Changes in 15 N natural abundance of biologically fixed N 2 in soybean due to shading, rhizobium strain and plant growth stage

Aims The evaluation of 15 N 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 15 N 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. Conclusion 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.