FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 404

2.4 Decomposer communities

2.4.1 MICRO-ORGANISMS

In the early stages of decomposition, well before leaf fall, colonies of bacteria, yeasts and
other fungi invade leaf surfaces. Amycelium then develops and, after some weeks,
penetrates the leaf tissues (Minderman and Daniels, 1967).
Fungi dominate the microflora of the litter system. As discussed earlier, these micro-
organisms have a pseudo mobility resulting from their capacity for rapid growth and
ability to translocate cytoplasm. They are also adapted to the rapid changes in temperature
and moisture conditions that may occur in the litter system. Their ability to perforate
cell-walls and feed preferentially on carbohydrates makes them efficient early litter
colonisers. Details of community structure and temporal changes are presented in
Chapter III (Section III.2.4.3) and the activities of white-rot fungi are considered in
Section IV.2.5.4 below.
After a succession of fungi belonging to different functional groups has taken place
(Table IV.4), the remaining secondary resources comprise a large proportion of dead
fungal material. By this stage, the remaining organic materials are located in the H layer
or Al horizons and further decomposition of this material will continue mainly through
bacterial activities (see Chapter III.2).

2.4.2 ROOTS

Roots are an important but often neglected component of the litter system in humid
climates. They grow principally in the lower parts of the system ( i.e., the F and H layers)
and absorb the nutrients released in these layers, either directly or through the medium
of the hyphae of their associated mycorrhizal fungi (Herrera et al ., 1978). In a Beech
( Fagus sylvatica ) forest growing on an inceptisol (acidic brown soil), fine roots actively
colonised the F2 and H layers of a moder humus system: 53.7 and 21.7 % of root-tips
were located in the F2 and H layers, respectively; 69.7 % of fine root biomass but only
34.1 % of small roots were concentrated in the same layers (Table IV.5) (Meyer and
Göttsche, 1971). In spodosols, all root tips were located in the F and H layers, whereas
in an inceptisol (eutrophic brown soil), root tips were less numerous but more evenly
distributed throughout the soil profile.
In soils with a particularly low nutrient status, roots may concentrate at the surface to
form a thick mat which may absorb virtually all the nutrients released from decomposing
leaves and dead roots (Cuevas and Medina, 1988). In an Amazonian spodosol, less than
0.1 % of the labelled calcium and phosphorus sprayed onto the surface of a litter system
with a thick root mat was leached through it (Stark and Jordan, 1978).
In most natural environments, certain zygomycote, ascomycote and basidiomycote
fungi (Section IV.3.1.2.2) enter into close relationships with plant roots to form largely-
mutualistic arbuscular mycorrhizal (AM) or ectomycorrhizal associations (ECM)
(Section III.3.2.1.2); these act as extensions of the root system and may create direct
links between decomposing leaves and roots. In a tierra firme or upland forest in
Amazonia, for example, St. John and Uhl (1983) estimated the proportion of infected