Impact of Microbial Strain on the Nitrogen Fixation of Soybean Varieties of Different Maturity Under Cool Climate Conditions of Northern Europe

Soybean inoculation with nitrogen (N) fixing bacteria can be highly promising for enhancing biological nitrogen fixation (BNF) and improving crop productivity. It helps to reduce dependency on chemical fertilizers, promotes sustainable agricultural practices, and minimizes environmental impacts. Therefore, understanding the specific aspects and conditions is essential for establishing the BNF process in particular environments. In this study, we investigated whether soybean inoculation is an effective strategy in cool-climate regions beyond their typical northern distribution, and which soybean varieties and microbial strains are the most effective for optimizing soybean productivity and performance in relatively cool environments. To address these questions, a natural abundance nitrogen stable isotope ratio analysis was conducted on two soybean varieties of different maturity groups, which were inoculated with three <i>Bradyrhizobium japonicum</i> strains, along with organic fertilizer and new promising endophyte treatments. This approach aimed to determine the differences in biological and chemical parameters of soybean, as well as the origin of N sources, its uptake, and the isotopic distribution within the plants. It was demonstrated that inoculation with <i>Bradyrhizobium japonicum</i> was more effective than fertilization, as the strains had a significant effect on nitrogen derived from the atmosphere (Ndfa), produced stable nitrogen isotope ratios close to 0‱, and substantially increased nitrogen content, particularly in beans. Soybean varieties Laulema and Merlin, representing different maturity groups, exhibited distinct nitrogen uptake patterns. <i>Bradyrhizobium japonicum</i> strain <i>AGF78</i> consistently produced the greatest effect on biological parameters in both varieties, particularly in seed yield and grain weight, with the later-maturing Merlin achieving the highest average yield of 3066.89 kg ha^-1. Notably, the Merlin inoculated with <i>AGF78</i> resulted in the highest nitrogen fixation in beans, with 66.8%NDFA and 134.0 kg/ha of fixed nitrogen. Similarly, Laulema inoculated with <i>AGF78</i> resulted in 88.2%NDFA and 123.2 kg/ha of fixed nitrogen. Inoculation with selected bacterial strains significantly increased protein content from 30% to 41%, with the effects being both strain- and variety-specific. Our study showed that establishing effective soybean-microbe interactions by choosing soybean variety and microbial strain is crucial for optimizing agricultural practices and improving crop performance, especially in sustainable and environmentally conscious farming systems under cool climatic conditions of Europe.