Is Soybean Yield Limited By Nitrogen Supply?.

As soybean yield continues to increase, it seems critical to know the yield level at which potential contribution of indigenous nitrogen (N) sources (N fixation and soil mineralization) becomes insufficient to meet crop N requirements for high-yields production environments, while still maintaining or increasing protein and oil concentration. We have hypothesized that, in absence of other limiting factors, the degree of N limitation increases with increasing yield potential (Yp) of the production environment. To test this hypothesis, we developed a protocol to ensure ample N supply during the entire crop season (full-N treatment). That protocol was applied to field-grown soybean in Balcarce (Argentina) and Nebraska (USA). The combination of locations, years, sowing dates, and N treatments resulted in a wide range of seed yields, from 2.5 to 6.5 Mg ha^-1. Overall, full-N seed yield averaged 11% higher than seed yield without N addition (zero-N). However, magnitude of yield difference between full-N and zero-N depended upon Yp, ranging from no detectable yield difference in low-Yp (ca. 2.5 Mg ha^-1) to up to 900 kg ha^-1 in high-Yp environments (ca. 6 Mg ha^-1). Seed yield differences were associated with higher aboveground dry matter, seed number, and seed weight in the full-N versus zero-N treatments. Seed protein (but not oil) concentration was higher in the full-N treatment, and both protein and oil yields were higher in the full-N versus zero-N treatments. Findings from this study indicate that (i) N limits soybean seed yield (as well as protein yield, and oil yield) in environments with high Yp, where indigenous N sources seem insufficient to fully satisfy crop N requirements, and (ii) yield response to N fertilizer can occur above a 2.5 Mg ha-1 Yp threshold and has upper limit of 250 kg seed per Mg increase in Yp.