Alley cropping on an Ultisol in subhumid Benin .3. nutrient budget of maize, cassava and trees

An alley cropping experiment was conducted from 1986 to 1992 near Cotonou, Benin Republic on a field previously used as four-year bush-fallow. A no-tree control was compared with Leucaena leucocephala (Lam.) de Wit and Cajanus cajan (L.) Millsp. grown in 4 m distant hedgerows. In the same experiment, application of 90-39-75 kg ha(-1) NPK fertiliser vs. no fertiliser and maize-cassava intercropping vs. rotation of sole crops was also tested. Topsoil samples (0-30 cm) were taken initially and again after four and six years. Alley cropping enhanced nitrogen recycling relative to the no-alley control by 253 kg ha(-1) yr(-1) for Leucaena and 131 kg ha(-1) yr(-1) for Cajanus in the overall average but a maximum of 22 kg N ha(-1) yr(-1) was recovered in total crop exports. Apparent fertiliser-N recovery increased from a low average of 14% in the first year to 66% in the second year up to a maximum of 109% in the no-tree control in the last year. Soil nutrient depletion during the six years was significant for all nutrients except P, K falling from 0.5 to 0.1 cmol(c) kg(-1) soil and N from 0.08 to 0.05%. Hedgerow accumulation of N in the topsoil-plant system calculated as difference to a no-tree control was only 18 to 37 kg ha(-1) yr(-1) during the first four years after clearing, increasing to 208 kg in Leucaena and 111 kg ha(-1) yr(-1) in Cajanus without NPK during the last two years. During the first four years, average N, K, Ca and Mg losses from the topsoil-plant system of 223 kg, 160 kg, 176 kg and 80 kg ha(-1) yr(-1) were high but probably stored in the subsoil because corresponding leaching losses were only 10, 1, 15 and 6 kg ha(-1) yr(-1) at 150 cm depth during the second to fourth cropping year. Beyond the fourth year, markedly reduced nutrient losses from the topsoil and stable food crop yields in no-tree plots suggest that continuous maize-cassava cropping using balanced rates of mineral fertiliser is producing sustainable yields with almost closed nutrient cycles of the soil-plant system on our site.