Agriculture Reference
In-Depth Information
Activation mainly results from fractionation and comminution of litter during transit
through the invertebrate gut, and by the activation and dissemination of micro-organisms.
Regulation of microbial activity
Two further basic processes, both mediated by invertebrates and leading to the enhancement
of decomposition rates are the grazing of microbial populations and dissemination
of their propagules. In litter-systems, most invertebrates appear to feed at least partly
on fungal material. Some Diptera and most Collembola are almost entirely fungivorous
(see
e.g.,
Anderson and Healey, 1972; Kilbertus and Vannier, 1979; Saur and Ponge,
1988; Bardgett
et al.,
1993). Furthermore, some specificity in feeding has been noted:
Collembola, for example, have demonstrated preferences for mycorrhizal fungi (Schultz;
1991; Thimm and Larink, 1995). Grazing has two complementary effects:
(i) The release of nutrients immobilised in the microbial biomass, through excretion
by the grazing organisms (see
e.g.,
Fenchel and Harrison, 1976; Ingham
et al
., 1986b).
As an example, Persson
et al.
(1980) calculated that invertebrates consumed 30 to 60 %
of the annual production of micro-organisms in a coniferous forest. This resulted in
the release of 10 to 49 % of total mineral nitrogen, 70 % of which was directly excreted
by micro-predators;
(ii) The maintenance of micro-organismal diversity by keeping population densities at
sufficiently low levels that competition is avoided (Paine, 1966; Wauthy, 1982). Grazing
is known to be selective (Proth, 1978) and may partially explain the high diversity of
fungal species observed in moder in comparison with mull litter systems (see
e.g.,
Martinez
et al.,
1980).
Propagule dissemination may be:
(i) Passive, when spores and other propagules are temporarily attached to the body
surfaces of soil animals and are later released (Touchot
et al.,
1983);
(ii) Active, when spores which have survived intestinal transit are released in faecal
pellets and subsequently germinate. This may involve a range of micro-organisms
including heterotrophs, autotrophs and organisms that form deleterious and beneficial
associations with plants and animals. For example, Reddell and Spain (1991a, b) reported
the successful transmission of viable spores of ecto and endomycorrhizal fungi and of
the actinobacterium
Frankia
sp. through the intestinal tracts of a number of earthworm
species. However, this process is selective with differential survival between the spores
of different species and many of them may loose their ability to germinate (see,
e.g.,
Cervek, 1971; Kozloskaja, 1971; Wynn-Williams, 1983). This selectivity has been used
in the control of some fungal plant pathogens (Ulber, 1983).
The micro-environments of the faecal pellets of epigeic species are generally con-
ducive to microbial digestion of their components (although some exceptions to this rule
exist,
e.g.,
in Enchytraeidae). Faeces therefore represent privileged incubation sites and
act in part as external rumens in that decomposition proceeds progressively as they
undergo ingestion and egestion by a succession of invertebrates. The invertebrate gut
may itself be a favourable, though temporary micro-environment for micro-organisms.
In one example, Hanlon (1981) observed a nine-fold increase of microbial biomass
during the passage of food through the gut of isopods and glomerid millipedes.
Spores are not the only propagules to retain viability after passage through the earth-
