Nutrient cycling in an agroforestry system with runoff irrigation in Northern Kenya

A nutrient balance was determined for sole and alley cropped Sorghum bicolo r and Acacia saligna in a runoff irrigation system in Northern Kenya. Nutri ent input including precipitation and runoff, and output through harvest an d leaching were measured for N, P, K, Ca and Mg using adsorption resins, te nsiometry and suction cups. Various management scenarios are discussed with respect to nutrient return. Nutrient input with rainfall was generally low in comparison to nutrient uptake or leaching losses. The irrigation water, however, constituted an important nutrient input, especially for Ca and Mg . Nutrient export with the harvest was large for N and K, but can effective ly be reduced by a nutrient return with mulch. Nutrient leaching losses fro m the topsoil (0-30 cm) were lower in the sorghum monoculture than in the t ree-based systems. In the subsoil (120 cm), however, leaching was effective ly reduced by the trees. In the agroforestry system, leaching losses of N u nder the sorghum were 53% lower than in the sorghum monoculture. This could be attributed to a higher root abundance and a higher ratio of nutrient up take-to-leaching in the agroforestry system than in the monocultures indica ting a higher nutrient efficiency. The lower leaching losses in the agrofor estry system compared to the crop monoculture could not compensate for the additional nutrient export in tree biomass. A nutrient return by mulching c rop residues and acacia leaves was essential for a positive nutrient balanc e in the agroforestry system. Combining annual and perennial crops provided a higher internal nutrient cycling than the monocultures.