Lowlands in West Africa are increasingly absorbing the growing pressure on land for food crop production. However, the current intensification of lowland cropping systems result in the mining of soil nutrient reserves, particularly of nitrogen (N) and phosphorus (P). While N can be supplied via biological N fixation (BNF) by legumes, P deficiency frequently limits both food crop growth as well as legume BNF, especially on acidic soils in the humid forest and savanna regions. Fortunately, large deposits of phosphate rocks (PR) exist in many African countries; and moreover, the PR efficacy can be enhanced when applied to submerged acidic soils. Thus the use of PR may not only supply the limiting P to soil and food crops, but also stimulate BNF-N by legumes including legumes used for green manuring in production systems. A screen house experiment was conducted to evaluate the effect of PR-P on the performance of N-fixing legume Aeschynomene afraspera L. during three successive cropping cycles under submerged conditions. BNF was estimated by the 15N-isotope dilution technique, using rice (Oryza sativa L.) as a non-fixing plant. The experimental soil was Ultisol low in available P. Unprocessed PR from Mali was applied at 60 kg P ha -1. Initial application of PR significantly increased legume above ground biomass and BNF over the unfertilized control by 1.4- and 3-fold respectively. This PR-induced growth stimulation further increased during the subsequent cropping period with a maximum biomass of 6.4 tons ha-1 and 80 kg of BNF-derived N ha-1 above the unamended control. The application of P significantly increased the percent N derived from the atmosphere (% Ndfa). However, compared to the first two cropping cycles, the efficacy of applied PR-P declined during the third cropping cycle, probably as a result of rise in soil pH following soil reduction. The results suggest that PR can be a useful P source for food crops in rotation with legumes.
Key words: Aeschynomene afraspera, phosphate rock, biological nitrogen fixation and flooded soils.
Abbreviations: BNF, Biological N fixation (BNF); %Ndfa,
Percent of N derived from the atmosphere; GM, Green
manure legume; PR, Phosphate rock; TSP, Triple super