Soybean Nitrogen Sources and Demand During the Seed‐Filling Period

Core Ideas Remobilization of N from vegetative organs fulfilled 59% of seed N, driven by plant biomass at R5.5. There was a trade‐off between N remobilization and N gain, connected to plant biomass and yield. Greater yields relied on both N remobilization and N gain during seed filling. Soybean [ Glycine max (L.) Merr.] seed N demand not met by biological N fixation is fulfilled by N remobilization from vegetative plant organs and N uptake from soil sources during the seed‐filling period (SFP), herein termed N gain. The objectives of this study were to: (i) quantify the contribution of plant organs to N remobilization during the SFP, (ii) determine the association between N gain and N remobilization, and (iii) study the effect of seed yield in both N gain and N remobilization processes. Three field experiments were conducted in 2015 and 2016 in Kansas. Three soybean genotypes: (i) non‐Roundup Ready (RR), released in 1997; (ii) RR‐1, released in 2009; and (iii) RR‐2, released in 2014 were tested under three N rates: (i) control without N application (zero‐N); (ii) 56 kg N ha −1 applied at R3 stage (late N); and (iii) 670 kg N ha −1 (full N; three applications of 223 kg N ha −1 ) applied at planting, R1, and R3 stages. The late‐N and full‐N rates increased yields by 9% (256 kg ha −1 ) relative to the zero N. Neither genotypes nor N rates affected N gain nor N remobilization. Nitrogen remobilization accounted for 59% of seed N demand at maturity, mainly driven by biomass at R5.5, with the leaf organ as the main contributor (52%) of the total remobilized N. Nitrogen remobilization was negatively related to N gain, and increases in N gain were linked to increases in biomass and yield.