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spherical shape. In alkaline medium with low salt concentration, they
develop network or linear structures (Chen et al. 2007).
Several agents stabilize the organic matter in soils. The major ones
are:
￿ Clay . In tropical and temperate soils, the organic matter content
is often positively correlated with clay content (Feller and Beare
1997). Both clay and the humic acids have negatively charged
surfaces but hydrogen bonds or cation bridges through divalent
or trivalent cations can form between them (Chamayou and
Legros 1989).
￿ Cations . Divalent and trivalent cations thus have a favourable
effect on preservation of humic substances in the soil. This is
particularly true of Ca ++ , the presence of which involves typical
humus forms: eumull and carbonate-rich eumull (French
terminology).
￿ Substances rich in iron and aluminium . These compounds can
block organic matter, making it harder to degrade (Chap. 10).
Aluminium is a cation and acts as one but its very marked
protective action seems to be also linked to its role as inhibitor
in biological processes (Bel Hadj Brahim 1987).
￿ Saturation with cold water . Organic substances can be degraded by
fermentation, in tropical swamps for example. But if the water is
at low temperature, microbial activity is minimal. Thus ancient
tree trunks are found intact in peats.
The order of magnitude of the time taken to degrade primary
compounds is given in Table 2.7. But the variations are large. For example,
a small proportion of lignin can persist for twenty years or more in a
soil however well aerated it might be (Bahri et al . 2006; Rumpel 2006).
The fulvic acids, humic acids and humin, as defined above, that is, rather
approximately, are degraded particularly slowly.
In short, when a dead leaf falls on the soil, some of its carbon atoms
return to the atmosphere after a few weeks or months. Others will be
retained for some time in the soil solution or will be incorporated in
microorganisms. Lastly, still others will be fixed in stable compounds
and will remain there for a thousand years! Under these conditions,
measurement of age with 14 C done in situ gives just a mean residence
time or MRT.
Stabilization and biodegradation
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