Yield can explain interannual variation in optimum nitrogen rates in continuous corn

Nitrogen (N) fertilizer decision support systems that rely primarily on corn grain yield often perform poorly because year-to-year variations in net soil N supply are not considered. However, there are environments where N mineralization and N losses are stable from year-to-year while variations in economically optimum N rates (EONR) are large. In such environments, interannual variations in EONR would be explained mainly by variation in yield, and weather effects on yield formation would be more important for EONR determination than weather effects on N cycling processes that govern N mineralization and loss. Data from a 10-year continuous corn N fertilization study (2009–2018) in Elora, Ontario, Canada were used to investigate the determinants of EONR in an environment with low and interannually stable N supply. EONR varied significantly over the study period, ranging between 157 and 273 kg N ha−1. Corn N uptake at maturity in check plots did not vary significantly over time, indicating stable soil N supply. Overwinter N losses effectively reset soil N supply to a baseline each season. Maximum economic yield was strongly correlated to EONR (R2 = 0.65). Most of the variability in the EONR could be explained by rainfall during V5-V12 (R2 = 0.84) and solar radiation during VT-R1 (R2 = 0.64), suggesting weather influenced EONR primarily by influencing crop N demand. Since grain yield can explain a large proportion of interannual variation in EONR, N rate recommendation systems should not ignore yield predictions, and greater efforts should be made to increase yield prediction accuracy.