Agriculture Reference
In-Depth Information
1.1.3.2
Physical and chemical characterisation
The chemical and other properties of soil organic matter differ considerably, depending
on the relative proportions of the five major components defined above. It may be
usefully fractionated into several components, defined by the methods used to
separate them.
One fraction is defined as the
labile organic matter
and is composed of relatively
simple organic compounds that may be rapidly assimilated by soil organisms; water-
soluble polysaccharides leached from the litter layers, root exudates and earthworm
mucus are some common examples. Such materials normally occur at low concentrations
due to their rapid decomposition; in temperate soils, water-soluble carbon seldom
exceeds 200 mg (Davidson
et al
., 1987). Similarly, fluxes of root exudates or earth-
worm mucus may be large although concentrations at any given moment are always low.
The microbial biomass is considered to be part of the labile pool of organic matter
although it is less labile than the above-mentioned materials. As discussed in Section
I.3.1.2.1, microbial biomass carbon concentrations in most soils average between 1 and 5
% of soil organic carbon.
Decomposing root and leaf litters comprise the
light fraction
of the soil organic matter
which may be separated by flotation on liquids in the density range 1.6 to 2.0 Mg
This fraction has a longer residence time in soils than the labile compounds and may
accumulate over time, depending on soil climatic conditions, clay mineralogy and
the presence of active decomposer invertebrates. It may comprise from 5 to 73 % of total
soil carbon (Figure I.12). The chemical composition of this fraction is dominated by
highly-condensed polysaccharides, together with high but variable concentrations of
cellulose and lignin.
