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 δ 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. Copyright 2011 by the American Geophysical Union.