Contemporary Biological N Fixation (BNF) for Soybean Genotypes during the Seed-Filling Period Under Different N Rates.

For soybeans (Glycine max L.), final seed weight is determined by the duration of the seed-filling period (SFP) and the rate of dry matter accumulation in the developing seeds. Biological N fixation (BNF) during the SFP represents a critical source of N to fulfill the seed N demand. However, addition of exogenous N sources represents a trade-off increasing N supply but inhibiting the BNF process. Seed weight formation and BNF were evaluated during the SFP under different genotypes and N conditions. Seven soybean genotypes released in the last 40 years were tested. Nitrogen treatments consisted in one condition of high-N availability (672 kg N ha^-1) and another condition with zero-N applied (only soil N availability). In overall, seed yield increased with the year of release primarily accompanied by increments on the seed number component (p<0.01). The N factor influenced seed yield primarily via affecting seed weight (p<0.05). The latter plant trait improved with the high-N treatment, which was related to changes in the duration of the SFP (p<0.05) rather than in the rate of the SFP. The BNF also reflected changes during the SFP related to the N treatment implemented, with high BNF (c.a. peak around 70-90%) under zero-N treatment, but still providing N-via BNF at a lower rate (c.a. peak around 40-50%) for the high-N treatment. The latter demonstrated that the N fertilization reduced BNF but did not completely inhibit this process. Thus, the high-N plants counted with three sources of N to satisfy seed N demand: N-BNF, N-soil, and N-fertilizer. The positive effect of the high-N treatment on the traits mentioned, could potentially demonstrate a plausible end-season N limitation during the SFP for soybeans. Further testing needs to be performed to provide an improved yield and BNF characterization under different growing seasons (weather).