Wednesday, May 28, 2008 - 11:45 AM
402

Linking human land-use with mechanisms of nitrate removal in streams using structural equation modeling

Robert O. Hall1, Jennifer L. Tank2, Daniel J. Sobota3, Patrick J. Mulholland4, Jackson R. Webster5, H. Maurice Valett6, Geoffrey C. Poole7, Bruce J. Peterson8, Judy L. Meyer9, William H. McDowell10, Sherri L. Johnson11, Stephen K. Hamilton12, Nancy B. Grimm13, Stanley V. Gregory14, Walter K. Dodds15, Clifford N. Dahm16, Lee W. Cooper17, Linda R. Ashkenas14, Suzanne M. Thomas8, Richard W. Sheibley18, Jody D. Potter10, Jonathan M. O'Brien12, Bobbie R. Niederlehner19, Laura T. Johnson2, Ashley M. Helton20, Chelsea L. Crenshaw21, Amy J. Burgin22, Melody J. Bernot23, Jake J. Beaulieu24, and Clay Arango25. (1) Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, (2) Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, (3) School of Earth and Environmental Sciences, Washington State University, Vancouver Campus, Vancouver, WA 98686, (4) Environmental Sciences Division, Oak Ridge National Laboratory, Bethel Valley Road, PO Box 2008, Oak Ridge, TN 37831-6036, (5) Department of Biological Sciences, Virginia Tech, Blacksburg, VA 24061, (6) Biological Sciences, Virginia Polytechnic Institute and State University, 2119 Derring Hall, Blacksburg, VA 24061, (7) Dept. of Land Resources and Environmental Sciences, Montana State University, 819 Leon Johnson Hall, P.O. Box 173120, Bozeman, MT 59717, (8) Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, (9) Institute of Ecology, University of Georgia, Athens, GA 30602, (10) Department of Natural Resources, University of New Hampshire, Durham, NH 03824, (11) Pacific NW Research Station, US Forest Service, 3200 SW Jefferson Way, Corvallis, OR 97330, (12) Kellogg Biological Station, 3700 East Gull Lake Drive, Hickory Corners, MI 49060, (13) School of Life Sciences, Arizona State University, Tempe, AZ 85287, (14) Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97331, (15) Division of Biology, Kansas State University, 104 Ackert Hall, Manhattan, KS 66506, (16) Department of Biology, University of New Mexico, Albuquerque, NM 87131, (17) Department of Ecology and Evolutionary Biology, The University of Tennessee, Knoxville, TN 37932, (18) Washington Water Science Center, U. S. Geological Survey, Tacoma, WA 98402, (19) Department of Biological Sciences, Virginia Polytechnic Institute and State University, 2119 Derring Hall, Blacksburg, VA 24061, (20) Odum School of Ecology, University of Georgia, Athens, GA 30602, (21) Department of Biology, Utah State University, Logan, UT 84322, (22) Cary Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545, (23) Department of Biology, Ball State University, Muncie, IN 42306, (24) US EPA, 26 W. Martin Luther King Drive, Cincinnati, OH 45268, (25) Depts of Biological Sciences/Geography, Central Washington University, Science Building, Room 236A, 400 E. University Way, Ellensburg, WA 98926-7537

Nitrate is highly reactive and has high concentrations in human-altered streams. Measuring nitrate removal from streams is central to understanding how they regulate nitrogen delivery to downstream ecosystems. We measured nitrate uptake in 72 streams with various human land uses across the U.S. using 15N tracer additions. Concomitantly we measured ecosystem metabolism, hydraulic parameters, and other chemical, biotic and riparian attributes as descriptor metrics. We tested a causal model describing controls on nitrate uptake length using structural equation modeling. Nitrate uptake lengths were longer with higher specific discharge and increasing nitrate concentrations, and were shorter with increasing gross primary production. The fraction of watershed area as agriculture or urban, did not statistically relate to nitrate uptake in bivariate correlation, nor was it a statistically-significant predictor variable in a set of multiple regression models. Adding land use to the structural equation model showed that it indirectly affected nitrate uptake lengths via increased gross primary production (which shortens nitrate uptake length) and increased nitrate availability (which lengthens nitrate uptake length). At first glance, land use appears to not control nitrate uptake, but in fact land use strongly regulates two counteracting direct controls leading to no net effect on nitrate uptake.


Web Page: nitrogen, land use, metabolism

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