FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 469

A similar situation occurs in the Colombian llanos when natural savanna is transformed
into pasture. In that environment, local species are adapted to grassland conditions and
benefit from the substantially-increased plant production, and the return of a significant
proportion of this production as cow dung (Decaëns et al., 1994; Jimenez Jaen, unpub-
lished data). Again, not all species respond equally to the new environmental conditions.
In Peruvian Amazonia, deforestation eliminates the native species and communities
establish comprising one or two highly-adaptable, aggressive, colonist species. These
species build up large populations with biomasses greater than 1 Mg fresh weight
maxima of have been found in a rich vertisol in Martinique (West Indies)
colonised by the pantropical species Polypheretima elongata (Barois et al ., 1988).

4.1.2 EARTHWORM DIGESTIVE SYSTEMS

A large part of the drilosphere effect on soil organic matter dynamics is due to the
mutualistic digestion of organic matter by earthworms acting in association with micro-
organisms. Although detailed information is still lacking, digestion in the earthworm gut
appears to result from a combination of 'direct' processes including enzyme production
by the worm and indirect processes involving some degree of mutualism with the
microflora (see Chapter III.4.4.1.2).
Epigeic lumbricids possess a rather diverse enzymatic capacity, including cellulases
(see e.g ., van Gansen, 1962; Parle, 1963; Neuhauser and Hartenstein, 1978), and this has
led to the hypothesis of direct digestion of litter by these worms. Lumbricus rubellus is
a common representative of this group in temperate regions and appears to digest
living bacteria, even when they are protected by polysaccharides and clay particles
(Kristufek et al ., 1994). However, it is still unclear which of these enzymes are produced
by the worm, and which by the ingested microflora (Loquet and Vinceslas, 1987).
Anecic earthworms appear to combine a form of direct digestion based on the
production of a few basic enzymes, with a mutualist digestion that occurs in combina-
tion with the soil and litter microflora. There is good evidence that the 'external rumen'
type of mutualist digestion is important, and perhaps essential, for these worms. Anecic
earthworms drag dead leaves from the litter into their burrows and ingest them some
weeks later, once microbial incubation has rendered them more palatable (see e.g.,
Wright, 1972; Cooke, 1983, Cortez et al., 1989). Other species concentrate litter around
the openings of their galleries thus creating 'hot spots' of increased activity where meso-
fauna and microbial populations are more numerous and active; such accumulations
are called 'middens' ( e.g., Nielsen and Hole, 1964; Dozsa-Farkas, 1978; Hamilton and
Sillman, 1989).
Tropical endogeic earthworms have a mutualistic digestive system based on interac-
tions that occur during gut transit between the worm and the microflora ingested with
the soil. Bacteria stimulated in the gut produce some enzymes that are not secreted by
the earthworm itself. Experiments on several tropical earthworm species have shown
that the enzymes cellulase and mananase (that digest important components of root
material) found in the gut content are produced by ingested bacteria and not by cells
of the gut wall (Zhang et al., 1993; Lattaud et al., 1997a, b) (Table IV. 15).