Soybean Nitrogen Fixation and Nitrogen Remobilization during Reproductive Development

Seed N is important for soybean [ Glycine max (L.) Merr.] because of the high grain protein concentration. However, the association between N 2 fixation and N remobilized from vegetative tissues to seed across different maturity groups (MGs) is not well understood. We hypothesized that increased amounts of biomass at the beginning of seedfill for late‐maturing genotypes would provide a larger pool of N that was available for remobilization during seedfill and would increase yield relative to earlier‐maturing genotypes. Near isolines for maturity (MG IV, V, and VI), a modern breeding line (R01‐416F, MG V), and a non‐nodulated soybean genotype (MG VI) were harvested periodically from beginning seedfill to physiological maturity to determine N remobilization and N 2 fixation. Nitrogen fixation contributed approximately 90% of the seed N. The MG VI isoline accumulated larger N amounts in vegetative tissues than the MG IV isoline. Maturity isolines had similar seed N content, but the MG IV genotype had higher N harvest index (2009) than the MG VI isoline. In 2009, R01‐416F had greater yields and N content than the isolines, but there was no indication that N remobilization during seedfill was greater. These data indicate that N 2 fixation continued until late seedfill and supplied nearly all of seed N. A large reservoir of N in the vegetative tissue of late MGs was not fully used, and N was not a limitation to yield under these conditions.