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

Dep. of Agronomy, Oklahoma State University, Stillwater, OK 74078

^* Corresponding author.

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.

Received for publication January 18, 1993.

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