Nitrogen complimentarity in timber based hedgerow intercropping systems on an acid upland soil in the Philippines

Complementarity in agroforestry systems is when trees take up water and nutrients, which would not be available to the associated annual food crops. Hedgerow systems of Acacia mangium (N2-fixing) and Gmelina arborea (non-N2-fixing) were studied to determine from which soil layers these trees take up N in relation to the associated maize crop, and to asses their N contribution (15NH4)2SO4 (40 atom percent) was injected at 20 and 100 cm depth using a ceramic porous cup in order to assess 15N uptake by trees and crop. This study was conducted at Claveria, Philippines (8 deg 30 N 124 deg 53' E) on a fine mixed isohyperthermic, Ultic Haplorthox soil with 3500 mm rainfall per year distributed throughout the year. Nitrogen recycled in tree systems was substantially higher than in the conventional maize monocrop system through pruning of lateral branches and N uptake from safety-net zone. A. mangium system provided two times more N recycling through prunings compared to the equally fast growing G. arborea. A. mangium derived 42 percent of its N from atmospheric N2 fixation and 52 percent from safety-net zone (greater than 100 cm depth). This counterbalanced the mineral N uptake by A. mangium from the upper soil depths, leading to higher maize yields and a positive system N balance. A. mangium had more fine roots even at lower depths, which provided good opportunities for safety-net and nutrient pumping functions compared to G. arborea. The maize in A. mangium system benefited with 28 percent of its N uptake through transfer of fixed N, e.g., through the application of lateral branches prunings and root turnover, as indicated by the 15N natural abundance method. A. mangium was less competitive with maize than G. arborea, and was thus more appropriate as a hedgerow species. Timber hedgerow system is a better farming option than the conventional maize system due to the N uptake by trees from lower soil depths regardless of species being used.