DROUGHT AND NITROGEN-SOURCE EFFECTS ON NITROGEN NUTRITION, SEED GROWTH, AND YIELD IN SOYBEAN

Drought in soybean [Glycine max (L.) Merr.] decreases yield-related pr ocesses and N-2 fixation is more sensitive to drought than are many ot her of these processes. Therefore, application of nitrogen (N) fertili zer may increase drought tolerance over those plants primarily depende nt 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 fertilize r (-N). In contrast, biomass and N accumulation rates were decreased f or the -N treatment. N fertilization increased seed growth rate and de creased seed fill duration for irrigated and drought treatments. In th e drought treatment, N application increased seed number per unit area , which resulted in higher yields. In a greenhouse experiment, fertili zation with either KNO3 or NH4Cl increased biomass and N accumulation rates during drought over those of plants dependent solely on N-2 fixa tion. It was concluded that application of N fertilizer to soybean inc reases drought tolerance because of the extreme sensitivity of N-2 fix ation to drought.