Recent research suggests that N recommendations developed using the linear plateau model have a poor relationship between yield and the economic optimum N rates. These results have been attributed to inaccurate assessment of N mineralization. The objective of this study was to determine the influence of soil moisture regime on yield, N responses, and soil N mineralization due to water availability where environmental conditions vary from year to year and from location to location. Field experiments were conducted in Aurora, South Dakota, 2002, 2003 and 2004. Treatments were natural rainfall, natural rainfall + irrigation, and four N rates (0, 56, 112, 168 kg N/ha). Plant samples were analyzed for ¹³C discrimination (D), ¹⁵N, and total N. An interaction between soil moisture regimes and N rates was not observed. Increasing N rates increased corn grain yield and applying supplemental irrigation increased corn grain yields and soil N mineralization. The optimum N rate was similar in both soil moisture regimes. This work suggests that the linear plateau model is not accurate because it does not account for increased N mineralization effect of soil moisture regimes.