Nitrogen Cycling in West African Ecosystems NITROGEN CYCLING IN A SOIL-TREE SYSTEM IN A SAHELIAN SAVANNA. EXAMPLE OF ACACIA SENEGAL

The cycling of N in the soil-tree system was studied in a Sahelian Savanna where Acacia senegal is large-ly represented as isolated trees or in clumps of a few trees. The herbaceous stratum is scarce and ir-regular in the open but it is dense under the trees. The return of organic matter and N from vegetation to soil by the litter was calculated and the herbaceous litter was found to be an important component under the trees, compared to the tree litter. The decay of organic matter seems to be fast, as only the first centimeters of soil show a greater accumulation of organic matter and N than in the open. The accumulation of N and its repartition was measured. The mineralization of organic N in the top soil was studied, and was found to be relatively important under the trees. Most of it occurred in the beginning of the rainy season and a short period of accumulation of mineral N took place at this time; the mineral N content of soil decreases afterwards during the growth of the herbaceous stratum. Mineral N produced was mainly in the form of NOs-N, and the possibility of N losses during periods of heavy rainfall is discussed.