Relative Sensitivity of Nitrogen and Biomass Accumulation to Drought in Field‐Grown Soybean1

Abstract

AbstractEvidence of differential sensitivity to drought between symbiotic N fixation and leaf photosynthesis in legumes has been measured under greenhouse conditions. If the greater sensitivity of N accumulation to drought is substantiated under field conditions, then important questions concerning legume production under water‐limited conditions must be considered. Consequently, the relative sensitivity of N and biomass accumulation to drought was examined in a 2‐yr study on ‘Biloxi’ soybean [Glycine max (L.) Merr.] grown in the field on Arredondo fine sand soil (loamy, siliceous, hyperthermic Grossarenic Paleudults). Various irrigation treatments were imposed during vegetative growth after canopy closure when solar radiation was almost fully intercepted. Shoot and root plus nodule biomass and N accumulation were determined from periodic harvests. Solar radiation use‐efficiencies were computed from regressions of biomass against cumulative solar radiation, and were found to range from 0.53 g MJ−1 under daily irrigation, to 0.23 g MJ−1 under the most severe drought treatment. Nitrogen accumulation rates were computed by regression of N accumulation against time, and were found to decline from 0.31 g N m−2 day−1 for the daily irrigation to 0.003 g N m−2 day−1 for the most severe drought treatment. In both years and all three drought treatments, the N accumulation rate was decreased relatively more than the solar radiation use‐efficiency for biomass accumulation. These results indicate that a high sensitivity of N accumulation to soil dehydration may be an important constraint on soybean productivity.