N₂O isotope approaches for source partitioning of N₂O production and estimation of N₂O reduction – validation with the ¹⁵N gas-flux method in laboratory and field studies

Abstract

The approaches based on natural abundance N2O stable isotopes are often applied for the estimation of mixing proportions between various N2O-producing pathways as well as for estimation of the extent of N2O reduction to N2. But such applications are associated with numerous uncertainties; hence, their limited accuracy needs to be considered. Here we present the first systematic validation of these methods for laboratory and field studies by applying the 15N gas-flux method as the reference approach. Besides applying dual-isotope plots for interpretation of N2O isotopic data, for the first time we propose a three dimensional N2O isotopocule model based on Bayesian statistics to estimate the N2O mixing proportions and reduction extent based simultaneously on three N2O isotopic signatures (δ15N, δ15NSP, and δ18O). Determination of the mixing proportions of individual pathways with N2O isotopic approaches often appears imprecise, mainly due to imperfect isotopic separation of the particular pathways. Nevertheless, the estimation of N2O reduction is much more robust, when applying an optimal calculation strategy, typically reaching an accuracy of N2O residual fraction determination of about 0.15.