Transport and fate of nitrate in headwater agricultural streams in Illinois

Nitrogen inputs to the Gulf of Mexico have increased during recent decades and agricultural regions in the upper Midwest, such as those in Illinois, are a major source of N to the Mississippi River. How strongly denitrification affects the transport of nitrate (NO₃-N) in Illinois streams has not been directly assessed. We used the nutrient spiraling model to assess the role of in-stream denitrification in affecting the concentration and downstream transport of NO₃-N in five headwater streams in agricultural areas of east-central Illinois. Denitrification in stream sediments was measured approximately monthly from April 2001 through January 2002. Denitrification rates tended to be high (up to 15 mg N m^-2 h^-1), but the concentration of NO₃-N in the streams was also high (>7 mg N L^-1). Uptake velocities for NO₃-N (uptake rate/concentration) were lower than reported for undisturbed streams, indicating that denitrification was not an efficient N sink relative to the concentration of NO₃-N in the water column. Denitrification uptake lengths (the average distance NO₃-N travels before being denitrified) were long and indicated that denitrification in the streambed did not affect the transport of NO₃-N. Loss rates for NO₃-N in the streams were <5% d^-1 except during periods of low discharge and low NO₃-N concentration, which occurred only in late summer and early autumn. Annually, most NO₃-N in these headwater sites appeared to be exported to downstream water bodies rather than denitrified, suggesting previous estimates of N losses through in-stream denitrification may have been overestimated.