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

al., 1998) experiments have shown that N2 fixation in soybean is especially sensitive to water deficits and deIncreasing N2 fixation tolerance to drought has been hindered by creases before transpiration or photosynthesis. Furtherthe labor and costs of quantifying N2 fixation using 15N methodologies. The relative abundance of ureides (RAU) in plant tissues has been more, soybean grain yield under drought conditions was used for estimating N2 fixation in soybean [Glycine max (L.) Merr.] increased by 18% by applying 336 kg N ha 1 of NH4NO3 grown under well-watered conditions, but it has not been evaluated compared with treatments receiving no N fertilizer (Purfor drought conditions. The present research evaluated the response cell and King, 1996). We are unaware of any soybean of N accumulation to N fertilization, the ability of the RAU technique data that address how the proportion of N derived from to predict N2 fixation under drought conditions, and the response of N2 fixation or from the soil may change in response yield to N fertilization under well-watered and drought conditions. to drought. Under drought, shoot N accumulation rate during vegetative growth Obstacles for measuring N2 fixation in field experiapproximately doubled as the amount of N fertilizer was increased ments include the labor required for collecting plant from 10 to 200 kg N ha 1, indicating a greater sensitivity of N2 fixation samples and the costs of 15N-enriched fertilizer and isoto drought than uptake and assimilation of inorganic N. Under wellwatered conditions, the relationship between estimates of N2 fixation tope analysis of plant samples. Typically, soybean and made by 15N-dilution and RAU agreed within 15% of published rea reference crop that does not fix N2 are grown on soil ports. Under drought conditions, however, this relationship was enriched with 15N fertilizer, plant samples are harvested greatly different (13 to 43%) from published reports. Fertilization from an area of the plots, and the samples are dried, with inorganic N in 1 yr increased grain yield 15 to 25% for the weighed, ground, and analyzed by mass spectroscopy. drought treatment and 12 to 15% for the well-watered treatment. In As the soybean crop fixes N2 from the atmosphere, 15N a second year, N fertilization increased yield of both drought and derived from the soil is diluted relative to that of the well-watered treatments approximately 9%. This research indicates reference crop, and this relationship can be used to that the RAU technique for estimating N2 fixation under drought estimate the N derived from the atmosphere (NDA) conditions may be invalid without further refinement, that N2 fixation (Fried and Middleboe, 1977; Herridge and Peoples, is more sensitive to drought than the uptake and assimilation of inorganic soil N, and that increasing the tolerance of N2 fixation to 1990): drought would likely result in yield increases. NDA 1 [(atom% excess of soybean)/ (atom% excess of reference)] [1] N nutrition in soybean is met by a combinaThe quantity of N2 fixed by the crop is represented by tion of the uptake and assimilation of inorganicthe product of the total N content per square meter soil N and symbiotic N2 fixation. The relative importance multiplied by NDA. Rates of N2 fixation are calculated of these two sources of N in meeting the crop’s N needs from the increase in the quantity of N2 fixed by the crop changes depending upon the availability of inorganicbetween two sampling dates divided by the number of soil N (Harper, 1987). Thus, when inorganic-soil N is days between samplings. abundant, N2 fixation is inhibited or delayed and the An alternative method for estimating NDA has been proportion of N in the crop derived from N2 fixation is developed for soybean (Herridge and Peoples, 1990) decreased. Conversely, when there is little inorganicand other legumes (Herridge and Peoples, 2002) that soil N available, N2 fixation provides the majority of the use allantoin and allantoate (collectively referred to as crop’s N needs. ureides) as their primary N-export product from nodThe proportion of a soybean crop’s N derived from ules. In this procedure, the concentration of ureides inorganic-soil N or from N2 fixation may also change from petiole-tissue extracts is expressed as the RAU, depending upon whether or not the crop is exposed which is the fraction of the N from ureides relative to to drought or other environmental constraints. Field the sum of the N found in ureides and NO 3 : (Sinclair et al., 1987; Sall and Sinclair, 1991; Serraj et al., 1997) and greenhouse (Sall and Sinclair, 1991; Purcell et RAU (4 ureide conc.)/[(4 ureide conc.) L.C. Purcell and A. De, Dep. of Crop, Soil, and Environmental Sci., NO 3 conc.] [2] 1366 W. Altheimer Drive, Univ. of Arkansas, Fayetteville, AR 72704; R. Serraj, International Crops Research Institute for the Semi-Arid In Eq. [2], ureide concentration is multiplied by four Tropics, Patancheru 502 324 Andhra Pradesh, India; T.R. Sinclair, because there are four N atoms in the ureide molecules. USDA-ARS, Univ. of Florida, Agronomy Physiology and Genetics A similar ratio of RAU may also be calculated for xylem Laboratory, IFAS Building no. 350, 2005 SW 23rd Street, P.O. Box sap. In this case, the denominator of Eq. [2] includes 110965, Gainesville, FL 32611-0965. Research supported in part by the United Soybean Board, project no. 4213. Received 3 July 2004. *Corresponding author ([email protected]). Abbreviations: DOY, Day of Year; NDA, N derived from atmosphere; NN-Hardee, nonnodulating ‘Hardee’; PNDA, proportion of N derived Published in Crop Sci. 44:484–492 (2004).  Crop Science Society of America from atmosphere between harvests; RAU, relative abundance of ureides. 677 S. Segoe Rd., Madison, WI 53711 USA