Assessment of Traits Related to Nitrogen Fixation in Soybean (Glycine max. [L.] Merril).

The high nitrogen (N) requirements of soybean crop are partially covered by its symbiotic relationship with the bacterium Bradyrhizobium japonicum. To meet the future N demand of high-yielding cultivars without the application of any fertilizer, it is necessary to develop genotypes with a higher capacity of biological nitrogen fixation (BNF). Thus, the objective of this study was to: (1) evaluate traits directly or indirectly related to BNF in 25 soybean genotypes with different genetic background; (2) test their association with BNF; and (3) assess their value as selection criteria for relatively high BNF capacity. The plants were assayed twice under greenhouse conditions using the ¹⁵N isotope dilution method. The results showed that leaf atom% ¹⁵N excess was moderately negatively correlated with SPAD readings (r=-0.45; p < 0.001) and leaf %N (r=-0.49; p < 0.001), whereas pod atom% ¹⁵N excess was moderately negatively correlated with shoot dry mass (r=-0.51; p < 0.001) and seed number per plant (r=-0.48; p < 0.001). A multiple regression model revealed that the number of days to maturity, seed number per plant, seed weight, leaf %N, and nodule dry weight significantly contributed to pod atom% ¹⁵N excess (R²=0.47; p< 0.001). Cluster analysis using the Ward's method was performed and grouped the 25 genotypes into three different clusters, revealing a high level of genetic diversity between the clusters. Overall, the results suggested that the evaluated traits could be used as selection criteria in breeding programs, aiming to improve soybean BNF capacity.