SOIL ORGANISMS - Soil Ecology - page 346

Micro-foodwebs mainly comprise microfauna that are predators of bacteria and fungi,
and their predators. Microfaunal species do not produce recognisable solid excrements
and the effects of these invertebrates on soil organic matter dynamics are not prolonged
within structures that remain stable for significant periods after deposition. However,
they have a significant impact on the population dynamics of micro-organisms and the
release of nutrients immobilised within the microbial biomass (Trofymow and Coleman,
1982; Clarholm, 1985). This process is particularly well developed in the rhizosphere.
Predatory Acari, Collembola, and even larger invertebrates (earthworms) may extend
this foodweb over several trophic levels.
Litter transformers mainly comprise mesofauna and large arthropods which normally
ingest purely organic material and develop an external ('exhabitational' sensu Lewis, 1985)
mutualism with microflora characterised by an 'external rumen' type of digestion, dissemi-
nation of spores or comminution (Swift et al ., 1979). Litter arthropods may digest part of
the microbial biomass or develop mutualistic interactions within their faecal pellets: in these
structures, organic resources which have been fragmented and moistened during gut
transit, are actively digested by the microflora. After some days of incubation, arthropods
often re-ingest their pellets and absorb the assimilable organic compounds that have been
released by microbial activity, and occasionally, part of the microbial biomass (see e.g.,
Hassal and Rushton, 1982). This specific type of exhabitational mutualism is known as
the 'external rumen' type of digestion (Swift et al., 1979). Within this general adaptive
strategy, a large diversity of behaviours may be identified (Vannier, 1985). Insects such as
Diptera (Sciaridae) have a comparatively efficient enzymatic machinery and produce liquid
faeces that are not re-ingested (Deleporte and Rouland, 1991). This digestion system is
unable to digest tannin-protein compounds and highly polymerised polysaccharide-aromatic
complexes (lignin) efficiently and these accumulate to hamper the progress of decomposition
(Minderman, 1968; Gourbière, 1982; Toutain, 1987a) (see also IV.2.5.3). The acid organic
compounds released in the course of decomposition are not flocculated in the presence of
mineral particles; they behave as aggressive compounds that may leach and actively
participate in mineral weathering, thereby favouring such processes as podzolisation
(Berthelin et al., 1979; Pedro, 1989).
Finally, the ecosystem engineers comprise macrofauna, mainly earthworms and
termites that are large enough to develop mutualistic relationships with microflora
within their own bodies. These interactions may involve obligate (such as the protists
contained in the posterior pouch of lower termites) or facultative symbionts; the latter
occur in the gut of higher termites and also in earthworms (Barois et al., 1987; Breznak,
1984). These organisms usually ingest a mixture of organic and mineral elements.
Organic acids produced by digestion and the subsequent incubation of organic matter
in casts, are normally flocculated in the presence of clay minerals and have a high microbial
activity. Digestion is efficient and complex organic compounds like cellulose, lignin and
tannin-protein complexes are at least partly assimilated ( e.g., Butler and Buckerfield, 1979;
Toutain, 1987b; Breznak, 1984; Rouland et al., 1990). The larger faecal pellets
(in the range of 0.1 to >2 cm) may form the component elements of macro-aggregate
structures; their properties may contribute prominently to those of the stable structures
through the regulation of porosity, aggregation, bulk density and surface features (Bal, 1982,
Blanchart et al., 1997). The large structures built by organisms, such as mounds and networks