The O-18 signature of biogenic nitrous oxide is determined by O exchange with water

To effectively mitigate emissions of the greenhouse gas nitrous oxide (N2O) it is essential to understand the biochemical pathways by which it is produced. The O-18 signature of N2O is increasingly used to characterize these processes. However, assumptions on the origin of the O atom and resultant isotopic composition of N2O that are based on reaction stoichiometry may be questioned. In particular, our deficient knowledge on O exchange between H2O and nitrogen oxides during N2O production complicates the interpretation of the O-18 signature of N2O. Here we studied O exchange during N2O formation in soil, using a novel combination of O-18 and N-15 tracing. Twelve soils were studied, covering soil and land-use variability across Europe. All soils demonstrated the significant presence of O exchange, as incorporation of O from O-18-enriched H2O into N2O exceeded their maxima achievable through reaction stoichiometry. Based on the retention of the enrichment ratio of O-18 and N-15 of NO3- into N2O, we quantified O exchange during denitrification. Up to 97% (median 85%) of the N2O-O originated from H2O instead of from the denitrification substrate NO3-. We conclude that in soil, the main source of atmospheric N2O, the O-18 signature of N2O is mainly determined by H2O due to O exchange between nitrogen oxides and H2O. This also challenges the assumption that the O of N2O originates from O-2 and NO3-, in ratios reflecting reaction stoichiometry. Copyright (C) 2008 John Wiley & Sons, Ltd.