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Understanding Nitrogen Limitation in Soybean
Meeting soybean demand on existing cropland area for a global population of 9.7 billion people by the year 2050 requires narrowing the existing gap between average producer yield and yield potential. Soybean relies on two sources on nitrogen (N): biological N2 fixation and indigenous soil N supply. As soybean yield continues to increase, it seems critical to know if there is a yield level at which potential contribution of indigenous nitrogen sources and fixation becomes insufficient to meet crop N requirements for high yields, while still maintaining or increasing protein and oil concentration. This study evaluated N limitation across 29 high-yield soybean environments in Argentina and Nebraska from 2015 to 2017. Each environment included a ‘zero-N’ treatment, which forced the crop to rely on biological N2 fixation and indigenous soil N, and a ‘full-N’ treatment, which provided an ample fertilizer N supply during the entire crop cycle based on novel protocol developed also in this study. Seed yield and protein concentration in full N were 11% and 3% higher than zero-N, respectively. The magnitude of the difference depended upon the yield level of the production environment, ranging from 0 kg ha-1 at 2.5 Mg ha-1 up to 900 kg ha-1 at 6 Mg ha-1. Seed yield responses were directly related with increases in accumulated N in aboveground biomass (70 kg N ha-1), without changes in nitrogen use efficiency. The N limitation was mitigated in environments with large contribution of indigenous soil N supply. The maximum rates of N limitation occurred before the seed filling and the plant mechanisms and processes underlying seed yield and protein concentrations were leaf area index, absorbed solar radiation, and N remobilization. Finally, there was a trade-off between biological N2 fixation and indigenous soil N supply with fixation reduced less than proportional per unit increase in indigenous N sources. There was a temporal asynchrony between biological N2 fixation and N demand, that is, biological N2 fixation was not sufficient to meet plant N demand as the latter increased and the contribution of indigenous soil N supply decreased. The peak of indigenous soil N supply was the most important factor explaining variation in the N limitation across environments. Findings from this study will help to narrow soybean yield gap to meet future food demand.
Cafaro La Menza, Nicolas, "Understanding Nitrogen Limitation in Soybean" (2019). ETD collection for University of Nebraska - Lincoln. AAI27665800.