ABSTRACT Accumulation of NH~ -N and NO3 -N in soils has not been thor. oughly evaluated in long-term continuous winter wheat (Triticum aes. tivum 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~ -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 KCI-extractable NH~ -N and NO~-N. At all locations, NH~ -N levels were not significantly different from the check (no fertilizer N) when rates were applied at or below yield goal requirements (90 or45 kg N ha -~ vs. 0 N). At N rates >90 kg N ha -~, surface (0-15 cm) NH~ -N increased compared with the check, while subsurface NH~ -N did not. Similarly, when N rates were <90 kg N ha -~, no significant differences in either surface or subsurface NO 3 -N were found. At N rates >90 kg N ha -1, NO5 -N accumulated in the subsurface soil profile (>30 cm). Estimates of N rates determined from simultaneous solutions of NO5 -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~ -N and NO5 -N occurred at recommended N rates where near maximum yields were obtained. p AST AND PRESENT use of N fertilizers for winter wheat production has been related to the potential for NOA--N contamination of surface and subsurface water. Although N fertilizers are essential for economic grain production, long-term N accumulation as a result of excessive N rates has not been monitored closely. Work by Liang et al. (1991) found that residual soil NO~--N did not increase in the soil profile (0-60 cm) over a 4-yr period when comparing N rates of 170 and 400 kg ha -1 applied to corn. MacDonald et al. (1989) indicated that following harvest, unfertilized wheat plots had inorganic N contents equal to those where 234 kg N ha -1 had been applied. This work further suggested that almost all of the NO~--N at risk to leaching over the winter period comes from mineralization of organic N and not from unused fertilizer applied in the spring; therefore, even a drastic reduction in N fertilizer use would have little effect on NO~--N leaching. Lamb et al. (1985) reported that the addition of N fertilizer increased the amount of NO~-N accumulated but did not change the accumulation pattern. Tillage system (no-till, stubble mulch, and plow) did not affect the time at which the NOA--N started to accumulate during the fallow period nor the rate of accumulation (Lamb et al., 1985). Sharpley al. (1991) reported no evidence of N accumulation the soil profile (0--180 cm) after 5 yr for either no-till or reduced-till cultural practices with N fertilizer applied to sorghum at recommended rates (0--146 kg ha-1 yr-1), although annual total N in surface runoff . was 0.76 kg N ha -1 for no-till, 0.99 kg N ha -1 for reduced-till, and 7.28 kg N ha -1 for conventional till. Smika (1990) reported that time must be allowed for the equilibration of soil conditions before evaluating NO~--N accumulation, citing research that showed less NO~--N accumulation to 120 cm for reduced-till methods compared with conventional tillage for short-term studies, but more NO~--N accumulation for reducedtill methods in long-term studies. Tracy et al. (1990) noted that tillage method (conventional, no-till) did not affect NO~--N accumulation below 5 cm; differences in NO~--N in the topsoil were attributed to organic matter incorporation over 16 yr of winter wheat farming. Varvel and Peterson (1990) reported that high N application rates (180 kg N ha-1) resulted in greater residual soil NO~--N to 150 cm for continuous corn and grain sorghum systems than for other cropping systems. This same study found that all systems had similar NO~--N accumulation at lower N application rates. Work by Liang et al. (1991) found that under irrigation, 100 kg NOA--N ha -~ was lost from the rooting zone (0-60 cm) during four growing seasons, with the majority coming from the surface 40 cm. The effects of N fertilizer rate (90 and 180 kg ha -x) and nitrification inhibitors on urea lSN leaching and balance on a irrigated sandy loam were summarized by Waiters and Malzer (1990). The higher N application rate resulted in 3.4 times more N leached over a 3-yr period (206 vs. 88 kg ha -1 to 1.2 m depth). Nitrification inhibitors delayed N losses, but did not decrease the total N lost. Westerman and Tucker (1979) noted that the presence of organic residue can lower denitrification by increased immobilization of inorganic or mineralized N. Immobilization was thus considered to be an N conserving process competing with denitrification for nitrate. Nitrate studies in field microplots showed that 17% of applied 15N (120 kg N ha -~ equivalent) was still in the 45-cm soil profile after 1 yr (Kowalenko, 1989). Webster et al. (1986) evaluated the movement (92 and 102 kg NHnNO 3 ha -~) in clay and sandy loam field microplots and found that < 1% of the fertilizer was leached beyond 130 cm in the first winter following application. Response of wheat grain yields to N fertilization has been documented in numerous soil fertility experiments. However, very few of these experiments have included evaluation for more than 3 to 5 yr that also accounted for accumulation of NOA--N and NH~--N within the soil profile. The objectives of this study were to determine the long-term response of 9
