Evaluation of cowpea genotypes for variations in their contribution of N and P to subsequent maize crop in three agro-ecological zones of West Africa

Cowpea is an important source of food, cash, and fodder in West Africa. It is perceived to be tolerant to low available;oil phosphorus (P) conditions and nodulates promiscuously contributing to soil fertility through its high nitrogen (N)-fixing capacity. Cowpea can contribute substantial amounts of N to a subsequent cereal crop but little is known of the effect of cowpea on P nutrition of a cereal crop grown in rotation. This study, therefore evaluated eight cowpea genotypes for N-2 fixation, tolerance to low soil P (0P), and response to P fertilizer application (90 kg P ha(-1) as Rock Phosphate (RP) or 30 kg P ha(-1) as Triple Superphosphate (TSP)), and their potential contribution of N and P to a subsequent maize crop in three agroecological zones (at Shika, northern Guinea savanna; at Fashola, derived Gavanna; and Davie, coastal savanna). Grain yield and P uptake of cowpea genotypes were influenced by location. Relative response of cowpea genotypes to P application was highest at Shika where soil resin P content was lowest. The responses to RP application across locations ranged from -14 to 94%; and 194 to 358% for TSP application. Variation between genotypes was minimal within the 0P and RP treatments but more pronounced within the TSP treatment at all the locations. The results showed that N2 fixation, N exported in grain, and N balances of cowpea genotypes were different depending on P nutrition conditions. Grain yield of maize following cowpea was influenced largely by the previous cowpea genotype through its N and P dynamics, location, and VAM colonization of roots. There was, however, no consistent evidence of a significant effect of P applied to the legume, on the residual value to the subsequent maize crop.