Nitrogen Fertilizer Timing and Riverine Nitrate Flux.

n east-central Illinois, fertilizer sales during the past twenty years indicate that more fertilizer nitrogen (N) is applied to corn (Zea mays L.) in the fall than in the spring; however, in 2009 fall fertilizer N sales were greatly reduced as wet soil conditions restricted fall field work, including fertilizer N applications. Using long-term Embarras River and Lake Fork of the Kaskaskia River nitrate data sets (1993 through 2012 and 1997 through 2012, respectively), we examined nitrate concentrations and developed models to evaluate the impact of fall fertilizer N application on riverine nitrate yields in these tile-drained watersheds. During these periods of record, annual riverine nitrate yields ranged from 8 to 57 kg N ha^-1 with an average of 29 kg N ha^-1 for the Embarras River, and ranged from 2.6 to 59 kg N ha^-1 yr^-1 with an average of 32 kg N ha^-1 yr^-1 for the Kaskaskia. In 2010 winter and spring nitrate concentrations were lower than the typical concentration pattern in both rivers. Flow-weighted nitrate N concentration for 2010 in the Embarras River was 5.7 mg N L^-1, and was 5.6 in the Kaskaskia, both the lowest during the period of record. We determined that a linear regression relationship with the current and previous year’s annual water yield and the 11-county fall fertilizer sales accounted for 93% of the annual variation in nitrate yield for Embarras River. A similar model accounted for 92% of the annual variation in nitrate yield for the Kaskaskia River. Running the regression models with fall fertilizer sales set to the 2009 amount suggested that the reduction in nitrate yield would be 22 and 27% for the Embarras and Kaskaskia Rivers, respectively. Reducing fall N applications to corn in central Illinois is a needed step to reduce tile nitrate exports.