Maintaining Long-Term Sustainability of Runoff Agroforestry System Through the Application of Composted Tree Trimmings.

Growing pressure on native vegetation by impoverished communities in developing countries to provide themselves with firewood, fodder and food can be partially met by planting trees and intercropping using runoff as source of water for irrigation. The long-term sustainability of agroforestry systems irrigated with runoff generated during episodic rainfall events can be ensured by pruning symbiotic nitrogen (N) fixing trees and applying the composted leaves to the soil, thus providing the necessary N to the annual crop grown in between the rows of the trees. In arid regions reasonable yield cannot be attained without additional N. Preliminary results indicated higher annual crop production if composted leaves were added rather than fresh leaves. We therefore set up a field trial in which runoff events were simulated and fast-growing acacia (Acacia saligna) shrubs were the woody component and maize (Zea mays L.) the intercrop. Ten treatments were applied to examine the effects of tree pruning, presence of an intercrop and application of acacia leaf compost on N and water balance, biomass production and the interactions between them.

Addition of compost in the first growth season mainly contributed to the organic matter reservoir in the soil. Flooding at the beginning of the second season together with the superficial soil tilling created favorable conditions for denitrification for 10 days after compost application. The source of N for the ammonification and nitrification that occurred thereafter was the soil organic matter that was built-up from the compost applied on the previous year. Maize yield on the second season was significantly higher in the plots to which compost was applied. Intercropping of maize between pruned trees, with the addition of composted leaves resulted in the highest water use efficiency. Moreover, this system resulted in a double complementary relation: the total above- and belowground production of the system increased due to an increase in maize production without affecting tree growth.