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

Abstract

AbstractNitrification 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 15N‐labeled urea with and without dicyandiamide (DCD) and split urea application in fall and spring. Grain yield (mean 4.26 Mg ha−1) increased with N rate (67, 101, and 134 kg N ha−1), but was not affected by DCD or time of N application. Recovery of 15N in wheat the first year (mean 53%) was not affected by DCD or time of N application. Immobilization of 15N 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 15N not accounted for, were not affected by DCD or timing of N. In the second year, 101 kg N ha−1 of unlabeled urea was split applied across the entire experiment. Grain yields averaged 2.07 Mg ha−1 and were not affected by N rate, DCD, or timing of N application from the previous year. Percent recovery of 15N averaged 1.0 and 0.9% in the grain and straw, respectively, or about 10% of the 15N immobilized the year before. Recovery of 15N 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.