FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 369

is the eradication of the original soil invertebrate communities and perennial roots
through the clear felling of forests and the cultivation of annual crops and grasses
(Coleman et al.‚ 1990).

1.3 Processes involved at each level of the hierarchy

Despite their dependence on other abiotic and biotic factors‚ both the natural histories
and life strategies of micro-organisms impact on the nature and dynamics of decompo-
sition processes. Such traits directly determine decomposition rates and the release of
soluble metabolites; they also control the succession of active and inactive phases and
affect their interactions with macro-organisms.

1.3.1 MICROBIAL PROCESSES (SEE ALSO CHAPTER III.2)

Micro-organisms may have huge population densities and biomasses‚ and wide
functional diversities. Densities of up to bacteria and several kilometres of fungal
hyphae per gram of dry soil have been measured over a wide range of tropical and
temperate climate soils. This represents a microbial biomass of several hundred kg
e.g.‚ 370 to 760 kg dry weight in early- and late-stage secondary Nigerian forests
(Ayanaba et al.‚ 1976) and 150-400 kg dryweight in pasture soils from tropical India
(Dash et al.‚ 1985). Despite this‚ microbial biomass comprises only a small percentage
of total soil carbon (1.7 to 4 % in the examples cited) with a considerable turnover time‚
of the order of a year (see for example‚ Jenkinson and Ladd‚ 1981; Voroney‚ 1983;
Chaussod et al.‚ 1988). Microbial communities are thus both numerous and diverse‚ but
largely dormant. The slight ability of most non-filamentous prokaryotes and protoctists
to move within the compact soil environment largely limits their individual spheres of
activity to their immediate microsites.
The huge discrepancy between high potential metabolic rates and the slow turnover
of biomass has been defined in Chapter III.2.4.3 as the 'Sleeping Beauty' paradox. Soil
macro-organisms act as 'Prince Charming' in that they provide assimilable substrates
(root exudates‚ earthworm mucus and other materials) which initiate their metabolic capa-
bilities. They may also transport propagules to locations where organic substrates are
favourable for colonisation and development.
Finally‚ the microbial biomass has a C:N ratio in the range of 5:1-7:1 (bacteria) to
7:1-25:1 (fungi) (Chapter I.3.1.4.3). Depending on the C:N ratio of the resource that they
exploit‚ mineral-N may be released during decomposition (net mineralisation) - and
possibly supplied to plants - or retained within microbial tissues (immobilisation or
reorganisation) and thereby made unavailable to plants (see Section IV.1.3.3).

1.3.2 ABIOTIC LIMITATIONS TO DECOMPOSITION

Climatic factors
In many parts of the world‚ some combination of low temperature and drought severely
limits the seasonal extent and rates of decomposition processes (see‚ for example‚