Wednesday, June 6, 2007 - 10:30 AM
233

Nitrate Uptake and Denitrification Rates in Streams Determined During the LINX II 15N Addition Experiments

Patrick J. Mulholland1, Linda R. Ashkenas2, Lee W. Cooper3, Clifford N. Dahm4, Walter K. Dodds5, Stuart Findlay6, Stan V. Gregory2, Nancy B. Grimm7, Robert O. Hall8, Stephen K. Hamilton9, Sherri L. Johnson10, William H. McDowell11, Judith L. Meyer12, Bruce J. Peterson13, Geoffrey C. Poole14, Jennifer L. Tank15, H. Maurice Valett16, Jackson R. Webster17, Clay Arango15, Melody J. Bernot18, Jake J. Beaulieu15, Amy J. Burgin19, Chelsea Crenshaw4, Ashley M. Helton12, Laura Johnson15, Jeff Merriam11, Bobbie R. Niederlehner16, Jonathan M. O'Brien9, Jody Potter11, Richard W. Sheibley7, Daniel J. Sobota2, and Suzanne M. Thomas13. (1) Environmental Sciences Division, Oak Ridge National Laboratory, Bethel Valley Road, Oak Ridge, TN 37831, (2) Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97331, (3) Department of Ecology and Evolutionary Biology, The University of Tennessee, Knoxville, TN 37932, (4) Department of Biology, University of New Mexico, Albuquerque, NM 87131, (5) Division of Biology, Kansas State University, 104 Ackert Hall, Manhattan, KS 66506, (6) Inst. of Ecosystem Studies, Millbrook, NY 12545, (7) School of Life Sciences, Arizona State University, Tempe, AZ 85287, (8) Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, (9) Kellogg Biological Station, 3700 East Gull Lake Drive, Hickory Corners, MI 49060, (10) Pacific NW Research Station, US Forest Service, 3200 SW Jefferson Way, Corvallis, OR 97331, (11) Department of Natural Resources, University of New Hampshire, Durham, NH 03824, (12) Institute of Ecology, University of Georgia, Athens, GA 30602, (13) Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, (14) Eco-metrics, Inc.; Institute of Ecology, University of Georgia; & Flathead Lake Biological Station, University of Montana, 2520 Pine Lake Road, Tucker, GA 30084, (15) Department of Biological Sciences, University of Notre Dame, Galvin Life Sciences, Notre Dame, IN 46556, (16) Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, (17) Department of Biological Sciences, Virginia Tech, 2119 Derring Hall, Blacksburg, VA 24061, (18) Department of Biological Sciences, Murray State University, Murray, KY 42071, (19) Kellogg Biological Station and Michigan State University, 3700 Gull Lake Dr., Hickory Corners, MI 49060

We conducted a large, intersite study of N cycling in streams using field 15N tracer additions to measure reach-scale nitrate uptake and denitrification rates.  Nitrate uptake was determined from the downstream decline in tracer 15N-nitrate flux and denitrification from analyses of tracer 15N appearing in dissolved N2 and N2O.  These rates were determined in 72 headwater streams draining reference, agricultural, and urbanized catchments.  A variety of physical, chemical, and biological variables were measured during each 15N experiment to identify controlling factors.  Nitrate uptake rates were highly variable among streams and land use types, with uptake lengths ranging from 20 to 31,000 m (median of 780 m).  Strongest predictors of nitrate uptake rate were nitrate concentration and gross primary production rate, together explaining 70% of the variation in uptake rate.   Denitrification accounted for 1 to100% of total nitrate uptake (median of 16%) and resulted almost exclusively in N2 rather than N2O production.   Strongest predictors of the rate of nitrate removal via denitrification were nitrate concentration and ecosystem respiration rate, together explaining 60% of the variation in denitrification rate.  Taken together, these results indicate the primary importance of biological processes in controlling nitrate dynamics across a wide range of streams.

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