Characterizing nitrous oxide sources by mass spectrometric analyses of several isotopologues

Monday, 15 December 2014: 2:40 PM
Paul Magyar, Victoria J Orphan and John M Eiler, California Institute of Technology, Pasadena, CA, United States
Nitrous oxide can be produced by a broad range of processes under various environmental conditions, making it generally difficult to establish the sources of N2O. We have developed high resolution mass spectrometric techniques to provide six-dimensional isotopic fingerprints of N2O. On a single sample, we measure δ15N, δ18O, ∆17O, 15N site preference (‘SP’), and the clumped isotopologues 14N15N18O and 15N14N18O. The relative abundance of these clumped isotopologues is expressed as (1) a 15N site preference for 18O-containing isotopologues (‘SP18’) and (2) an overall preference for clumped isotopologues, ∆(14N15N18O+15N14N18O), over the expectation for a random distribution of isotopes among isotopologues. Each of these six isotopic parameters distinctively combines information about substrates, equilibrium exchange reactions, diffusive transport, mixing, and the kinetics of production and consumption mechanisms. We will describe the preliminary characterization of N2O produced by various biotic and abiotic sources. In particular, nitrous oxide from a genetic mutant of the denitrifying bacterium Pseudomonas aeruginosa strain PA14 that lacks the ability to reduce N2O to N2 is shown to have a site preference consistent with past measurements of denitrifiers. Its clumped isotopic composition is also distinct from predicted values for equilibrium and provides a signature for N2O production by a denitrifier. Measurements of ∆17O from two experiments with different starting nitrate ∆17O can be used to determine both the effective 18O/16O fractionation factor for reduction of nitrate to N2O (37±4‰), as well as the fraction of O atoms exchanged with water for this organism (52±6%). N2O collected from Lake Vida, Antarctica, has a site preference and clumped isotope composition distinct from that produced by P. aeruginosa. Also, both SP and ∆(14N15N18O+15N14N18O) in Lake Vida N2O are negative, which places constraints on its production and processing, and suggests synthesis at a transition state quite distinct from either the known NO reductase enzymes or the structure of N2O itself.