Reach‐scale isotope tracer experiment to quantify denitrification and related processes in a nitrate‐rich stream, midcontinent United States

Abstract

We conducted an in‐stream tracer experiment with Br and 15N‐enriched NO3− to determine the rates of denitrification and related processes in a gaining NO3− ‐rich stream in an agricultural watershed in the upper Mississippi basin in September 2001. We determined reach‐averaged rates of N fluxes and reactions from isotopic analyses of NO3−, NO2−, N2, and suspended particulate N in conjunction with other data in a 1.2‐km reach by using a forward time‐stepping numerical simulation that included groundwater discharge, denitrification, nitrification, assimilation, and air‐water gas exchange with changing temperature. Denitrification was indicated by a systematic downstream increase in the δ15N values of dissolved N2. The reach‐averaged rate of denitrification of surface‐water NO3− indicated by the isotope tracer was approximately 120 ± 20 µmol m−2 h−1 (corresponding to zero‐ and first‐order rate constants of 0.63 µmol L−1 h−1 and 0.009 h−1, respectively). The overall rate of NO3− loss by processes other than denitrification (between 0 and about 200 µmol m−2 h−1) probably was less than the denitrification rate but had a large relative uncertainty because the NO3− load was large and was increasing through the reach. The rates of denitrification and other losses would have been sufficient to reduce the stream NO3− load substantially in the absence of NO3− sources, but the losses were more than offset by nitrification and groundwater NO3− inputs at a combined rate of about 500‐700 µmol m−2 h−1. Despite the importance of denitrification, the overall mass fluxes of N2 were dominated by discharge of denitrified groundwater and air‐water gas exchange in response to changing temperature, whereas the flux of N2 attributed to denitrification was relatively small. The in‐stream isotope tracer experiment provided a sensitive direct reach‐scale measurement of denitrification and related processes in a NO3− ‐rich stream where other mass‐balance methods were not suitable because of insufficient sensitivity or offsetting sources and sinks. Despite the increasing NO3− load in the experimental reach, the isotope tracer data indicate that denitrification was a substantial permanent sink for N leaving this agricultural watershed during low‐flow conditions.