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

We conducted an in‐stream tracer experiment with Br and 15 N‐enriched NO 3 − to determine the rates of denitrification and related processes in a gaining NO 3 − ‐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 NO 3 − , NO 2 − , N 2 , 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 δ 15 N values of dissolved N 2 . The reach‐averaged rate of denitrification of surface‐water NO 3 − 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 NO 3 − 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 NO 3 − load was large and was increasing through the reach. The rates of denitrification and other losses would have been sufficient to reduce the stream NO 3 − load substantially in the absence of NO 3 − sources, but the losses were more than offset by nitrification and groundwater NO 3 − inputs at a combined rate of about 500‐700 µmol m −2 h −1 . Despite the importance of denitrification, the overall mass fluxes of N 2 were dominated by discharge of denitrified groundwater and air‐water gas exchange in response to changing temperature, whereas the flux of N 2 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 NO 3 − ‐rich stream where other mass‐balance methods were not suitable because of insufficient sensitivity or offsetting sources and sinks. Despite the increasing NO 3 − 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.