Soybean Nitrogen Limitation in High-Yield Production Environments.

Soybean [Glycine max (L.) Merr.] is the most important legume crop in the world accounting for 56% of global oilseed production. It is uncertain to which degree soybean crops that approach yield potential can meet their large nitrogen (N) requirement relying on indigenous N sources which include N from fixation and soil organic matter mineralization. The objective of the study was to test the hypothesis that soybean is limited by N supply in high-yield production environments. Replicated experiments were installed in four producer irrigated fields in Nebraska (USA) planted with soybean in 2016. These fields had consistently achieved yields >5 Mg ha^-1 in previous years (>90% of their yield potential). A protocol was implemented to ensure ample N supply during the entire crop season (‘full-N treatment’) based on the simulated yield potential. Crops received a total of ca. 870 kg N ha^-1, split into six applications (V1, V2, V4, R1, R3, and R5) with amount in each application adjusted to match the seasonal dynamics of crop N demand. The full-N treatment was compared with crop that did not received fertilizer inputs (zero-N treatment). Aboveground dry matter (ADM), N uptake, and ADM in energy basis were measured every week from emergence to physiological maturity. Nitrogen use efficiency (NUE) was calculated by relating the ADM and crop N uptake. ADM at physiological maturity ranged from 1093 to 1708 g m^-2 across fields and treatments, with N uptake ranging from 364 to 581 kg ha^-1. Average seed yield was 5.5 and 6.1 Mg ha^-1 for the zero and full-N treatments respectively. The yield difference between treatments was explained by an increase in N uptake (7% higher in full-N treatments) while NUE remained fairly stable between treatments (±1%). To summarize, this study indicates that soybean productivity is limited by N supply in high-yield production environments.