The 18O 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 18O 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 18O signature of N2O. Here we studied O exchange during N2O formation in soil, using a novel combination of 18O and 15N 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 18O-enriched H2O into N2O exceeded their maxima achievable through reaction stoichiometry. Based on the retention of the enrichment ratio of 18O and 15N of NO 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 NO. We conclude that in soil, the main source of atmospheric N2O, the 18O 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 O2 and NO, in ratios reflecting reaction stoichiometr