Drought and nitrogen source effects on nitrogen nutrition, seed growth, and yield in soybean

Drought in soybean [Glycine max (L.) Merr.] decreases yield-related processes and N-2 fixation is more sensitive to drought than are many other of these processes. Therefore, application of nitrogen (N) fertilizer may increase drought tolerance over those plants primarily dependent on N-2 fixation. In a field experiment, NH4NO3 applications (+N) to drought-stressed soybean resulted in biomass and N accumulation rates similar to those rates for an irrigated treatment without N fertilizer (-N). In contrast, biomass and N accumulation rates were decreased for the -N treatment. N fertilization increased seed growth rate and decreased seed fill duration for irrigated and drought treatments. In the drought treatment, N application increased seed number per unit area, which resulted in higher yields. In a greenhouse experiment, fertilization with either KNO3 or NH4Cl increased biomass and N accumulation rates during drought over those of plants dependent solely on N-2 fixation. It was concluded that application of N fertilizer to soybean increases drought tolerance because of the extreme sensitivity of N-2 fixation to drought.