Biological N fixation activity in soybean can be estimated based on nodule dry weight and is increased by additional inoculation

The response of soybean to biological nitrogen fixation (BNF) varies due to factors such as plant variety, soil, and environmental conditions, which can compromise the N2 fixation capacity. The objective of this study was to evaluate additional inoculation with B. japonicum at different soybean growth stages as a strategy to improve N2 fixation. This was determined by studying plant nodulation and BNF with the isotope dilution method. The experiment was carried out under greenhouse conditions in a soil without soybean cropping history. Inoculated soybean increased nodulation, plant biomass, BNF and yield components compared to un-inoculated soybean. Additional inoculation at the V3 growth stage showed promising results by maintaining BNF at later growth stages. A strong correlation was observed between nodule dry weight and N2 fixation, providing an easily available method to estimate differences in BNF activity between treatments. Moreover, efficient nodulation is crucial to sustain high yields with BNF, as shown by the correlation between nodule dry weight and pod dry weight.