SOIL ORGANISMS - Soil Ecology - page 214

dependent on water than other micro-organisms and while there is a considerable range
of tolerances to low moisture potentials, some xerophytic fungi such as Aspergillus spp.
survive in pure culture at potentials as low as -40 MPa (Parr et al ., 1981). Facultative
anaerobiosis is also widespread in fungal species. However, in the field, tolerance is
effectively reduced due to interactions with other micro-organisms. Temperature, solute
concentrations in the soil solution and the nutritive resources available will also influence
the relationships between fungal activity and soil moisture levels (Griffin, 1972).
Their oxidative capacity, which is in the range of 10 to 12 dry cells is

substantially inferior to that of the bacteria.
Digestion is external. In contrast to bacteria which attack organic substrates from the
outside by direct contact, some fungi produce mycelial extensions which breach the cell-
walls (Figure III.2a). These then penetrate the living or dead tissues allowing a rapid
digestion of cell contents and a further breakdown of cell walls. Digestive capacities and
nutritional requirements are diverse and Garrett (1951) has classified the fungi into five
trophic groups: saprotrophic sugar fungi which live on simple sugars and the products of
cellulolysis, lignin-decomposing, coprophilous, root associated (mutualistic or antagonistic)
and predaceous fungi that feed on nematodes. The activity of fungi starts in the living
leaves and roots, is at a maximum in decomposing leaves and wood and tends to diminish
in the later stages of decomposition.
It is known that the fungi active in the soil environment may simultaneously use a
wide range of carbon sources including gases and volatiles. Attempts to classify fungi
into exclusive ecological categories, have proved difficult due to the wide versatility of
their metabolic capacities and the diverse patterns of growth, reproduction and dispersal
(Swift 1976; Pugh, 1980; Wainwright, 1988).
Fungi have the ability to move by growing new mycelium and may translocate living
protoplasm towards the growing ends (Dowding, 1976; Schnürer, 1985). This leads to
an accumulation of large masses of dead mycelium which may exceed the biomass of
living mycelium by several times. In six European temperate forest soils, live mycelium
comprised from 2.4 to 82 % of the total. Average maximum values ranged from 50 to
70 % in the L and F layers and minimum values of approximately 17 % were found in
the H layer and A horizon (Kjöller and Struwe, 1982).
Five main growth patterns have been distinguished (Burges, 1960):
(i) In the Penicillium type, the fungi densely colonise organic substrates but mycelia
do not extend into the surrounding soil;
(ii) In contrast, in the Mucor ramannianus type, the fungal mycelia grow into the
surrounding soil and produce numerous spores following colonisation and exploitation
of an organic substrate;
(iii) Fungi of the Zygorrynchus type produce isolated hyphae in the soil although these
have no obvious relationship with organic substrates;
(iv) In the Basidiomycota, growth from a colonised substrate occurs by the production
of mycelial strands and rhizomorphs;
(v) In the “fairy ring” type, fungi form an expanding annulus with a dense network
of mycelium unrelated to substrate accumulation. Associated changes in soil chemical
properties make such zones conspicuous, especially in grasslands.