J. W. Harvey M.A. Voytek J.K. Böhlke 2004 <p><span>We conducted an in-stream tracer experiment with Br and </span>¹⁵<span>N-enriched NO</span>₃^-<span> to determine the rates of denitrification and related processes in a gaining NO</span>₃^-<span> -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</span>₃^-<span>, NO</span>₂^-<span>, N</span>₂<span>, 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 d15N values of dissolved N</span>₂<span>. The reach-averaged rate of denitrification of surface-water NO</span>₃^-<span> indicated by the isotope tracer was approximately 120 ± 20 µmol m</span>^-2<span> h</span>^-1<span> (corresponding to zero- and first-order rate constants of 0.63 µmol L</span>^-1<span> h</span>^-1<span> and 0.009 h</span>^-1<span>, respectively). The overall rate of NO</span>₃^-<span> loss by processes other than denitrification (between 0 and about 200 µmol m</span>^-2<span> h</span>^-1<span>) probably was less than the denitrification rate but had a large relative uncertainty because the NO</span>₃^-<span> 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</span>₃^-<span> load substantially in the absence of NO</span>₃^-<span> sources, but the losses were more than offset by nitrification and groundwater NO</span>₃^-<span> inputs at a combined rate of about 500-700 µmol m</span>^-2<span> h</span>^-1<span>. Despite the importance of denitrification, the overall mass fluxes of N</span>₂<span> were dominated by discharge of denitrified groundwater and air-water gas exchange in response to changing temperature, whereas the flux of N</span>₂<span> 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</span>₃^-<span> -rich stream where other mass-balance methods were not suitable because of insufficient sensitivity or offsetting sources and sinks. Despite the increasing NO</span>₃^-<span> 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.</span></p> application/pdf 10.4319/lo.2004.49.3.0821 en ASLO Reach-scale isotope tracer experiment to quantify denitrification and related processes in a nitrate-rich stream, midcontinent United States article