Estimating N₂-fixation by Pueraria phaseoloides in rubber interrows using the ¹⁵N isotope dilution technique

Symbiotic N2-fixation by field-grown Pueraria phaseoloides was assessed using the 15N-dilution method. The legume was established on an Oxisol between interrows of rubber which had been growing for three years. The effect of + shading by the expanding canopies of rubber trees was also evaluated. The test legume and the non-fixing reference crop (Ipomoea batatas) were grown in separate plots, and labelled N applied in equal amounts to both crops. At periodic harvests of30, 63 + and 97 days following I5N application, the isotopic composition of fresh shoot and decomposing litter samples of both crops were determined to evaluate the contribution of N2 fixed to total plant N. The effect of time of harvest or light regime on shoot-+ % Ndfa was not significant except during the third harvest where a significant reduction of % Ndfa occurred in plants grown under pruned (light) canopies. The proportion of P. phaseoloides N derived from fixation for shoot averaged 73.7% (range, + 59.9-83.8) and this estimate corresponded to 49 kg N/ha fixed after 97 days. Estimates of N2-fixation using the classical N-difference method gave values averaging 53 kg N/ha. Estimated N2 -fixation declined from the first to the third harvest that could+ be due to litter production being higher than the assimilation of N. The effect of light regime on legume shoot dry matter yields or on N concentrations and contents were also not significant at all harvests, although shoot yields were significantly + lower for legumes grown under shade at the second harvest. However, litter production was much higher for legumes grown in light.