Abstract
A study was undertaken on a clayey ferrallitic soil from Congo, exploring techniques of particle-size fractionation and gas chromatography for characterizing the carbon distribution and carbohydrate (sugar) composition of soil organic matter-mineral associations. The whole soil under savannah vegetation contained 43 g C kg−1 soil, considerably more than under long-term cultivation, the clay-sized fraction accounting for most of the total soil carbon in both sites. Amongst the sugar monomers in the savannah site, glucose was the most abundant, followed by mannose with surprisingly high concentrations of ribose. The same observation held for the particle-size fractions of the soil with the exception of the coarse sand-sized fraction (>200 μm), where xylose (a sugar essentially of plant origin) was the next most abundant sugar after glucose; this was in accordance with the particulate plant character of this fraction. The ratio galactose + mannose/arabinose + xylose, an indicator of the decomposition of plant residues and the accumulation of microbial metabolites, increased 4- to 6-fold from coarse to finer fractions. The same trends persisted after a 17-year cultivation of cassava. Results were evaluated in comparison with data provided in the literature for temperate and tropical soils.
Published Version
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