Denitrification is an important process in aquatic sediments, but its role has not been assessed in the N mass balance of upper-Midwestern (USA) reservoirs that receive large agricultural riverine N inputs. We used a 4400-ha reservoir to determine the role of denitrification in the N mass balance and effectiveness in reducing downstream transport of NO(3-)N. Sediment denitrification was (1) measured monthly (March 2002-March 2003) at eight sites in the Lake Shelbyville reservoir in central Illinois using the acetylene inhibition, chloramphenicol technique, (2) scaled to the overall reservoir and compared to N not accounted for in a mass balance, and (3) estimated indirectly using long-term (1981-2003) mass balances of N in the reservoir. Denitrification rates in the reservoir were high during spring and early summer of 2002, when maximum NO(3-)N concentrations were measured (10-14 mg NO(3-)N/L). We estimated that denitrification for the year was between 2580 and 5150 Mg N. Missing N from the mass balance was 3004 Mg N, suggesting that sediment denitrification was the sink. Areal rates of sediment denitrification in the reservoir ranged from 62 to 225 g N x m(-2) x yr(-1), with rates a function of both denitrification intensity (microg N x g dry mass x h(-1)) and the overall mass of sediment present. From 1981 to 2003 the average NO(3-)N inlet flux was 8900 Mg N/yr. About 58% of the total NO(3-)N input was removed, and annual NO(3-)N removed as a percentage of inputs was significantly related to reservoir retention time (average = 0.36 yr for the 23 years, range = 0.21-0.84 yr). By scaling denitrification in Lake Shelbyville to other reservoirs in Illinois, we estimated a sink of 48900 Mg N/yr. When combined with estimated in-stream denitrification, 60900 Mg N/yr was estimated to be removed by sediment denitrification. This reduces riverine export from Illinois to the Gulf of Mexico, where the flux during the 1990s was about 244000 Mg N/yr, and illustrates the importance of reservoir denitrification as an N sink in Midwestern agricultural landscapes.

Reservoirs are intrinsically linked to the rivers that feed them, creating a river-reservoir continuum in which water and sediment inputs are a function of the surrounding watershed land use. We examined the spatial and temporal variability of sediment denitrification rates by sampling longitudinally along an agriculturally influenced river-reservoir continuum monthly for 13 months. Sediment denitrification rates ranged from 0 to 63 lg N 2 O g ash free dry mass of sediments (AFDM) À1 h À1 or 0-2.7 lg N 2 O g dry mass of sediments (DM) À1 h À1 at reservoir sites, vs. 0-12 lg N 2 O gAFDM À1 h À1 or 0-0.27 lg N 2 O gDM À1 h À1 at riverine sites. Temporally, highest denitrification activity traveled through the reservoir from upper reservoir sites to the dam, following the load of high nitrate (NO 3 À -N) water associated with spring runoff. Annual mean sediment denitrification rates at different reservoir sites were consistently higher than at riverine sites, yet significant relationships among theses sites differed when denitrification rates were expressed per gDM vs. per gAFDM. There was a significant positive relationship between sediment denitrification rates and NO 3 À -N concentration up to a threshold of 0.88 mg NO 3 À -N l À1 , above which it appeared NO 3 À -N was no longer limiting. Denitrification assays were amended seasonally with NO 3 À -N and an organic carbon source (glucose) to determine nutrient limitation of sediment denitrification. While organic carbon never limited sediment denitrification, all sites were significantly limited by NO 3 À -N during fall and winter when ambient NO 3 À -N was low.