Propriétés chimiques des sols sableux sahéliens sous parcs à "Faidherbia albida" (Del.) A. Chev.

Résumé

Faidherbia albida, a multipurpose tree in semiarid zones of West Africa, is fully integrated in traditional Sahelian farming systems. F. albida is known as a "reverse phenology" tree, i.e. avoiding competition for soil nutrients, water and solar radiation by shedding its leaves at the onset of the cropping season, with positive effects on the soil and physical factors. In Niger, we monitored changes in soil chemical properties at high or sparse F. albida tree densities, at the beginning and end of the rainy season. Two sites with different F. albida tree density (Guilleny, 32 trees.ha-1; and Tilly, 4 trees.ha-1) were assessed and 10 trees with a mean trunk diameter of 95 cm were selected on each site. Soil was sampled in the four transects around trees at three distances from the trunk (1, 5 and 11 m), at 0-25 cm depth, in July and November 1992, 1993 and 1994. Increases in pH, total N, K and Bray PI were noted at the beginning of the rainy season. Bray P1 content decreased with the distance from the trunk, contrary to pH which was higher 11 m from the trunk. There were no effects of distance on total N and K contents. Soil fertility improvement was more significant for Bray P1 at Tilly and for total N and K at Guilleny. Soil nutrient gradients around F. albida frees were similar at the beginning and end of the rainy season. The high soil fertility under the tree canopy could be explained by several factors, e.g. F. albida litterfall, animals, dust, etc. Dissociating the contribution of these inputs would provide a better understanding of tree/crop/livestock interactions, and improve F. albida tree management.