Agriculture Reference
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and at sites where compaction is occurring. Seasonal variation in soil properties has
sometimes been related to changes in the abundance of earthworm populations (Hopp,
1973; Marinissen, 1995). Soil porosity increases of the order of 75 to 100 % may follow the
colonisation of new soils by earthworms (Hoeksema and Jongerius, 1959) with a predictable
increase in the proportion of large pores (Edwards and Lofty, 1982; Knight
et al.,
1992).
Infiltration of water and gas exchange
Burrows have a significant effect on water infiltration despite their small contribution
(generally less than 5 %) to the total soil volume (Edwards and Bohlen, 1996). Experiments
in arable land have demonstrated that when direct-drilling is substituted for ploughing,
anecic earthworms are favoured. Hydraulic conductivities at the interfaces of the A
and B horizons may be doubled through their enhanced activities (Douglas
et al.,
1980;
Urbanek and Dolezal, 1992; Joschko
et al.,
1992; Ligthart 1996) although many other
authors have emphasised the fact that burrow systems may only affect the flux of water
through the soil if ponding occurs at the soil surface. When this occurs, infiltration rates
will depend on the connectivity of the pore space and the properties of the burrow walls.
These may have linings of fine panicles, a compressed zone around the periphery, or no
modifications compared with the surrounding soils. Gas diffusion is affected in the same
way although diffusion into the soil surrounding the galleries is greater than direct
diffusion along the burrows (Kretzschmar, 1989).
In microcosms inoculated with the epi-anecic species
Lumbricus rubellus,
Kladivko
et al.
(1986) concluded that these earthworms could diminish surface crusting through
the formation of stable macro-aggregates.
4.3
The endogeic drilosphere
4.3.1
DISTRIBUTION OF ENDOGEIC EARTHWORMS
Endogeic earthworms form part of most earthworm communities in environments
distributed from the cold temperate areas to the humid tropics (Figure III.45). Polyhumic
endogeic species are the most populous worms in temperate and semi-dry tropical areas.
They represent a functional group intermediate between true endogeics and anecics
since some species actually feed on surface-litter that they drag from underneath and mix
with the soil. The large proportion of plant remains sometimes found in their guts (up to
50-70 %) attests this behaviour. These earthworms also create galleries which are
mainly subhorizontal (Kretzschmar, 1982).
Mesohumic and oligohumic endogeics are most typical as they feed solely on soil
organic matter and do not build burrow systems. Gut content analysis has shown no
selective ingestion of dead plant tissues.
At a broad geographical scale, the distribution of true endogeics is mainly determined
by temperature. Since they feed on low quality soil organic matter, they can only thrive
where high temperatures allow an efficient digestion through a mutualistic association
with the soil microflora (see Section IV.4.1.2).
At a regional scale, for example in tropical rainforests, endogeics mostly occur
