Late-season nitrogen application enhances amino acid concentrations in soybean seeds

Abstract

Soybean ( Glycine max ) seed composition is crucial for global food security and high-quality animal feed. Historic genetic selection and innovation have led to improved yield potential, but this has concurrently resulted in decreased seed quality. Biological nitrogen fixation is often not sufficient to meet the total nitrogen demand of the modern high-yielding cultivars, especially during seed filling. This study quantifies the effect of a moderate dose of supplemental nitrogen applied at distinct reproductive stages on individual amino acid, fatty acid, and carbohydrate profiles to determine if late-season nitrogen can selectively alter seed composition. A 2-year field experiment was conducted under rainfed conditions using a randomized controlled block design. Treatments consisted of a single application of 80 kg N ha −1 at five distinct reproductive stages. Seed composition traits were quantified using near-infrared reflectance spectroscopy. Supplemental nitrogen applied at the rapid seed-filling stage (R5) significantly increased the concentration of half of the amino acids (isoleucine, leucine, lysine, phenylalanine, valine, aspartic acid, glutamic acid, alanine, and arginine) compared to unfertilized plots. In contrast, nitrogen application had no significant effect on fatty acid profile or soluble and structural carbohydrates, and these traits were primarily driven by year-to-year environmental differences, such as precipitation patterns and drought stress severity. The results suggest that a moderate single dose of nitrogen, if applied at R5 stage, can be used to selectively enhance amino acid concentration. The effectiveness of the late-season nitrogen remobilization can be dependent on sufficient moisture during the seed-filling period. However, the lack of response in oil quality emphasizes that nitrogen supplementation must be paired with proper crop management practices.