Influence of growth stage and seed nitrogen on B values and potential contributions to error in estimating biological N2 fixation using the 15N natural abundance method

Aims The 15N natural abundance technique is increasingly being used to biological nitrogen (N2) fixation by legume crops, but requires the determination of a ‘B value’ which describes the isotopic fractionation for a given plant species and strain of rhizobium. Most B values in the literature are derived for shoot tissue harvested at mid pod filling, when plant N content is generally at its maximum. However, if legumes are grown as green manure inter-crops and growth is terminated at earlier stages, to obtain accurate values for total N2 fixed growth-stage specific B values and correction for seed N contributions may be necessary. This study aimed to evaluate the magnitude of the impact of different values of the N content and 15N abundance of seeds and of B values on the estimates of the proportion of legume N derived from N2 fixation.