Quantification of B-Values and Symbiotic Efficiency of Bradyrhizobium-Inoculated Soybean Varieties Using the δ15N Natural Abundance Method

Biological nitrogen fixation (BNF) is crucial for enhancing soybean productivity while reducing reliance on mineral nitrogen fertilizers. The accurate estimation of BNF via the δ15N natural abundance method depends on reliable B-values, which represent plants that derive all their nitrogen from fixation. This study aimed to assess the B-values and symbiotic efficiency of Bradyrhizobium-inoculated soybean varieties using δ15N natural abundance techniques. Eight strains and five soybean varieties were evaluated in sterilized sand culture using a factorial completely randomized design under lath-house conditions. Plants were analyzed for δ15N, shoot N concentration, shoot N content, and symbiotic efficiency (SE). The applied treatments showed highly significant effects with strong interactions, reflecting substantial genotypic variation in symbiotic performance. Strain SD-53 produced the lowest δ15N values (−0.24 to 0.14‰) and the highest SE, with several strain–variety combinations surpassing N-fertilized controls. Shoot N concentration and content ranged from 0.96–3.16% and 9.90–52.73 mg plant−1, respectively, and SE varied from 29.07 to 136.29%. δ15N showed strong negative correlations with SE and plant N traits. The study identified SD-53 as a promising inoculant candidate and generated the first regional soybean B-values (−0.24 to 0.14‰ for each tested variety and a mean of −0.08 ± 0.14‰) for Ethiopia. These values provide an important baseline for %Ndfa calculations and support future field validation and inoculant formulation.