Response of Illinois Streams to Nitrogen Fertilization Timing and Balances.

Illinois streams are a major source of nitrate to the Gulf of Mexico, with most of the N due to corn and soybean agriculture on tile drained fields. Long-term monitoring has shown the increase in nitrate concentrations from the 1950s to the late 1970s, with high concentrations since about 1980 with no trends through time. We utilized long-term stream monitoring data from several east-central Illinois agricultural watersheds to examine the recent response in nitrate concentrations and loads. Due to wet conditions in the fall of 2009, little fall N fertilizer was applied in this area (where typically 50 to 75% is fall applied). In addition, net N balances have greatly declined in this region during the past 10 years as fertilizer rates have been steady and yields have increased. We observed the lowest stream water nitrate concentrations (< 10 mg nitrate-N/L in all streams, with many only 5-6 mg N/L) during the winter and spring of 2010 in our period of record (1993 through 2010), with a multifaceted response during the higher flows of spring and early summer of 2010. Our long-term record and the events of 2009-2010 demonstrates the complex nature of nitrate leaching from tile drained agricultural fields, and how quickly changes can occur.