Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 1, 2005
Publication Date: January 1, 2006
Citation: Ray, J.D., Fritschi, F.B., Heatherly, L.G. 2006. Influence of large amounts of nitrogen applied at planting on nonirrigated and irrigated soybean. Crop Science. Vol 46: 52-60 Interpretive Summary: Soybean producers rely on soybean to supply their own nitrogen through the process of nitrogen fixation. The amount of nitrogen supplied by this process may not be sufficient for maximizing yield, especially under drought conditions. Alternatively, the soybean requirements for nitrogen can be supplied by nitrogen fertilizer. When large amounts of nitrogen fertilizer was applied at planting in order to replace the nitrogen supplied by nitrogen fixation, plant growth and seed yield increased in both irrigated and nonirrigated fields in 2002-2004. The results confirm the sensitivity of nitrogen fixation in soybean to drought and indicate the amount of nitrogen produced by the nitrogen fixation process may limit soybean yield in the midsouthern USA. These experiments provide the foundation for future research to determine ways of overcoming the limitations of nitrogen fixation and increasing soybean yields in nonirrigated environments without producers expending additional resources.
Technical Abstract: 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 at Stoneville, MS (lat. 33 26'N). The objective was to determine the effect of high rates of N applied as a replacement for nitrogen fixation in nonirrigated and irrigated environments. Eight cultivars ranging from Maturity Group II-IV were planted on 17 April 2002, 2 April 2003, and 25 March 2004. All cultivars were not evaluated in all three years. Glyphosate herbicide was used in all three years 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, nitrogen 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 nitrogen fixation to drought and indicate that nitrogen fixation may limit yield of soybean grown in both irrigated and nonirrigated environments of the midsouthern USA, and that nitrogen fixation deficiencies occur before the beginning of processes that determine number of seed.