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 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 $\text{NO}_{3}^{-},\text{NO}_{2}^{-},\text{N}_{2}$, 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 15N 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 $\text{L}^{-1}\ \text{h}^{-1}$ 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.