Short-term effect of cultivation on organic status and aggregation of a clayey oxisol in Congo.

Cultivation of oxisols of the Niari Valley (Congo) leads to considerable decline in the soil organic carbon content and structural stability in a few years. The objective of this study was to estimate the very short-term effects of cultivation on organic status and macroaggregate (> 0.2 mm) stability of a soil which had been under savanna for 20 yr. Another objective was to identify the determinants of macroaggregate stability. On one hand, after 5 mo of manual or mechanized cultivation, stable macroaggregate content MA decreased significantly in the 0- to 10-cm layer (7% on average); soil organic carbon stock S did not (6% on average), whereas the decrease in soil organic carbon content C was significant only under mechanized cultivation (13.5% on average, vs. 7% under manual cultivation). On the other hand, the influence of cultivation on MA, S and C was not significant in the 10- to 20-cm layer (variations < 5%, on average). MA was correlated with exchangeable aluminum content (r = 0.6), though its contribution to stability was low, but was neither With ''free'' or ''amorphous'' iron and aluminum contents (r < 0.3), nor with C (r < 0.1); moreover, the influence of hot water-extractable compounds on MA was not significant. Therefore, macroaggregation did not seem to be controlled by organic matter.