Nitrogen budgets in crop sequences with or without phosphorus-fertilised cowpea in the maize-based cropping systems of semi-arid eastern Africa

Advantages of cowpea were tested in terms of net N-input by cowpea mono- or intercropped with maize in a crop sequence where sparingly soluble P sources were added to the first year crop. The grain yield of maize grown after cowpea monocrop was doubled and the N uptake increased by 60% compared to maize following maize. The N-value of growing cowpea monocropped prior to maize monocrop was equivalent to the application of 50 kg N ha-1 as mineral fertiliser. When maize followed a maize-cowpea intercrop, grain yield was increased by 67%. Around 34% of the N contained in cowpea residues was recovered in the following maize as estimated by 15N. There was however no benefit on the second season maize of applying sparingly soluble P sources to the first season legume.   Key words: Cowpea, crop sequences, maize, nitrogen budgets, sparingly soluble phosphorus.