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

To effectively mitigate emissions of the greenhouse gas nitrous oxide (N 2 O) it is essential to understand the biochemical pathways by which it is produced. The 18 O signature of N 2 O is increasingly used to characterize these processes. However, assumptions on the origin of the O atom and resultant isotopic composition of N 2 O that are based on reaction stoichiometry may be questioned. In particular, our deficient knowledge on O exchange between H 2 O and nitrogen oxides during N 2 O production complicates the interpretation of the 18 O signature of N 2 O. Here we studied O exchange during N 2 O formation in soil, using a novel combination of 18 O and 15 N 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 18 O‐enriched H 2 O into N 2 O exceeded their maxima achievable through reaction stoichiometry. Based on the retention of the enrichment ratio of 18 O and 15 N of NO   3 −into N 2 O, we quantified O exchange during denitrification. Up to 97% (median 85%) of the N 2 O‐O originated from H 2 O instead of from the denitrification substrate NO   3 − . We conclude that in soil, the main source of atmospheric N 2 O, the 18 O signature of N 2 O is mainly determined by H 2 O due to O exchange between nitrogen oxides and H 2 O. This also challenges the assumption that the O of N 2 O originates from O 2 and NO   3 − , in ratios reflecting reaction stoichiometry. Copyright © 2008 John Wiley & Sons, Ltd.