Regional Characterization of Soybean Seed Yield and Biological N Fixation Responses to N Fertilizer in the US Midwest Region.

Biological N fixation (BNF) process in soybean (Glycine max L.) can represent 50% or more of the total N uptake during the growing season. This process is responsible for the adequate N supply necessary to achieve high yields without compromising seed composition (e.g., protein concentration) as well. A concern related to N supply in high-yield cropping systems has been raised. The objective of this work is to characterize soybean response to N fertilization and the effect in seed yield, protein concentration, and BNF process for the US Midwest region. The experiment consisted in three timings of fertilizer N application (N at planting, N at V4, and N at R2) with the same N rates (112 kg N ha^-1) and a control without N added. The experiment was successfully replicated in 23 locations across the US Midwest region. Seed yield was significantly (p<0.05) affected in nine locations in response to N application; however, the yield response was not consistent across locations or treatments. For protein concentration only four locations showed significant response to N addition. To characterize BNF response, locations were separated in three groups according to their BNF levels (from low <72%, medium 72-88%, and high >88% BNF). The zero-N treatment presented the highest BNF in all groups with 91, 80, and 68% BNF for the high, medium, and low groups, respectively. Fertilizer N applications partially inhibited the BNF process according to the application timing. The results presented in this study showed that the soybean yield response to N fertilization is complex and not consistent, but the same is its interaction with the BNF process. Lastly, for the first time a characterization of the BNF at a regional level was documented showing the range of BNF values that high-yielding productive systems are currently achieving. A more comprehensive analysis dissecting the intrinsic effects of the environment (soil and weather) should be addressed to acquire a better understanding of the BNF process.