Agriculture Reference
In-Depth Information
Dead organisms quickly come in contact with the soil surface, or die within it, and
immediately start to decompose. The biomass of these organisms is arbitrarily divided
into decomposable and resistant materials (Jenkinson and Rayner, 1977). Metabolites and
decomposable organic matter are considered to degrade rapidly and are incorporated in
the 'soil biomass' compartment, which is the sum of the biomasses of all living organisms,
including the micro-organisms and soil fauna. Because of physical or chemical protec-
tion from decomposer attack, a considerable proportion of the original organic matter
accumulates in forms resistant to further decomposition and undergoes protracted
decomposition.
Thus, as a result of this extended decomposition, organic matter gradually accumulates
in pools with increasing levels of resistance to decomposition (Jenkinson and Rayner,
1977) (Table II.5). Balesdent et al. (1987) estimated the origin of soil carbon by using
natural isotopic labelling in temperate soils that had been cropped first to C3 forages
and then to com ( Zea mays ) (a C4 plant) for more than 30 years. They showed the
remarkable stability of the older organic matter from the original grassland (57 to 75 %
of total C) and a gradual replacement of C3 carbon by C4 organic matter produced by
the corn crop. After 35 years, less than 20 % of C3 carbon remained and an equivalent
amount of C4 carbon from the maize had accumulated (Figure II.5).
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