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

Abstract

Stable isotopes ( 15N and 18O) of the greenhouse gas N2O provide information about the sources and processes leading to N2O production and emission from aquatic ecosystems to the atmosphere. In turn, this describes the fate of nitrogen in the aquatic environment since N2O is an obligate intermediate of denitrification and can be a by-product of nitrification. However, due to exchange with the atmosphere, the values at typical concentrations in aquatic ecosystems differ significantly from both the source of N2O and the N2O emitted to the atmosphere. A dynamic model, SIDNO, was developed to explore the relationship between the isotopic ratios of N2O, N2O source, and the emitted N2O. If the N2O production rate or isotopic ratios vary, then the N2O 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 N2O 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, N2O production rate, and the production-weighted values of the N2O source in aquatic ecosystems. Combining field data with these modelling efforts allows this critical piece of nitrogen cycling and N2O flux to the atmosphere to be assessed.

From the Ground Up: Global Nitrous Oxide Sources are Constrained by Stable Isotope Values

“…6 ). To do this we filtered the data to include: (i) all reports of emitted N 2 O, regardless of the strength of the flux (two freshwater studies [ 13 , 46 ], two marine studies [ 16 , 43 ] and several soil studies) (see S2 Dataset ); (ii) isotope data in the soil profile that had concentrations of N 2 O >650 ppb v/v (or 200% ambient); and (iii) isotope data in freshwater and near-surface marine environments (depths >100 m) with dissolved N 2 O concentrations >200% saturation with respect to atmospheric N 2 O [ 70 ].…”

Controls on nitrous oxide production and consumption in reservoirs of the Ohio River Basin

“…Denitrification and nitrification have been identified as the major N 2 O‐producing pathways in soils and waters [ Firestone and Davidson , ; Thuss et al ., ]. Denitrification is a form of anaerobic respiration in which nitrate (NO 3 − ) is sequentially reduced to nitrite (NO 2 − ), nitric oxide (NO), nitrous oxide (N 2 O), and dinitrogen (N 2 ).…”

Hydrologic Connectivity Regulates Riverine N2O Sources and Dynamics

“…Stream-Emitted N 2 O Isotopes and N 2 O Source Characterization. We calculated the isotope ratios of instantaneous N 2 O emission from the stream surface using the method of Baulch et al 36 and Thuss et al 37 This method employs the kinetic isotope fractionation factors for N 2 O invasion and evasion to account for the differential transfer velocities of N 2 O isotopes during stream−atmosphere exchange. 38 To infer the sources and cycling mechanisms of N 2 O, we adopted the SP-versus-δ 15 N mapping approach originally developed by Koba et al 39 and later extended by Ibraim et al 40 to estimate the mixing fraction of N 2 O produced from nitrification (f nit ) and the partial reduction of N 2 O to N 2 by denitrification (f red ).…”