Fig. 5. Plateau-linear response functions describing the response of the post-harvest residual soil NO₃–N content within a 0.9-m depth to the EONR in the six experimental fields. The EONR is the difference between the total N applied and economically optimal N rate (EONR). Location abbreviations are CP = claypan soil region, DL = deep loess soil region, MD = Mississippi delta soil region, 00 = 2000, and 01 = 2001.