Changes in 15N natural abundance of biologically fixed N2 in soybean due to shading, rhizobium strain and plant growth stage

Aims The evaluation of N-15 abundance of N derived from biological N-2 fixation (BNF) in legume shoots ('B-S' value) is essential for quantifying BNF inputs to field-grown legumes. The aim of this study was to investigate the impact of shading, development stage of soybean (Glycine max) and rhizobium strain on the 'B-S' value. Methods Soybean plants were grown in pots of autoclaved sand/perlite mixture in the open field. Plants were harvested at weekly intervals from 46 days after planting (DAP) to 75 DAP. All material was analysed for N and N-15 abundance. 'B-S' was calculated assuming 50% of seed N was translocated to the shoots. Results 'B-S' was stable until 60 DAP but subsequently increased for the three strains tested. Nodule efficiency (N-2 fixed g DM nodule(-1)) was greatly increased by shading and was significantly different between Bradyrhizobium species. 'B-S' was greatly increased by shading. Conclusions We recommend that 'B-S' should be evaluated on plants of the same development stage and light intensity as those where BNF is quantified in the field. Different Bradyrhizobium strains make a large impact on 'B-S' and may lead to considerable errors in estimation of BNF inputs to plants with high %N derived from BNF.