Nitrogen fertilizer effects on soybean physiology, yield components, seed yield and protein content in the Southeastern United States

Soybean physiology, seed yield and composition can vary greatly and are a function of genetics, environment, and management practices. N fertilizer is not often applied to soybeans, but it can affect yield and seed composition, and the outcome of N fertilization has varied widely among studies. To test if certain physiological responses might be linked with positive agronomic responses to N fertilizer, a field study was conducted in varied environments unique to the Southeast United States (US). Five genotypes from maturity groups V-VII were evaluated. Although the environment and genotype had significant effects on most of the physiological characteristics and yield components measured, nitrogen application interacted significantly with environment for seed protein concentration, seed oil concentration, yield, and percent of nitrogen derived from the atmosphere in leaves during the seed fill period. The direction of the nitrogen effect on protein concentration differed among environments, increasing protein concentration in one environment and reducing protein concentration in another environment. Nitrogen application effects did not vary among the genotypes included in this study. Photosynthetic and nitrogen fixation responses to nitrogen application were not clearly linked with seed protein concentration response. A tailored nitrogen recommendation for environment is the best option for improving seed protein in this region with diverse environmental conditions.