Agriculture Reference
In-Depth Information
1.4
Biological systems of regulation
1.4.1
DEFINITION AND GENERAL PROPERTIES
Soil organisms directly control decomposition processes and rates (Lavelle‚ 1987).
Microbial respiration makes by far the most important contribution to overall respiration
contributing between 80 and 95 % of the total (see‚
e.g.‚
Satchell‚ 1971; Ryszkowski‚
1975; Reichle
et al.‚
1975; Persson and Lohm‚ 1977; Lamotte‚ 1975 (Table IV.2)).
However‚ microbial activity is limited by their relative immobilities and their high
sensitivities to environment constraints (see the 'Sleeping Beauty paradox' in Chapter
III.2.4.3). Their activities are therefore largely determined by processes operating at
larger spatial scales‚ particularly those associated with the larger soil organisms‚ or macro-
organisms. Biological systems of regulation include three components: (i) a decomposing
resource‚
e.g.‚
leaf or root litter‚ or soil organic matter; (ii) micro-organisms‚ which mediate
most chemical transformations; and (iii) macro-organisms‚ which create suitable conditions
for microbial activity‚ at defined scales of space and time (Figure IV. 11).
With the exception of termites and leaf-cutting ants‚ the direct effects of macro-organ-
isms on decomposing resources are generally limited (see Chapter IV.5). The transport
of micro-organisms to new substrates and the priming of their activities by the produc-
tion of limited amounts of readily-assimilable organic matter (
e.g.‚
root exudates and
earthworm intestinal mucus) are key processes in the functioning of these systems.
They generally enhance microbial activity and hence accelerate decomposition through
a priming effect (see details in Section IV. 1.5.2) although inhibitory feed-back mecha-
nisms may sometimes develop‚ either in parallel or at different scales of time. For example‚
increased humification may chemically limit mineralisation rates or the compact structure
of earthworm casts and the walls of termite nests may physically protect organic matter
from digestion (see‚ for example‚ Okello-Oloya
et al.‚
1985; Garnier-Sillam
et al.‚
1987;
Martin‚ 1989 and Sections IV.4 and 5).
The functioning of biological systems of regulation depends directly on the physical‚
chemical and biological characteristics of all three major components. It is therefore
essential to separately consider (Figure IV.12):
