Late-season N fertilization effects on soybean seed protein and biological N-2 fixation

Soybean [Glycine max (L.) Merr.] seed protein concentration has been declining over the past decades with protein values falling below the similar to 40% (dry weight base) required for optimal industrial processing. Further refinement of management practices is needed to help maintain concentrations above market standards. The objective of this work was to evaluate the impact of N fertilization during late reproductive stage on soybean seed protein on two cultivars differing in seed protein concentration in order to untangle the role of biological nitrogen fixation (BNF) to mediate seed protein responses. In experiments at two planting dates, late N fertilization was applied as either a foliar or soil-applied fertilizer and consisted of applying 80 kg ha(-1) of N from R3 to R6 with four 20 kg ha(-1) N applications. Both cultivar and planting date affected seed yield and protein concentration, ranging from 3,242 to 3,949 kg ha(-1) and 34.8 to 38.4 g 100 g(-1), respectively. Soil and foliar N fertilizations had no effect on yield or N uptake but had positive effects on seed protein concentration (+2 g 100 g(-1)) only in the late-planted experiments. Applying N in the soil inhibited BNF. The amount of N-2 fixed during the seed-filling period, expressed per unit seed, is a relatively good predictor of the response of seed protein to N fertilization suggesting a critical role of BNF during this stage for securing a high concentration of seed protein.