A stable isotope technique for measuring production and consumption rates of nitrous oxide in coastal waters

The coastal ocean is a globally important source of atmospheric nitrous oxide (N2O), yet understanding the behaviour of N2O in the near-shore water column is often hampered by allochthonous inputs of N2O and the potential for concurrent production and loss processes. A method has been developed to simultaneously measure rates of N2O production resulting from ammonium oxidation, and rates of production and loss due to denitrification, in regions of high N2O activity. Water samples are incubated with 15N-labelled substrates (15NH4+, 15NO3− or 15N2O) and time-series measurements of 15N2O concentrations are made by purge-and-trap gas chromatography/mass spectrometry (P+T GC/MS). Analytical precision is maintained at ±1% by the use of a fully labelled N2O internal standard (15N218O). Some N2O production rates obtained as part of a series of measurements in a coastal embayment (Bedford Basin, Nova Scotia) are shown.