Denitrification in nitrate-rich streams: Application of N2:Ar and 15N-tracer methods in intact cores

Rates of benthic denitrification were measured using two techniques, membrane inlet mass spectrometry (MIMS) and isotope ratio mass spectrometry (IRMS), applied to sediment cores from two NO₃⁻‐rich streams draining agricultural land in the upper Mississippi River Basin. Denitrification was estimated simultaneously from measurements of N₂:Ar (MIMS) and ¹⁵N[N₂] (IRMS) after the addition of low‐level ¹⁵NO₃⁻ tracer (¹⁵N:N = 0.03–0.08) in stream water overlying intact sediment cores. Denitrification rates ranged from about 0 to 4400 μmol N·m⁻²·h⁻¹ in Sugar Creek and from 0 to 1300 μmol N·m⁻²·h⁻¹ in Iroquois River, the latter of which possesses greater streamflow discharge and a more homogeneous streambed and water column. Within the uncertainties of the two techniques, there is good agreement between the MIMS and IRMS results, which indicates that the production of N₂ by the coupled process of nitrification/denitrification was relatively unimportant and surface‐water NO₃⁻ was the dominant source of NO₃⁻ for benthic denitrification in these streams. Variation in stream NO₃⁻ concentration (from about 20 μmol/L during low discharge to 1000 μmol/L during high discharge) was a significant control of benthic denitrification rates, judging from the more abundant MIMS data. The interpretation that NO₃⁻ concentration directly affects denitrification rate was corroborated by increased rates of denitrification in cores amended with NO₃⁻. Denitrification in Sugar Creek removed ≤11% per day of the in‐stream NO₃⁻ in late spring and removed roughly 15–20% in late summer. The fraction of NO₃⁻ removed in Iroquois River was less than that of Sugar Creek. Although benthic denitrification rates were relatively high during periods of high stream flow, when NO₃concentrations were also high, the increase in benthic denitrification could not compensate for the much larger increase in stream NO₃⁻ fluxes during high flow. Consequently, fractional NO₃⁻ losses were relatively low during high flow.