Fractionation of N2O isotopomers during production by denitrifier

Isotopomer ratios of N2O, which include intramolecular 15N-site preference in addition to conventional isotope ratios for N and O in NNO (we designate Nα and Nβ for the center and end N atom, respectively, in the asymmetric molecule), reflect production and consumption processes of this greenhouse gas. Therefore, they are useful parameters for deducing global N2O budget. This paper reports the first precise measurement of 15N-site preference in N2O produced by two species of denitrifying bacteria, Pseudomonas fluorescens (ATCC 13525) and Paracoccus denitrificans (ATCC 17741). es were incubated in a batch mode with a liquid medium that contains KNO3 as unique nitrogen supply under acetylene/helium (10% v/v) atmosphere at 27 °C. Enrichment factors for 15N in bulk nitrogen in N2O (average for Nα and Nβ) fluctuated in a few tens permil showing a slight difference between the species. In contrast, 15N-site preference (difference in isotope ratios between Nα and Nβ) showed nearly constant and distinct value for the two species (23.3±4.2 and −5.1±1.8‰ for P. fluorescens and P. denitrificans, respectively). The site preference was also measured for N2O produced by inorganic reactions (nitrite reduction and hydroxylamine oxidation); a unique value (about 30‰ for the both reactions) was obtained. These results and those recently reported for nitrifying bacteria suggest that 15N-site preference in N2O can be used to identify the production processes of N2O on the level of bacterial species or enzymes involved.