The Isotopomers of Nitrous Oxide: Analytical Considerations and Application to Resolution of Microbial Production Pathways

Abstract

Nitrous oxide (N2O) is an important greenhouse gas and the most important substance involved in ozone depletion. The N2O molecule consists of two N atoms that differ in the nature of their covalent bonds and consequently tend to acquire distinct isotopic compositions. Thus, N2O contains both bulk (δ15N and δ18O) and site specific isotopic information. Site preference (SP) is defined as the difference in δ15N between the central (α) and outer (β) N atoms in N2O. SP has emerged as a potential conservative tracer of microbial N2O production because (1) it is independent of the isotopologue composition of the substrates of nitrification and denitrification and (2) does not exhibit fractionation during production. In pure microbial culture distinct SP values for N2O production from bacterial denitrification, including nitrifier denitrification (−10 to 0 ‰), relative to hydroxylamine oxidation and fungal denitrification (33–37 ‰) provides a basis to resolve production pathways in the natural environment. Future directions for isotopomer research include (1) improvements in calibration and mass overlap corrections, (2) evaluation of SP signals of microbial production pathways not yet studied including heterotrophic nitrification, codenitrification, and dissimilatory reduction of nitrate to ammonium, and (3) evaluation of the potential for biological production to produce a 17O anomaly in atmospheric N2O.