Illinois Stream Nitrate Concentrations and Loads: Response to Fall Fertilization.

Tile drained fields in the upper Midwest are a major source of nitrate to the Gulf of Mexico. Winter/spring nitrate loads in the Mississippi River are a major cause of the Gulf of Mexico hypoxic zone that forms each summer. We utilized long-term stream monitoring data from several east-central Illinois agricultural watersheds to examine the recent response in nitrate concentrations and loads. These watersheds are dominantly corn and soybean, with average nitrate-N loads of 30 kg N/ha/yr. Due to wet conditions in the fall of 2009, little fall N fertilizer was applied in this area (where typically > 50 is fall applied). Fall 2010 had typical fertilization patterns, with extensive application of anhydrous ammonia to many fields. In 2010 we observed the lowest stream water nitrate concentrations (< 10 mg nitrate-N/L in all streams, with many only 4-6 mg N/L) in our period of record (1993 through 2010). In early 2011 nitrate concentrations in all monitored rivers were 7 to 56% greater, with several above 10 mg N/L. Our long-term record and the events of 2009-2011 demonstrates the complex nature of nitrate leaching from tile drained agricultural fields, and how quickly changes can occur in response to a factor such as fall N.