This study set out to (i) evaluate the effect on the nutrient and water status and growth of trees of growing P. phaseoloides in the interrrows of a young rubber tree plantation in northeast Thailand, (ii) evaluate the N2 fixation by the legume and transfer of the fixed N to the trees using the 15N natural abundance method, and (iii) evaluate the variability of these results along a gradient of soil depth which induced a soil water storage gradient