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.