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Denitrification and the Nitrogen Budget of a Reservoir in an Agricultural Landscape

Mark B. David, Lareina G. Wall, Todd V. Royer and Jennifer L. Tank
Ecological Applications
Vol. 16, No. 6 (Dec., 2006), pp. 2177-2190
Stable URL: http://www.jstor.org/stable/40061950
Page Count: 14
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Denitrification and the Nitrogen Budget of a Reservoir in an Agricultural Landscape
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Abstract

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 NO₃-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 NO₃-N concentrations were measured (10-14 mg NO₃-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.m⁻².yr⁻¹, with rates a function of both denitrification intensity (microgram N.g dry mass.h⁻¹ and the overall mass of sediment present. From 1981 to 2003 the average NO₃-N inlet flux was 8900 Mg N/yr. About 58% of the total NO₃-N input was removed, and annual NO₃-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 48 900 Mg N/yr. When combined with estimated in-stream denitrification, 60 900 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.

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