Sources of nitrate in rivers draining sixteen watersheds in the northeastern U.S.: Isotopic constraints |
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| Authors: | Mayer Bernhard Boyer Elizabeth W Goodale Christine Jaworski Norbert A van Breemen Nico Howarth Robert W Seitzinger Sybil Billen Gilles Lajtha Kate Nadelhoffer Knute Van Dam Douwe Hetling Leo J Nosal Miloslav Paustian Keith |
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| Institution: | (1) Departments of Geology & Geophysics and Physics & Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4;(2) College of Environmental Science and Forestry, State University of New York, 1 Forestry Drive, Syracuse, NY, 13210, U.S.A;(3) Department of Plant Biology, Carnegie Institution of Washington, 260 Panama St., Stanford, CA, 94305, U.S.A;(4) USEPA (retired), 2004 S. Magnolia Ave., Sanford, FL, 32771, U.S.A;(5) Laboratory of Soil Science and Geology and Wageningen Institute for Environment and Climate Research, Wageningen University, Wageningen, the Netherlands;(6) Department of Ecology & Environmental Biology, Cornell University, Corson Hall, Ithaca, NY, 14853, U.S.A;(7) Rutgers University, Institute of Marine and Coastal Sciences, Rutgers/NOAA CMER Program, 71 Dudley Road, New Brunswick, NJ, 08901, U.S.A;(8) UMR Sisyphe, University of Paris VI, 4 Place Jussieu, 75005 Paris, France;(9) Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR, 97331, U.S.A;(10) Marine Biological Laboratory, The Ecosystems Center, Woods Hole, MA, 02543, U.S.A;(11) Department of Energy and Environmental Engineering, Rensselaer Polytechnic Institute, Troy, New York, U.S.A;(12) Department of Mathematics & Statistics, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4;(13) Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado, 80523, U.S.A |
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| Abstract: | The feasibility of using nitrogen and oxygenisotope ratios of nitrate (NO3
–) forelucidating sources and transformations ofriverine nitrate was evaluated in a comparativestudy of 16 watersheds in the northeastern U.S.A. Stream water was sampled repeatedly at theoutlets of the watersheds between January andDecember 1999 for determining concentrations, 15N values, and 18Ovalues of riverine nitrate.In conjunction with information about land useand nitrogen fluxes,15Nnitrate and18Onitrate values providedmainly information about sources of riverinenitrate. In predominantly forested watersheds,riverine nitrate had mean concentrations ofless than 0.4 mg NO3
–-N L–1,15Nnitrate values of lessthan +5 , and 18Onitratevalues between +12 and +19. This indicatesthat riverine nitrate was almost exclusivelyderived from soil nitrification processes withpotentially minor nitrate contributions fromatmospheric deposition in some catchments. Inwatersheds with significant agricultural andurban land use, concentrations of riverinenitrate were as high as 2.6 mg NO3
–-NL–1 with 15Nnitratevalues between +5 and +8 and18Onitrate values generallybelow +15. Correlations between nitrateconcentrations, 15Nnitratevalues, and N fluxes suggest that nitrate inwaste water constituted a major, and nitrate inmanure a minor additional source of riverinenitrate. Atmospheric nitrate deposition ornitrate-containing fertilizers were not asignificant source of riverine nitrate inwatersheds with significant agricultural andurban land use. Although complementary studiesindicate that in-stream denitrification wassignificant in all rivers, the isotopiccomposition of riverine nitrate sampled at theoutlet of the 16 watersheds did not provideevidence for denitrification in the form ofelevated 15Nnitrate and18Onitrate values. Relativelylow isotopic enrichment factors for nitrogenand oxygen during in-stream denitrification andcontinuous admixture of nitrate from theabove-described sources are thought to beresponsible for this finding. |
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| Keywords: | denitrification nitrate nitrate sources rivers stable isotopes  15Nnitrate" target="_blank">gif" alt="delta" align="BASELINE" BORDER="0">15Nnitrate 18Onitrate" target="_blank">gif" alt="delta" align="BASELINE" BORDER="0">18Onitrate |
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