Ammonium and Nitrate Nitrogen in Soil Profiles of Long‐Term Winter Wheat Fertilization Experiments

Accumulation of NH + 4 ‐N and NO ‐ 3 ‐N in soils has not been thoroughly evaluated in long‐term continuous winter wheat ( Triticum aestivum L.) production systems. The objectives of this study were to determine long‐term response of winter wheat to N fertilization and to evaluate accumulation of NH + 4 ‐N and NO ‐ 3 ‐N in the soil profile. Four long‐term winter wheat soil fertility experiments on thermic Ustoll soils that received annual applications of N for > 18 yr at selected N rates were sampled. At each location, one soil core 4.4 cm in diameter was taken to a depth of 240 cm from plots receiving variable N rates. Cores were separated into 30‐cm increments and analyzed for 2 M KCI‐extractable NH + 4 ‐N and NO ‐ 3 ‐N. At all locations, NH + 4 ‐N levels were not significantly different from the check (no fertilizer N) when rates were applied at or below yield goal requirements (90 or 45 kg N ha −1 vs. 0 N). At N rates >90 kg N ha −1 , surface (0–15 cm) NH + 4 ‐N increased compared with the check, while subsurface NH + 4 ‐N did not. Similarly, when N rates were <90 kg N ha −1 , no significant differences in either surface or subsurface NO ‐ 3 ‐N were found. At N rates >90 kg N ha −1 , NO ‐ 3 ‐N accumulated in the subsurface soil profile (>30 cm). Estimates of N rates determined from simultaneous solutions of NO ‐ 3 ‐N accumulation minimums and yield maximums generated from quadratic regression were greater than N rates currently recommended to achieve yield goals at all locations. For these long‐term continuous winter wheat experiments, no accumulation of NH + 4 ‐N and NO ‐ 3 ‐N occurred at recommended N rates where near maximum yields were obtained.