Genotype and Fertilizer N Effects on Reproductive N and N Remobilization in Soybeans.

Soybean yield improvement has been documented in the past decades. Maximum yields depend on a balanced nutrition, with nitrogen (N) as one of the main limiting factors. The process of remobilization of N provides significant amounts of N; accounting for 50 to 60% of total N in the seed organ. The overall objective of this project was to investigate the main N sources of N for the plant during the seed filling period in soybean, N remobilization and N uptake during the seed filling under different genotypes and contrasting N conditions. Three field experiments were conducted during the 2015-2016 growing seasons in Kansas, US. Three genotypes were utilized: genotype 1, non-RR, released in 1997; genotype 2, RR-1, released in 2009; and genotype 3, RR-2, released in 2014. All tested under three N management strategies: zero-N, control; late-N, 56 kg ha^-1 applied at R3 growth stage; and full-N, 600 kg ha^-1 with an equal split application at planting, R1, and R3 growth stages. Aboveground biomass samples were collected at the beginning (R5) and end (R8) of the seed filling period (SFP), plants were fractioned in stems, leaves, pod walls, and seeds. Across all N levels, genotype 3 outyielded (9%) other materials. The application of N fertilizer increased yields by 9% relative to the control, without yield difference between late- and full-N. As for yields, a comparable increase was observed for total N content. No treatment differences were observed for remobilized and reproductive N. Remobilization accounted for 59% of total N content in the seed, with the leaf organ (52%) as the main plant fraction. A positive strong correlation (R²=0.90) was observed between vegetative N content at R5 and N remobilization. Reproductive N during SFP was negatively related to N remobilization. Lastly, greater reproductive N was primarily related to biomass accumulation during SFP, and consequently, connected to greater yields.