Date of this Version
Cafaro La Menza, N. (2019). Understanding Nitrogen Limitation In Soybean (Doctoral dissertation). University of Nebraska-Lincoln.
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.
Advisor: Patricio Grassini