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Nitrogen supplied by N2 fixation to soybean [Glycine max (L.) Merr.] may not be sufficient to maximize yield. Field studies were conducted in 2002, 2003, and 2004 on Sharkey clay soil (very-fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33 269 N lat). The objective was to determine the effect of high rates of N applied as a replacement for N2 fixation in nonirrigated and irrigated environments. Eight cultivars ranging from Maturity Group II to IV were planted on 17 Apr. 2002, 2 Apr. 2003, and 25 Mar. 2004. Not all cultivars were evaluated in all 3 yr. Glyphosate herbicide was used in all 3 yr and a non-glyphosate herbicide treatment was applied in 2002. Cultivars grown in 2003 were also evaluated under an application of 21.3 kg ha of Mn. All cultivar, herbicide, and Mn treatments were evaluated in irrigated and nonirrigated environments with fertilizer N (PlusN treatment) or without fertilizer N (ZeroN treatment). In the PlusN treatment, granular NH4NO3 was surface applied at soybean emergence at rates of 290 kg ha in 2002, 310 kg ha in 2003, and 360 kg ha in 2004. When analyzed over all management practices (years, cultivars, herbicide, and Mn treatments), the PlusN treatment resulted in significantly decreased ureide concentration (57.2 and 53.5% reduction) and significantly increased biomass accumulation (14.1 and 16.7%), N accumulation (12.8 and 28.1%), and seed yield (7.7 and 15.5%) for the irrigated and nonirrigated environments, respectively. The majority of the yield increase in each environment resulted from increased number of seed (9.5% irrigated and 16.2% nonirrigated). These results confirm the sensitivity of N2 fixation to drought and indicate that N2 fixation may limit yield of soybean grown in both irrigated and nonirrigated environments of the midsouthern USA, and that N2 fixation deficiencies occur before the beginning of processes that determine number of seed. WATER DEFICIT STRESS (drought) is difficult to define and quantify because the magnitude of its effect depends on numerous crop (species, cultivar, phenology, etc.) and environmental factors (intensity, duration, evaporative demand, etc.), as well as their interactions. A deficit of certain intensity and duration may have differing effects on crop performance depending on the stage of development at which it occurs. Nonetheless, there is wide consensus that most plant physiological processes are unaffected by water deficits until .60% of the available soil water has been lost (Weisz et al., 1994; Sadras and Milroy, 1996). An exception to this generalization is N2 fixation in soybean which has been shown to be more sensitive to water deficits than other physiological processes such as transpiration, photosynthesis, and biomass accumulation (Sinclair, 1986; Durand et al., 1987; Sinclair et al., 1987; Kirda et al., 1989; Djekoun and Planchon, 1991; Serraj et al., 1999). Increased sensitivity to water deficits is primarily concluded from experiments showing that N2 fixation begins to respond (decline) at greater soil water contents than other physiological processes. The implication of this sensitivity is that even relatively limited water deficits can affect N accumulation and yield. Purcell and King (1996) hypothesized that the uptake and assimilation of soil N was less sensitive to water deficits than was N2 fixation, and that N fertilizer might ameliorate the effects of drought. They evaluated this hypothesis in a 1-yr field experiment using one cultivar (Hutcheson) in irrigated and nonirrigated (drought) plots with split applications of 224 kg N ha at V6 and 112 kg N ha at full bloom (R2) (stages according to Fehr et al., 1971). They found that N fertilizer applied to nonirrigated plots increased yield by about 18% over nonirrigated plots without fertilizer, but saw no effect of added fertilizer in irrigated plots. They concluded that N fertilizer applications do result in increased water deficit (drought) tolerance.Hutchesonwas further examined by Purcell et al. (2004) in a 2-yr experiment of similar design to that reported by Purcell and King (1996). As with the previous experiment, yields of Hutcheson were increased in the nonirrigated treatment (9–25% increase), but unlike in the previous experiment, yields were also increased in the irrigated treatment by 9 to 15%. Over the years, numerous studies have been conducted examining the application of N to soybean. In most cases these have been application of low rates of ‘‘Starter N.’’ However, soybean grown on most soils does not respond to low rates (25–35 kg N ha) of preplant N fertilization (Johnson, 1987; Varco, 1999; Hoeft et al., 2000; Heatherly et al., 2003; Scharf and Wiebold, 2003). The exceptions cited by Johnson (1987) were applications made to soils that were somewhat poorly drained, were low in organic matter, and/or were strongly acid below the plow layer. Other significant responses have been observed in soils with low residual soil nitrate (Lamb et al., 1990) or in situations where inorganic N is temporarily immobilized by soil microorganisms decomposing wheat straw (Whitney, 1997). In most cases, N fertilization of soybean is an unnecessary expenditure (Varco, 1999; Hoeft et al., 2000). Additionally, concentrations of N surrounding soybean roots can delay or impede nodulation (Harper and Gibson, 1984; Gibson and Harper, 1985), and thus reduce N2 fixation. Larger amounts of N fertilizer (100 to .500 kg ha) have been applied to soybean with effects on yield rangJeffery D. Ray, Research Geneticist, Larry G. Heatherly, Research Agronomist, and Felix B. Fritschi, Research Physiologist, USDAARS, Crop Genetics and Production Research Unit, P.O. Box 345, Stoneville, MS (662-686-3036). Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply approval or the exclusion of other products that may also be suitable. Received 13 Jan. 2005. *Corresponding author ([email protected]). Published in Crop Sci. 46:52–60 (2006). Crop Physiology & Metabolism doi:10.2135/cropsci2005.0043 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DAP, days after planting; DTM, days to maturity; ESPS, early soybean production system; MG, maturity group. R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 52 Published online December 2, 2005