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Keywords:

  • denitrification;
  • dissolved organic carbon;
  • nitrogen;
  • nutrient cycling;
  • stable isotope;
  • surface water-groundwater interaction

[1] 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 AcOinfluences 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.