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

Abstract

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 NO 3 − ‐rich streams draining agricultural land in the upper Mississippi River Basin. Denitrification was estimated simultaneously from measurements of N 2 :Ar (MIMS) and 15 N[N 2 ] (IRMS) after the addition of low‐level 15 NO 3 − tracer ( 15 N: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 N 2 by the coupled process of nitrification/denitrification was relatively unimportant and surface‐water NO 3 − was the dominant source of NO 3 − for benthic denitrification in these streams. Variation in stream NO 3 − 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 NO 3 − concentration directly affects denitrification rate was corroborated by increased rates of denitrification in cores amended with NO 3 − . Denitrification in Sugar Creek removed ≤11% per day of the in‐stream NO 3 − in late spring and removed roughly 15–20% in late summer. The fraction of NO 3 − 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 NO 3 concentrations were also high, the increase in benthic denitrification could not compensate for the much larger increase in stream NO 3 − fluxes during high flow. Consequently, fractional NO 3 − losses were relatively low during high flow.