AMMONIUM AND NITRATE-NITROGEN IN SOIL PROFILES OF LONG-TERM WINTER-WHEAT FERTILIZATION EXPERIMENTS

Accumulation of NH4+-N and NO3--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 NH4+-N and NO3--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 KCl-extractable NH4+ -N and NO2--N. At all locations, NH4+ -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) NH4+-N increased compared with the check, while subsurface NH4+-N did not. Similarly, when N rates were < 90 kg N ha-1, no significant differences in either surface or subsurface NO3--N were found. At N rates > 90 kg N ha-1, NO3--N accumulated in the subsurface soil profile (>30 cm). Estimates of N rates determined from simultaneous solutions of NO3--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 NH4+-N and NO3--N occurred at recommended N rates where near maximum yields were obtained.