Proper Interpretation of Dissolved Nitrous Oxide Isotopes, Production Pathways, and Emissions Requires a Modelling Approach

Stable isotopes ([Image: see text] (15)N and [Image: see text] (18)O) of the greenhouse gas N(2)O provide information about the sources and processes leading to N(2)O production and emission from aquatic ecosystems to the atmosphere. In turn, this describes the fate of nitrogen in the aquatic environment since N(2)O is an obligate intermediate of denitrification and can be a by-product of nitrification. However, due to exchange with the atmosphere, the [Image: see text] values at typical concentrations in aquatic ecosystems differ significantly from both the source of N(2)O and the N(2)O emitted to the atmosphere. A dynamic model, SIDNO, was developed to explore the relationship between the isotopic ratios of N(2)O, N(2)O source, and the emitted N(2)O. If the N(2)O production rate or isotopic ratios vary, then the N(2)O concentration and isotopic ratios may vary or be constant, not necessarily concomitantly, depending on the synchronicity of production rate and source isotopic ratios. Thus prima facie interpretation of patterns in dissolved N(2)O concentrations and isotopic ratios is difficult. The dynamic model may be used to correctly interpret diel field data and allows for the estimation of the gas exchange coefficient, N(2)O production rate, and the production-weighted [Image: see text] values of the N(2)O source in aquatic ecosystems. Combining field data with these modelling efforts allows this critical piece of nitrogen cycling and N(2)O flux to the atmosphere to be assessed.