Estimation of Early and Late Nitrogen Fixation in Soybean (Glycine max[L.] Merril).

The soybean genetic control of biological nitrogen fixation (BNF) has been investigated both qualitatively (i.e., traits related to nodulation) and quantitatively (i.e., nitrogen fixation performance);  however, little progress has been made in the enhancement of BNF capacity via breeding and genetics. Thus, the objective of this study was to evaluate the BNF capacity of 25 soybean lines with diverse genetic background during the early and late growth stages. The experiment was designed as a randomized complete block with four blocks and three replications per block and performed under greenhouse conditions. A small amount of ¹⁵N fertilizer was applied to each pot at sowing, whereas leaves and pods were sampled at five weeks after germination and at the R7 developmental stage, respectively. Atom % ¹⁵N excess (AT%) was used to perform multiple comparison tests and estimate the percentage of N derived from atmosphere (Ndfa%) in each genotype. The results from the first and second sampling showed highly significant differences (p < 0.001) in AT% between genotypes in both sampling times. Ndfa% ranged from 0 to 53% and 0 to 91% at the first and second sampling time, respectively. These results suggested that there is a significant genetic variation in BNF capacity at early and late soybean growth stages, and consequently it could be used as an effective tool for improving N fixation in soybean.