Effect of additional inoculation on in situ biomass, nodule formation and N2 fixation in soybean under Northern European conditions

Abstract Background and Aims Adequate nitrogen (N) supply, often met through biological nitrogen fixation (BNF) in soybean plants, has been widely studied in North and South America and parts of Europe, but remains largely unexplored in Northern Europe. This study was conducted over two cropping seasons in the central Jutland region of Denmark to investigate the effectiveness of dual inoculation strategy with Bradyrhizobium japonicum for soybean, where local production faces challenges such as climate adaptation. Methods Building on previous greenhouse findings, we assessed the impact of inoculation treatments on soybean yield, nodulation, root traits, and nitrogen fixation in soils with no prior history of soybean cultivation. Results Variability in rainfall over the cropping seasons influenced pod development and N accumulation. In 2022, pod development was well-established, whereas the wetter conditions in 2023 promoted vegetative growth and nodule formation. The combination of seed inoculation plus additional inoculation at the V3 growth stage (third node) increased nodule number by 82.3%, nodule dry weight by 20.6%, and the quantity of N derived from the atmosphere by 30.2% compared with seed inoculation alone. Conclusion Efficient nodulation is crucial for ensuring N 2 fixation, highlighting that optimized inoculation practices, rather than N fertilizers, may offer a sustainable strategy to meet soybean’s N demands during the growing season were weather conditions challenge growth dynamics.