Nitrogen‐15 Recovery in Winter Wheat as Affected by Application Timing and Dicyandiamide

Nitrification inhibitor amendment to fall-applied N for winter wheat may reduce denitrification and leaching losses of N by maintaining N in ammoniacal form. A field study on a Norfolk loamy sand (fine-loamy, siliceous, thermic Typic Paleudult) compared fall application of "N-labeled urea with and without dicyandiamide (DCD) and split urea application in fall and spring. Grain yield (mean 4.26 Mg ha-) increased with N rate (67,101, and 134 kg N ha^), but was not affected by DCD or time of N application. Recovery of N in wheat the first year (mean 53%) was not affected by DCD or time of N application. Immobilization of N was 21, 20, and 15% with the fall N plus DCD, split-applied N, and fall N treatments, respectively. Nitrogen-15 leached at harvest was less with fall N plus DCD or split-applied N than with fall N only. Denitrification estimates, calculated as *N not accounted for, were not affected by DCD or riming of N. In the second year, 101 kg N ha ' of unlabeled urea was split applied across the entire experiment. Grain yields averaged 2.07 Mg haand were not affected by N rate, DCD, or timing of N application from the previous year. Percent recovery of N averaged 1.0 and 0.9% in the grain and straw, respectively, or about 10% of the N immobilized the year before. Recovery of N in the second year was significantly higher in the straw and roots following split-applied N or fall N plus DCD, compared with following fall N alone. This suggests that nitrification inhibitors can affect N uptake of a second-year crop by enhancing biological immobilization of fertilizer N. E USE OF N continues to be mandated in light of energy and environmental concerns. Urea fertilizer makes up 40% of the N fertilizer used worldwide (Voss, 1984). However, biological oxidation quickly converts NH4 to NO3, which is subject to loss through leaching and denitrification (Huber et al., 1977). Nitrification inhibitors can maintain applied N in the NH4 form and possibly improve N-use efficiency by reducing losses. In the southeastern USA, split (fall and spring) applications of N fertilizer are currently recommended for winter wheat (Touchton and Hargrove, 1983). Use of a nitrification inhibitor with fallapplied N fertilizer on winter wheat could possibly minimize losses of N fertilizer through denitrification and leaching. The nitrification inhibitor that has received the most attention in recent years is nitrapyrin [2-chloro-6(trichloromethyl)-pyridine], which cannot be granulated with solid fertilizers because of its high vapor K.F. Bronson, USDA-ARS, Northern Plains Area, P.O. Box E, Fort Collins, CO 80522; J.T. Touchton, Dep. of Agronomy and Soils, Auburn Univ., Auburn, AL 36849; and R.D. Hauck and K.R. Kelley, Tennessee Valley Authority-National Fertilizer and Environmental Research Center, Muscle Shoals, AL 35660. Contribution from Auburn Univ., Alabama Agric. Exp. Stn. and Agricultural Research Dep. , TVA-NFERC. Journal no. 3-902458P. Part of a thesis presented to Auburn Univ. by the senior author in partial fulfillment of the requirements for the Ph.D. Received 2 Mar. 1990. Corresponding author. Published in Soil Sci. Soc. Am. J. 55:130-135 (1991). pressure. Dicyandiamide (DCD) does not have this limitation. A few positive responses of winter wheat to nitrification inhibitors with fall-applied N have been reported in Canada (Yadvinder-Singh and Beauchamp, 1988) and in the Midwest (Huber et al., 1980). In the southeast, responses of wheat to fall-applied N with nitrification inhibitors are less frequent (Boswell et al., 1976 and Sharpe and Touchton, 1986). Touchton and Boswell (1980) have stated that the longevity of nitrification inhibitors is limited in the warm winters of the southeast. Other workers have attributed a lack of crop response to nitrification inhibitors to enhanced biological immobilization of fertilizer N by inhibitors (Juma and Paul, 1983; Ashworth, 1986). In the first year of this experiment, a N source was used to determine the effect of DCD and time of N application on N transformations in soil and on the recovery of fertilizer N by winter wheat. It was hypothesized that DCD applied in the first year might conserve fertilizer N in organic forms and affect wheat yields or fertilizer-N recovery in the second year. MATERIALS AND METHODS