M.A. Voytek J.K. Böhlke J. W. Harvey Lesley K. Smith 2006 <div class="article-section__content en main"><p>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<span>&nbsp;</span>¹⁵N[N₂] (IRMS) after the addition of low‐level<span>&nbsp;</span>¹⁵NO₃⁻<span>&nbsp;</span>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<span>&nbsp;</span>in Sugar Creek and from 0 to 1300 μmol N·m⁻²·h⁻¹<span>&nbsp;</span>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<span>&nbsp;</span>by the coupled process of nitrification/denitrification was relatively unimportant and surface‐water NO₃⁻<span>&nbsp;</span>was the dominant source of NO₃⁻<span>&nbsp;</span>for benthic denitrification in these streams. Variation in stream NO₃⁻<span>&nbsp;</span>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⁻<span>&nbsp;</span>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⁻<span>&nbsp;</span>in late spring and removed roughly 15–20% in late summer. The fraction of NO₃⁻<span>&nbsp;</span>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⁻<span>&nbsp;</span>fluxes during high flow. Consequently, fractional NO₃⁻<span>&nbsp;</span>losses were relatively low during high flow.</p></div> application/pdf 10.1890/1051-0761(2006)016[2191:DINSAO]2.0.CO;2 en Ecological Society of America Denitrification in nitrate-rich streams: Application of N2:Ar and 15N-tracer methods in intact cores article