AGUSTIN MERCADO1, MEINE VAN NOORDWIJK2, THOMAS HILGER1, GEORG CADISCH1
1University of Hohenheim, Plant Production and Agroecology in the Tropics and Subtropics, Germany
2World Agroforestry Centre (ICRAF), Southeast Asian Research Program, Indonesia
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 profiles these trees take up N in relation to the maize crop, and to assess their N contribution. (15NH4)2SO4 (40 atom %) was injected at 20 and 100cm depth using a ceramic porous cup in order to assess 15N uptake by trees and crop. This study was conducted at Claveria, Philippines (8°30 N 124°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 wase 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% of its N from atmospheric N2 fixation and 52% from safety-net zone (>100cm 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% of its N uptake through transfer of fixed N2, 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.
Keywords: Hedgerow intercropping, N complementarity, timber trees