times greater on burrow walls than in the soil,
whereas fungal biomass decreased in litter deposited at the soil surface (Binet, 1993).
The anecic drilosphere community is also characterised by an increased proportion of
actinobacteria and protists with lesser (or adverse) effects on fungal populations.
Hendrix
et al.
(1986) observed that minimum tillage stimulates earthworm activities and
the microbial communities that were dominated by fungi in conventional systems,
become dominated by bacteria in 'no-till' plots, as a consequence of earthworm activities.
Nonetheless, a detailed survey of the available literature shows that the results differ
between soils and with earthworm species; the hierarchy of determinants that influence
the composition of microbial communities is not yet fully understood (Brown, 1995;
Edwards and Bohlen, 1996).
The fate of the ingested micro-organisms varies broadly; some groups may be digested
by the worms, especially protists, algae and some fungi or bacteria whereas a large pro-
portion remains unharmed (see
e.g.,
Cooke and Luxton, 1980; Piearce and Phillips,
1980; Rouelle, 1983; Brusewitz, 1959). Marked differences seem to exist between
species since bacterial densities increase during gut transit in
Lumbricus terrestris
and
Lumbricus rubellus,
but decrease in the gut of
Aporectodea caliginosa
(Kristufek
et al.,
1992; Parle, 1963). Finally, some astomatous ciliates are permanent residents of the gut
of anecic earthworms (de Puytorac, 1954; Piearce and Phillips, 1980; Rouelle, 1983).
The effects of earthworms on other soil faunalgroups are variable. Earthworm activities
tend to depress nematode populations, especially those of phytoparasitic species
(Roessner, 1986; Boyer). This effect is probably due to changes in the soil environment
(Yeates, 1981) and positive effects on nematophagous fungi (Edwards and Fletcher,
1988), rather than direct feeding on nematodes (Dash
et al.,
1980). Litter arthropod
populations are generally depleted when anecic earthworms ingest most of the litter
present and mix it with the soil. Nonetheless, their populations may increase locally in
litter accumulated as 'middens' around the gallery openings into the soil (see, for example,
Szlavecz, 1985).
4.2.5
FLUX OF LITTER AND SOIL THROUGH THE ANECIC DRILOSPHERE
Where abundant, anecic earthworms may incorporate the total annual litter mass produced
into the soil within as little as two to three months (Lee, 1985). Their activity seems to
be limited more by food availability than by the environmental determinants of their activ-
ity. In a microcosm experiment conducted at 12 °C with a population of
Lumbricus ter-
restris
equivalent to
ca.
800 kg fresh weight 85 % of litter deposited at the soil sur-
face had disappeared in three months (Binet, 1993). In southern France, Bouché (1983)
estimated that earthworms had assimilated 30 % of the carbon available in litter in 17
weeks. Such a figure is only acceptable if earthworms re-ingest the litter contained in
their casts several times.
In the humid savannas of the Côte d'Ivoire, anecic earthworms annually incorporate
the equivalent of 180 to 510 kg of dry litter into the soil, in regularly burned savannas.
This represents
ca.
30 % of the weight of litter annually decomposed (
i.e.,
mineralised
or incorporated into the soil instead of being destroyed by fire), but less than 10 % of
