HR: 11:15h
AN: NB22E-04    [Abstracts]
TI: Denitrification Rates in a Midwestern Stream Containing High Nitrate: In Situ Assessment Using Tracers in Dome-Shaped Incubation Chambers
AU: * Smith, R L
EM: rlsmith@usgs.gov
AF: US Geological Survey, 3215 Marine Street, Boulder, CO 80303 United States
AU: Böhlke, J K
EM: jkbohlke@usgs.gov
AF: US Geological Survey, 431 National Center, Reston, VA 20192 United States
AU: Repert, D A
EM: darepert@usgs.gov
AF: US Geological Survey, 3215 Marine Street, Boulder, CO 80303 United States
AU: Hart, C P
EM: cphart@usgs.gov
AF: US Geological Survey, 3215 Marine Street, Boulder, CO 80303 United States
AB: The extent to which in-stream processes alter or remove nutrient loads in agriculturally-impacted streams is critically important to the function of a watershed and the delivery of those loads to coastal waters. Accurate assessment and characterization of key biogeochemical processes, such as nitrification and denitrification, are needed for establishing links between land use and watershed response and for developing predictive tools for agricultural management practices. In this study, patch-scale rates of in-stream benthic processes were determined within a second order stream reach (Sugar Creek, IN) dominated by drainage from tiled, row-crop fields. Rates of denitrification, nitrification, and net oxygen production/consumption were quantified using open-bottomed incubation chambers (0.6 m diam. acrylic hemispheres inserted 5-10 cm into the stream-channel sediment) that were fitted with water sampling/mixing ports, a volume compensation bladder, and pore-water piezometers. Incubations were conducted by injecting tracers (NaBr as a conservative tracer, along with ¹⁵N-enriched nitrate, nitrite or ammonium) into the chambers and collecting samples at 1-5 hr intervals for up to 48 hrs. Also assessed were in situ responses to increased nitrate concentrations, light vs. dark incubations, rates in various sediment types, and companion incubations during a reach-scale, in-stream tracer test with ¹⁵N-enriched nitrate. Overall, chamber denitrification rates ranged from about 25 to 155 μmol N m^-2 hr^-1 for nitrate concentrations from 90 to 1300 μM and were in general agreement with modeled rates from the reach-scale test. Increased nitrate concentrations resulted in increased denitrification rates, increased nitrite and nitrous oxide fluxes, and decreased methane fluxes. Oxygen consumption rates and nitrate loss rates based solely on concentration changes were much more variable than denitrification rates based on the isotope tracer results. Measured rates of nitrification were relatively low, even with added ammonium. The chamber incubations confirmed earlier studies that denitrification was a substantial sink for nitrate in this watershed and provided additional information about in situ responses to experimental manipulations.
DE: 1806 Chemistry of fresh water
DE: 1871 Surface water quality
SC: North American Benthological Society [NB]
MN: 2005 Joint Assembly