The Oxygen Isotopic Signature of Nitrous Oxide is Determined by Oxygen Exchange

In order to derive accurate budget estimations and effective mitigation strategies for the greenhouse gas nitrous oxide (N2O), it is essential to identify the processes involved in its production. Analyses of the isotopic composition of N2O are increasingly used to characterize the importance of these processes. However, we argue that the reliability of results based on oxygen (O) isotopic analysis of N2O may be questioned due to insufficient consideration of O exchange between H2O and nitrogen oxides. We studied the process of O exchange in 12 widely varying soils using a novel combination of 18O and 15N tracing experiments. Incorporation of O from 18O-enriched H2O into N2O exceeded theoretical maxima based on reaction stoichiometry, revealing the presence of O exchange. Novel methodology based on the retention of the 18O:15N-enrichment ratio of NO3- in N2O allowed to quantify O exchange during denitrification: up to 97% of N2O-O originated from H2O instead of NO3-. Our results show that in soil, the main source of N2O, the conventional assumption that the O isotopic composition of N2O is determined by reaction stoichiometry and isotopic fractionation during its production does not hold. In all cases, the O isotopic signature of N2O was found to be dominated by the effect of O exchange between nitrogen oxides and water. We speculate that the implications of O exchange will extend across terrestrial and aquatic ecosystems, and possibly to other nitrogen oxides as well. Especially, a potential effect of O exchange on the O isotopic signature of NO3- needs to be studied, as this is routinely used for NO3- source determination. Our results may facilitate the development of improved methodology to study and understand the global N cycle.