Nutrient cycling in agroforestry systems of the semi-arid tropics of Africa

Trees integrated with crops in time and/or space mediate nutrient cycling o n farms by increasing the supply and availability of nutrients in the crop root zone and reducing nutrient losses. However, quantitative information i s lacking on biological nitrogen fixation and nutrients recycled in semi-ar id agroforestry due to the control of losses by trees, and most information available is on nutrients recycled through their above- and below-ground b iomass. The potential for exploiting nutrient cycling in simultaneous agrof orestry systems is constrained by a low and erratic water supply in the sem i-arid areas of Africa. In parkland systems, there is a limited scope for i ncreasing nutrient cycling because of the small quantity of biomass added i nto soil from low tree density in the system. However, trees in these tradi tional systems are more important for economic products in terms of poles; timber, fruits, etc., and have ecological benefits such as the reduction of nutrient and water losses. Although hedgerow intercropping uses high tree density and increases the nitrogen (N) supply in the soil, crops neverthele ss fail to benefit due to competition between trees and crops for water. Co ntour hedgerows can conserve nutrients by arresting soil erosion on moderat ely sloping lands and are worthy of consideration for both soil conservatio n and fodder production. Mixed intercropping of Gliricidia septum and maize with efficient tree management appears to be promising in some areas and n eeds further testing before widespread dissemination. Sequential systems wi th short-duration leguminous trees and shrubs, exemplified by 2-year Sesban ia sesban rotated with crops, and biomass transfer systems are both efficie nt in nutrient cycling and increasing crop yields. Therefore, these systems can be considered for dissemination depending on the socio-econornic condi tions. The potential for using biomass transfer systems is constrained by t he availability of biomass outside the farms, land for its production on fa rms and labor for its application. Although these agroforestry systems prov ide an adequate quantity of N for moderate cereal yields, they cannot meet the phosphorus (P) requirements of crops in P-deficient soils, which should be supplemented through inorganic P sources. Nutrient cycling using livest ock is less efficient due to N losses by volatilization and leaching than d irect use of biomass as green manure. The use of palatable and high quality biomass as fodder is three to five times more economical than green manure . While agroforestry clearly has a definite role to increase nutrient cycli ng in low input systems, no single technology can guarantee controlling nut rient depletion in the long term; The choice of different technologies to s uit different niches of the farm and their integration with inorganic nutri ents is important for sustainable production even on smallholders' farms in the semi-arid Africa.