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Labile dissolved organic carbon supply limits hyporheic denitrification

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Abstract
  • We used an in situ steady state ¹⁵N-labeled nitrate (¹⁵NO₃⁻) and acetate (AcO⁻) well-to-wells injection experiment to determine how the availability of labile dissolved organic carbon (DOC) as AcO⁻ influences microbial denitrification in the hyporheic zone of an upland (third-order) agricultural stream. The experimental wells receiving conservative (Cl⁻ and Br) and reactive (¹⁵NO₃⁻) solute tracers had hyporheic median residence times of 7.0 to 13.1 h, nominal flowpath lengths of 0.7 to 3.7 m, and hypoxic conditions (<1.5 mg O₂ L⁻¹). All receiving wells demonstrated ¹⁵N₂ production during ambient conditions, indicating that the hyporheic zone was an environment with active denitrification. The subsequent addition of AcO⁻ stimulated more denitrification as evidenced by significant δ¹⁵N₂ increases by factors of 2.7 to 26.1 in receiving wells and significant decreases of NO₃⁻ and DO in the two wells most hydrologically connected to the injection. The rate of nitrate removal in the hyporheic zone increased from 218 kg ha⁻¹ yr⁻¹ to 521 kg ha⁻¹ yr⁻¹ under elevated AcO⁻ conditions. In all receiving wells, increases of bromide and ¹⁵N₂ occurred without concurrent increases in AcO⁻, indicating that 100% of AcO⁻ was retained or lost in the hyporheic zone. These results support the hypothesis that denitrification in anaerobic portions of the hyporheic zone is limited by labile DOC supply.
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  • Zarnetske, J. P., R. Haggerty, S. M. Wondzell, and M. A. Baker (2011), Labile dissolved organic carbon supply limits hyporheic denitrification, Journal of Geophysical Research, 116, G04036, doi:10.1029/2011JG001730.
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  • 116
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  • G4
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  • The published article can be found at AGU - American Geophysical Union.
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  • Support for this project was primarily provided by a NSF Ecosystem Informatics IGERT fellowship (grant DGE-0333257) to J.P.Z.; research grants from the OSU Institute for Water and Watersheds and a North American Benthological Society Endowment Fund to J.P.Z.; and NSF grants EAR-0409534 and EAR-0409591 to R.H., S.M.W., and M.A.B. Additional support was provided by the Hollis M. Dole Environmental Geology Fund at OSU.
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