SHORT-TERM EFFECT OF CULTIVATION ON ORGAN IC STATUS AND AGGREGATION OF A CLAYEY OXISOL IN CONGO

Cultivation of oxisols of the Niari Valley (Congo) leads to considerab le 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 cultivatio n, 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 w as significant only under mechanized cultivation (13.5% on average, vs . 7% under manual cultivation). On the other hand, the influence of cu ltivation on MA, S and C was not significant in the 10- to 20-cm layer (variations < 5%, on average). MA was correlated with exchangeable al uminum content (r = 0.6), though its contribution to stability was low , but was neither With ''free'' or ''amorphous'' iron and aluminum con tents (r < 0.3), nor with C (r < 0.1); moreover, the influence of hot water-extractable compounds on MA was not significant. Therefore, macr oaggregation did not seem to be controlled by organic matter.