Effects of Rock Phosphate on Indigenous Rhizobia Associated with Sesbania sesban
Low soil fertility, mainly low levels of P and N, seems to be a major constraint to the establishment and growth of leguminous trees used in agroforestry systems (Sanginga, 1992; Bâ et al., 1996). Sesbania sesban is an important multipurpose legume, which is often used as cover tree (Desaeger and Rao, 2001) or green manure (Carsky et al., 2001), but its effectiveness may be hindered by a P deficiency (Carsky et al., 2001). P may be in soluble or insoluble forms. Although the former can be immediately taken up by plants, the latter is used mostly because of lower cost. In Africa, the latter form exists in great quantity with Tilemsi rock phosphate (PNT) from Mali as one of the best in West Africa (Truong et al., 1978). In this work at LCM, Dakar, we studied over 105 days, the effects of 150 kg P2O5.ha−1 of PNT on the diversity of indigenous rhizobia associated with Sesbania sesban on soil from Nioro (Senegal), which contains 23.4 ppm available P. Nodules were harvested every 21 days, crushed, and PCR-RFLP used to extract DNA from the 16S-23S IGS region. The PCR products were digested and analysed. Sixteen IGS types were detected, but they varied depending on whether insoluble-P was present or not. It seems that PNT has an effect on rhizobia diversity in contrast to soluble phosphate (TSP) (O. Sacko et al., unpublished data, 2005). Insoluble-P, despite its low cost and its usefulness in agroforestry (Bâ et al., 2001), disturbed the indigenous rhizobia associated with S. sesban. This result may be due to competition for soluble-P, which is insufficiently in most soils of West Africa. However, this idea must be confirmed.