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.
ASJC Scopus subject areas
- Environmental Engineering
- Water Science and Technology
- Waste Management and Disposal
- Management, Monitoring, Policy and Law