Testing the safety-net role of hedgerow tree roots by N-15 placement at different soil depths

Trees which root below crops may have a beneficial role in simultaneous agr oforestry systems by intercepting and recycling nutrients which leach below the crop rooting zone. They may also compete less strongly for nutrients t han trees which root mainly within the same zone as crops. To test these hy potheses we placed highly enriched N-15-labelled ammonium sulphate at three depths in the soil between mixed hedgerows of the shallow-rooting Gliricid ia sepium and the deep rooting Peltophorum dasyrrhachis. A year after the i sotope application most of the residual N-15 in the soil remained close to the injection points due to the joint application with a carbon source whic h promoted N-15 immobilization. Temporal N-15 uptake patterns (two-weekly l eaf sub-sampling) as well as total N-15 recovery measurements suggested tha t Peltophorum obtained more N from the subsoil than Gliricidia. Despite thi s Gliricidia appeared to compete weakly with the crop for N as it recovered little N-15 from any depth but obtained an estimated 44-58% of its N from atmospheric N-2-fixation. Gliricidia took up an estimated 21 kg N ha(-1) an d Peltophorum an estimated 42 kg N ha(-1) from beneath the main crop rootin g zone. The results demonstrate that direct placement of N-15 can be used t o identify N sourcing by trees and crops in simultaneous agroforestry syste ms, although the heterogeneity of tree root distributions needs to be taken into account when designing experiments.