Biochar and Late-Season Nitrogen Fertilization Effects on Soybean Yield and Seed Quality

Declining seed protein concentration in modern high-yielding soybean Glycine max (L.) Merr. cultivars is a major concern for the U.S. soybean industry. Nitrogen limitation during the seed-filling period is considered the primary reason for the decline. This two-year field study (2023 and 2024) evaluated the effects of biochar application (0 and 12 t ha -1 ) and late-season nitrogen (N) fertilization (0, 40, 80, and 120 kg N ha -1 split equally at R5, R5+1 week, and R5+2 weeks) on grain yield and seed composition in two maturity groups (MG2 and MG4) soybean cultivars. The experiment was arranged in a split-split randomized block design with four replications. Late-season N fertilization significantly increased seed protein concentration in both years without affecting oil concentration. All N rates increased protein concentration by 1.2 to 2.8% over the unfertilized control, with lower doses producing a response comparable to highest rate. The yield response was year-dependent and cultivar-specific. Nitrogen significantly increased seed yield only in 2024 under drought stress in MG4, where all N rates increased yield by 17 to 22% over the control. MG2 showed no significant yield response in either year. Nitrogen-treated plots maintained significantly higher leaf SPAD chlorophyll content during seed filling in 2024. Biochar did not significantly affect any measured parameter. These findings demonstrate that late-season N fertilization during seed filling period could be an effective strategy for improving soybean seed protein concentration. • Biochar and late-season N impacts on yield and seed composition were evaluated • Biochar addition did not affect soybean yield and seed composition. • Late-season N fertilization increased seed protein concentration. • Late-season N had minimal effect on yield and no effect on oil concentration