Reach-Scale Application of 15N-Enriched Nitrate Tracer for Measuring Denitrification in a Nitrate-Rich Stream

Denitrification is one of the most important processes removing nitrate from streams and rivers before it is delivered to coastal ecosystems, but denitrification is difficult to quantify at the reach scale. We conducted a reach-scale isotopic tracer test to measure denitrification and other related processes in a nitrate-rich stream in the upper Mississippi River basin in September 2001. Bromide and ¹⁵N-enriched nitrate were injected into the stream for 7 hours, yielding plateau concentrations in the stream for a distance of approximately 1 km. Concentrations of bromide were used to determine transport parameters and ground-water inputs. The concentrations and isotopic compositions of nitrate, nitrite, and nitrogen gas were used to determine reach-scale rates of nitrate addition and denitrification. This was done by combining the data in a forward-stepping numerical model that simulated a parcel of water moving through the reach, subject to denitrification, nitrification, ground-water discharge, and air-water gas exchange with changing temperature. The mass fluxes of nitrogen gas were dominated by discharge of denitrified ground water and air-water gas exchange, whereas the flux attributable to denitrification was relatively small. Nevertheless, there was systematic ¹⁵N enrichment of the nitrogen gas going downstream indicating denitrification of surface-water nitrate. The reach-averaged rate of denitrification indicated by the isotopic tracer was equivalent to 130 μ mol/m²/hr (as benthic flux), 0.7 μ mol/L/hr (as zero-order rate constant), or 0.01/hr (as first-order rate constant). This rate of denitrification should have been sufficient to reduce the nitrate load substantially within the tracer reach; however, the denitrification loss was more than offset by addition of nitrate from a combination of nitrification and discharge of ground-water nitrate. Consequently, there was a net increase in the nitrate load even though denitrification was a major sink for N in the stream. The in-stream nitrate isotopic tracer experiment provided a sensitive direct reach-scale measurement of denitrification in a nitrate-rich stream where other mass balance methods were not definitive because of insufficient sensitivity or offsetting sources and sinks.