Relationship between aggregate stability and total and hot-water-extractable organic matter in clayey ferrallitic soils (Congo, Brazil).

Organic matter (OM) generally plays an important role in soil aggregate sta bility. The objective of this work was to characterize the hot water-extrac table OM and its role in the aggregate stability of clayey ferrallitic soil s under different land use management. The macroaggregate (>200 mu m) stabi lity of these soils was determined before (AS) and after hot-water extracti on (ASe). The contents in total organic carbon and in carbon present as car bohydrates, as well as the sugar composition, were determined on the bulk s oils and their hot-water extracts. The carbon and macroaggregate contents decreased upon land-clearing and cul tivation, but to a lesser extent when some cultural practices were used. Wh atever the situation considered, the-hot-water extract always presented a h igher carbohydrate content than the bulk soil. The high values of ratios r = (galactose + mannose)/(arabinose + xylose) suggested that a large proport ion of carbohydrates was of microbial origin. There were significant correl ations between AS and the carbon content of bulk soils or hot-water extract s, and between AS and the soil carbohydrate content. On the opposite, whate ver the soil sample studied, no significant difference could be found betwe en AS and ASe, showing that hot-water extractable OM had thus no clear aggr egating role in these soils.