Stream denitrification and total nitrate uptake rates measured using a field 15N tracer addition approach

We measured denitrification and total nitrate uptake rates in a small stream (East Fork of Walker Branch in eastern Tennessee) using a new field 15N tracer addition and modeling approach that quantifies these rates for entire stream reaches. The field experiment consisted of an 8-h addition of 99 atom% KNO3 and a conservative solute tracer. Two 15N tracer addition experiments were performed on consecutive days, the first under ambient NO concentra3 tions (23 mg N L21) and the second with a NO addition of approximately 500 mg N L21. We fit first-order NO 2 2 3 3 uptake and two-box denitrification models to the longitudinal measurements of tracer 15N in dissolved NO , N2, 3 and N2O in stream water to determine rates. Total NO uptake rates were 0.028 m21 (0.32 mg N m22 s21) and 0.01 3 m21 (1.6 mg N m22 s21) under ambient NO and with NO addition, respectively. Denitrification rates were 0.0046 2 2 3 3 m21 (uncertainty range of 0.002 to 0.008 m21) and 9 3 1025 m21 (uncertainty range of 3 3 1025 to 21 3 1025 m21) under ambient NO and with NO addition, respectively. Denitrification resulted almost exclusively in N2 production 2 2 3 3 (.99%) and comprised about 16% (610%) of total NO uptake rate under ambient NO concentrations and about 2 2 3 3 1% (61%) of total NO3 uptake rate with NO addition. Denitrification rate expressed on a mass flux basis was 3 about 12 mmol m22 h21 under ambient NO concentrations, a value within the range reported for other streams 3 with low NO concentrations. 3 Humans have greatly altered the nitrogen (N) cycle in recent decades, more than doubling the inputs of fixed N to the biosphere (Vitousek et al. 1997). The increased inputs 1 Corresponding author ([email protected]). 2 Present address: Department of Ecology and Evolutionary Biology, Cornell University, Corson Hall, Ithaca, New York 148532701. Acknowledgments We thank Jeff Houser, Ramie Wilkerson, and Erica Lewis for their help in the field and laboratory; Suzanne Thomas for analysis of 15N samples at the Marine Biological Lab; and Melody Bernot and Jennifer Tank for kindly providing the gas sampling vials. We also appreciate advice on sampling and sample analysis from Jennifer Tank and Melody Bernot. We thank David Harris, Stable Isotope Laboratory, University of California, Davis, for performing most of the 15N analysis. We benefited greatly from discussions with Jim McClelland on transformations of the isotope data and with Wil Wollheim on model development. We also thank two anonymous reviewers for their constructive comments on an earlier version of the manuscript. This work was supported by grants from the U.S. National Science Foundation (DEB-9815868 and DEB-0111410). have led to increased hydrologic export of N from landscapes and consequent large increases in the inputs of N, primarily as nitrate–nitrogen (NO -N), via rivers to estuaries 3 and coastal oceans (Howarth et al. 1996; Jordan and Weller 1996). The increases in N loading to streams and rivers have accelerated rates of eutrophication and the development of extensive areas of anoxia and may be linked to harmful algal blooms in a number of coastal ecosystems (Turner and Rabalais 1994; Nixon et al. 1996; Glasgow and Burkholder