Soybean N₂ Fixation Estimates, Ureide Concentration, and Yield Responses to Drought

Increasing N 2 fixation tolerance to drought has been hindered by the labor and costs of quantifying N 2 fixation using 15 N methodologies. The relative abundance of ureides (RAU) in plant tissues has been used for estimating N 2 fixation in soybean [ Glycine max (L.) Merr.] grown under well‐watered conditions, but it has not been evaluated for drought conditions. The present research evaluated the response of N accumulation to N fertilization, the ability of the RAU technique to predict N 2 fixation under drought conditions, and the response of yield to N fertilization under well‐watered and drought conditions. Under drought, shoot N accumulation rate during vegetative growth approximately doubled as the amount of N fertilizer was increased from 10 to 200 kg N ha −1 , indicating a greater sensitivity of N 2 fixation to drought than uptake and assimilation of inorganic N. Under well‐watered conditions, the relationship between estimates of N 2 fixation made by 15 N‐dilution and RAU agreed within 15% of published reports. Under drought conditions, however, this relationship was greatly different (13 to 43%) from published reports. Fertilization with inorganic N in 1 yr increased grain yield 15 to 25% for the drought treatment and 12 to 15% for the well‐watered treatment. In a second year, N fertilization increased yield of both drought and well‐watered treatments approximately 9%. This research indicates that the RAU technique for estimating N 2 fixation under drought conditions may be invalid without further refinement, that N 2 fixation is more sensitive to drought than the uptake and assimilation of inorganic soil N, and that increasing the tolerance of N 2 fixation to drought would likely result in yield increases.