Effet à court terme de la mise en culture sur le statut organique et l’agrégation d’un sol ferrallitique argileux du Congo

Abstract

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. Key words: Macroaggregate stability, soil organic carbon, oxisol, setting into cultivation, exchangeable aluminum, hot water pretreatment