N-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-15-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 N-15 in wheat the first year (mean 53%) was not affected by DCD or time of N application. Immobilization of N-15 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-15 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 N-15 averaged 1.0 and 0.9% in the grain and straw, respectively, or about 10% of the N-15 immobilized the year before. Recovery of N-15 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.