Title: Soil fertility under four tree species in the semi-arid climate of Niger

A field study was conducted to investigate the impact of four tree species (Acacia albida, Acacia senegalensis, Combretum aculeatum and Piliostigma reticulatum) on soil fertility in semi-arid Niger. The species are common trees in farmers’ fields used for restoration of soil fertility. Soil composite samples were collected along four transects at different distances from the tree trunk, at five levels of soil depth (0-10, 10-20, 20-30, 30-40 and 40-50 cm) and two age classes of trees (young and mature) to assess the effects of trees on the chemical properties of the soil (pH, organic carbon, ammonium-N, P, Na, K, Ca and Mg). The vertical distribution of all parameters, except for P, were significant, and decreased with depth. The results also showed that tree species influenced differently the distribution of the soil parameters analyzed. Soil associated with C. aculeatum was more acidic than for the other species. The organic carbon was higher beneath leguminous trees (A. albida and P. reticulatum) than non-leguminous trees (A. senegalensis and C. aculeatum). The concentration of ammonium-N, P and K were generally lower for C. aculeatum than other tree species. However, Na, Ca and Mg concentrations were generally lower with A. senegalensis than for the other tree species. Under A. albida, phosphorus concentration decreased with increasing distance from the tree trunk. And under P. reticulatum, phosphorus concentration and pH increased with tree age. Based on the present project, all the four tree species under study can be recommended for use in farmers’ fields, both for reforestation and for improving soil fertility in the semi-arid zone of Niger. Farmers can also be advised to maintain more mature trees in their fields for improving crop production. Nevertheless, further research is needed to understand better the effect of distance from tree trunk and impact of tree age on soil chemical properties and crop yields.