1. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
2. Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996, USA
3. Odum School of Ecology, University of Georgia, Athens, Georgia 30602, USA
4. Eco-metrics, Inc., Tucker, Georgia 30084, USA
5. Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming 82071, USA
6. Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan 49060, USA
7. Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA
8. Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, USA
9. Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oregon 97331, USA
10. Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA
11. Division of Biology, Kansas State University, Manhattan, Kansas 66506, USA
12. Institute of Ecosystem Studies, Millbrook, New York 12545, USA
13. School of Life Sciences, Arizona State University, Tempe, Arizona 85287, USA
14. Pacific Northwest Research Station, US Forest Service, Corvallis, Oregon 97331, USA
15. Department of Natural Resources, University of New Hampshire, Durham, New Hampshire 03824, USA
16. Department of Biological Sciences, Virginia Tech, Blacksburg, Virginia 24061, USA
17. Department of Biology, Ball State University, Muncie, Indiana 47306, USA
18. Present addresses: US Environmental Protection Agency, Cincinnati, Ohio 45268, USA (J.J.B.); US Geological Survey, Tacoma, Washington 98402, USA (R.W.S.); School of Earth and Environmental Sciences, Washington State University, Vancouver Campus, Vancouver, Washington 98686, USA (D.J.S.).

Correspondence to: Patrick J. Mulholland^1,2 Correspondence and requests for materials should be addressed to P.J.M. (Email: mulhollandpj@ornl.gov).

Abstract

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing^{1, 2} and terrestrial ecosystems are becoming increasingly nitrogen-saturated³, causing more bioavailable nitrogen to enter groundwater and surface waters^{4, 5, 6}. Large-scale nitrogen budgets show that an average of about 20–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins^{7, 8}, indicating that substantial sinks for nitrogen must exist in the landscape⁹. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification^{6, 10, 11}. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.

1. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
2. Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996, USA
3. Odum School of Ecology, University of Georgia, Athens, Georgia 30602, USA
4. Eco-metrics, Inc., Tucker, Georgia 30084, USA
5. Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming 82071, USA
6. Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan 49060, USA
7. Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA
8. Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, USA
9. Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oregon 97331, USA
10. Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA
11. Division of Biology, Kansas State University, Manhattan, Kansas 66506, USA
12. Institute of Ecosystem Studies, Millbrook, New York 12545, USA
13. School of Life Sciences, Arizona State University, Tempe, Arizona 85287, USA
14. Pacific Northwest Research Station, US Forest Service, Corvallis, Oregon 97331, USA
15. Department of Natural Resources, University of New Hampshire, Durham, New Hampshire 03824, USA
16. Department of Biological Sciences, Virginia Tech, Blacksburg, Virginia 24061, USA
17. Department of Biology, Ball State University, Muncie, Indiana 47306, USA
18. Present addresses: US Environmental Protection Agency, Cincinnati, Ohio 45268, USA (J.J.B.); US Geological Survey, Tacoma, Washington 98402, USA (R.W.S.); School of Earth and Environmental Sciences, Washington State University, Vancouver Campus, Vancouver, Washington 98686, USA (D.J.S.).

Correspondence to: Patrick J. Mulholland^1,2 Correspondence and requests for materials should be addressed to P.J.M. (Email: mulhollandpj@ornl.gov).

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