Carbon and monosaccharide distribution in particle-size fractions from a clayey ferrallitic soil (Congo)

A study was undertaken on a clayey ferrallitic soil from Congo, exploring t echniques of particle-size fractionation and gas chromatography for charact erizing the carbon distribution and carbohydrate (sugar) composition of soi l organic matter-mineral associations. The whole soil under savannah vegeta tion contained 43g C kg(-1) soil, considerably more than under long-term cu ltivation, the clay-sized fraction accounting for most of the total soil ca rbon in both sites. Amongst the sugar monomers in the savannah site, glucos e was the most abundant, followed by mannose with surprisingly high concent rations of ribose. The same observation held for the particle-size fraction s of the soil with the exception of the coarse sand-sized fraction (> 200 m um), where xylose (a sugar essentially of plant origin) was the next most a bundant sugar after glucose; this was in accordance with the particulate pl ant character of this fraction. The ratio galactose + mannose/arabinose + x ylose, an indicator of the decomposition of plant residues and the accumula tion 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.