Nitrogen Cycling and Root Dynamics in an Agroforestry System

Primary productivity in arid zones is limited by the lack of water and soil nutrients. Conveying and storing flood water in plots surrounded by embankments allows agricultural activity in areas where there is generally insufficient rainfall to sustain agricultural production. The efficient exploitation of the stored water was achieved by intercropping trees with an annual crop and pruning the former before planting the intercrop. This approach minimized competition for water and solar radiation. However, in order to ensure the long-term viability of such a system nutrients have to be added to the soil in order to compensate for the uptake of the intercrop, Nitrogen being the main element. The composted leaves of a leguminous shrub-like tree incorporated into the soil could satisfy the nitrogen demand of the intercrop.We tested this approach in a simulated runoff agroforestry system with fast-growing acacia (A. saligna) trees as the woody component and maize (Zea mays L.) as intercrop for two consecutive seasons. Ten treatments were applied (radical pruning before intercrop planting, compost application and planting of the intercrop as factors) and the below- and above-ground effects and interactions examined. Pruning the trees canopies changed the trees root spatial and temporal distribution, allowing the annual crop to develop between the trees. Addition of compost significantly increased intercrop yield irrespective of the presence of the woody component while the presence of the intercrop did not affect the productivity of the trees. The highest productivity was obtained for the pruned trees, intercrop and added compost treatment. A significant increase in the presence of tree roots was observed for the deeper parts of the soil profile for the pruned trees, intercrop and added compost treatment. The addition of composted leaves from the leguminous woody component to the intercrop resulted in a very high water use efficiency of the water stored in the soil.