Fractionation of N2O isotopomers during production by denitrifier

Isotopomer ratios of N2O, which include intramolecular N-15-site preference in addition to conventional isotope ratios for N and O in NNO (we designate N-alpha and N-beta 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 N-15-site preference in N2O produced by two species of denitrifying bacteria, Pseudomonas fluorescens (ATCC 13525) and Paracoccus denitrificans (ATCC 17741). Cultures 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 degrees C. Enrichment factors for N-15 in bulk nitrogen in N2O (average for N-alpha and N-beta) fluctuated in a few tens permil showing a slight difference between the species. In contrast, N-15-site preference (difference in isotope ratios between N-alpha and N-beta) showed nearly constant and distinct value for the two species (23.3 +/- 4.2 and -5.1 +/- 1.8 parts per thousand 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 parts per thousand for the both reactions) was obtained. These results and those recently reported for nitrifying bacteria suggest that N-15-site preference in N2O can be used to identify the production processes of N2O on the level of bacterial species or enzymes involved. (c) 2005 Elsevier Ltd. All rights reserved.