Fractionation of N sub(2)O isotopomers during production by denitrifier

Isotopomer ratios of N sub(2)O, which include intramolecular super(15)N-site preference in addition to conventional isotope ratios for N and O in NNO (we designate N super( alpha ) and N super( 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 N sub(2)O budget. This paper reports the first precise measurement of super(15)N-site preference in N sub(2)O 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 KNO sub(3) as unique nitrogen supply under acetylene/helium (10% v/v) atmosphere at 27 C. Enrichment factors for super(15)N in bulk nitrogen in N sub(2)O (average for N super( alpha ) and N super( beta )) fluctuated in a few tens permil showing a slight difference between the species. In contrast, super(15)N-site preference (difference in isotope ratios between N super( alpha ) and N super( beta )) showed nearly constant and distinct value for the two species (23.3 plus or minus 4.2 and-5.1 plus or minus 1.8 for P. fluorescens and P. denitrificans, respectively). The site preference was also measured for N sub(2)O 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 super(15)N-site preference in N sub(2)O can be used to identify the production processes of N sub(2)O on the level of bacterial species or enzymes involved.