Agriculture Reference
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Decomposition rates subsequently increase. In the initial stages, fungi predominate,
since they are more capable than bacteria and actinomycetes of degrading the sugars
and polysaccharides of the primary resources. Fungi colonise fresh litter, dead roots
and logs. Unlike bacteria, they may tolerate relatively unfavourable environmental
conditions in order to penetrate cell walls and metabolise the condensed cytoplasm and
cell wall constituents.
Primary resources often have a mosaic pattern of distribution and communities of
a few species may develop on each part of this. Early colonisers may be 'sugar-fungi' as
defined by Garrett (1963). Although polysaccharolytic fungi soon colonise the substrate,
Swift (1976) points out that even though simple sugars decompose more rapidly than
cellulose, more cellulose than sugar disappears at the beginning of decomposition
because of its much greater abundance. Beside sugar-fungi which colonise the leaf before
its death, resource-specific polysaccharolytic fungi develop. The metabolites released by
their external digestion create a trophic attraction (' appel trophique ') and commensal
secondary sugar-fungi, also defined as 'resource non-specific', may develop (Garrett,
1963; Reisinger and Kilbertus, 1980). Under some circumstances, the presence of
secondary sugar fungi may benefit the polysaccharolytic organism by preventing the
catabolic suppression of enzyme production that can follow the accumulation of
simple carbohydrates (Figure III.12).
During the later stages of this original phase of decomposition, fungal communities
include increasing numbers of resource non-specific organisms, species diversity
decreases and unit-communities tend to be more homogenous (Hogg and Hudson, 1966;
Swift, 1976). As the substrate changes, specific resources become exhausted leading to
competition. The most persistent fungal flora includes components that have the highest
capacity to produce antibiotics ( Penicillium spp.) and the widest range of enzymatic
capacities (Basidiomycota) (Webster, 1970; Swift, 1976).
Finally, fungi produce spores and die. New colonisers spread which are better adapted
to the changed environmental and trophic conditions of the substrate.
The decomposing substrate changes and its relative nitrogen content increases as
the proportion of mycelial material increases. After three months of decomposition in
temperate forests, 50 to 80 % of biomass of decomposing hornbeam ( Carpinus betulus )
foliage is composed of fungi mycelium. Two months later, in early spring, the proportion
increases up to 90 to 95 % (Proth, 1978). Environmental conditions, change as this
material is progressively incorporated into the soil. Bacteria and actinobacteria then become
predominant. They are more capable than fungi of using substrates with relatively high
nitrogen contents and many of them possess chitinases to decompose mycelial cell-walls.
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