Comparison of isotope pairing and N2:Ar methods for measuring sediment-denitrification-assumptions, modifications, and implications

Denitrification has been measured during the last few years using two different methods in particular: isotope pairing measured on a triple-collector isotopic ratio mass spectrometer and N-2:Ar ratios measured on a membrane inlet mass spectrometer (MIMS). This study compares these two techniques in short-term batch experiments. Rates obtained using the original N-2:Ar method were up to 3 to 4 times higher than rates obtained using the isotope pairing technique due to O-2 reacting with the N-2 during MIMS analysis. Oxygen combines with N-2 within the mass spectrometer ion source forming NO+ which reduces the N-2 concentration. The decrease in N-2 is least at lower O-2 concentrations and since oxygen is typically consumed during incubations of sediment cores, the result is often a pseudo-increase in N-2 concentration being interpreted as denitrification activity. The magnitude of, this oxygen effect may be instrument, specific. The reaction of O-2 with N-2 and the subsequent decrease in N-2 was only partly corrected using an O-2 correction curve for the relationship between N-2 and O-2 concentrations. The O-2 corrected N-2:Ar denitrification rates were-lower, but still did not match the isotope pairing rates and the variability between replicates was much higher. Using a copper reduction column heated to 600degreesC to remove all of the O-2 from the sample before MIMS analysis resulted in comparable rates (slightly-lower), and comparable variability between replicates, to the isotope pairing technique. The N-2:Ar technique determines the net N-2 production as the difference between N-2 production by denitrification and N-2 Consumption by N-fixation, while N-fixation has little effect on the isotope pairing technique which determines a rate very close to the gross N-2 production. When. the two different techniques were applied on the same sediment, the small difference in rates obtained by the two methods seemed to reflect N-fixation as also supported from measurements of ethylene production in acetylene enriched sediment cores. The N-2:Ar and isotope pairing techniques may be combined to provide simultaneous measurements of denitrification and N-fixation. Both techniques have several assumptions that must be met to achieve accurate rates; a number of tests are outlined that can be applied to demonstrate that these assumptions are being meet.