Evaluating the impact of cover cropping and late-season N applications on soybean growth and biological N fixation

Context or problem: Developing sustainable cultural practices that enhance soybean (Glycine max L. (Merr.)) productivity and seed protein concentration is critical. Previous research conducted during 2019 and 2020 (Fayetteville, Arkansas; Lexington, Kentucky; and St. Paul, Minnesota) found a significant effect of winter rotation (fallow vs. cereal cover crop with residue removed, CC) and N fertilizer applications during seed growth (N_fert-R5; 202 kg N ha⁻¹ applied vs. unfertilized control) on seed yield and protein concentration. Objective: This follow-up research aimed at quantifying the effect of CC and N_fert-R5 on soybean in-season aboveground biomass and total N content, the percentage of N derived from the atmosphere (%, pNDFA), total biological N fixation (kg N ha⁻¹, BNF), and N concentration in developing seeds (mg N g^-1, N_seed). We hypothesized that CC would enhance soybean BNF compared to fallow and that N_fert-R5 would increase N_seed without compromising BNF. Methods: Soybean aboveground biomass was sampled at the R2 and R6 developmental stages, and developing seeds were sampled four times from mid-R5 to R8 for analysis of total N concentration. The δ¹⁵N was analyzed in aboveground biomass and R8 seed samples from all treatments and in reference non-nodulating plants, and pNDFA was estimated based on the natural abundance technique. Results: The CC increased pNDFA measured at the R2, R6, or R8 developmental stages in most instances but only increased BNF in harvested seed (BNF_R8) in Arkansas due to an overall reduction in aboveground biomass and N content (10 – 50 % average reduction depending on the location and developmental stage) relative to soybean after winter fallow. Of interest, pronounced reductions in soybean in-season growth after the CC had a relatively small impact on soybean final yield. The N_fert-R5 increased N_seed relative to the unfertilized control in the R5.5 developmental stage. However, N_fert-R5 reduced BNF at R6 (BNF_R6) by 51–88 kg N ha⁻¹ in six out of 12 locations, year, and cultivar MG combinations, and increased BNF_R6 by 63–71 kg N ha⁻¹ in two cases. At harvest, N_fert-R5 reduced BNF_R8 by 23–96 kg N ha⁻¹ in six out of twelve locations, year, and MG combinations (p < 0.10), and increased BNF_R8 by 30 kg ha⁻¹ in one case. Conclusions: Our hypotheses were only partially supported. The CC increased pNDFA but did not enhance BNF in most cases. Similarly, N_fert-R5 increased seed N concentration but reduced both pNDFA and BNF in most cases. Implications or significance: While CC and N_fert-R5 have specific benefits, their overall impact on in-season biomass, nitrogen dynamics, and yield is complex and environment-specific.