Can biochar and late-season nitrogen fertilization overcome the sulfur-containing amino acid deficiency and improve oil stability in soybean seeds?

Soybean meal quality for monogastric animals is limited by low sulfur-containing amino acids, especially methionine and cysteine, while oil oxidative stability is restricted by the high polyunsaturated fatty acid content. This two-year field study (2023-2024) evaluated the effects of biochar (0 and 12 t ha -1 ), cultivars differing in maturity group (MG2 and MG4), and late-season nitrogen (0, 40, 80, and 120 kg N ha -1 at R5) on ten essential amino acids and five major fatty acids in soybean seeds using a split-split plot randomized complete block design. Late-season nitrogen at 80 kg ha -1 (N80) and 120 kg ha -1 (N120) increased the concentration of seven out of ten essential amino acids, including threonine and lysine, but did not improve the levels of methionine, cysteine, or tryptophan. Amino acid concentrations varied by year, and their responses were most pronounced in the warmer 2024 season. The growing season also affected seven amino acids between cultivars, with the MG2 cultivar accumulating 3.4-5.9% higher concentrations than the MG4 cultivar under 2024 conditions. For fatty acids, cultivar and year interaction was the dominant factor shaping seed oil quality. MG2 cultivar had a higher (55.3%) oleic to polyunsaturated fatty acids ratio [Oleic/(Linoleic + Linolenic)] compared to MG4 cultivar in 2023, while nitrogen (N80 and N120) only changed oleic acid (p = 0.048) without improving the overall O/(L+Ln) ratio. Biochar at 12 t ha -1 had no significant influence on both amino acid and fatty acid concentrations. These results highlight that biochar and nitrogen fertilization may not address the sulfur-containing amino acid deficiency or oil stability, indicating that alternative strategies will be necessary to improve soybean seed quality.