Labile dissolved organic carbon supply limits hyporheic denitrification

Abstract

We used an in situ steady state 15 N-labeled nitrate ( 15 NO 3 ) 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 ( 15 NO 3 ) 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 O 2 L 1 ). All receiving wells demonstrated 15 N 2 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 d 15 N 2 increases by factors of 2.7 to 26.1 in receiving wells and significant decreases of NO 3 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 15 N 2 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.