Characterizing nitrous oxide sources by mass spectrometric analyses of several isotopologues

Abstract:

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.