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

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 (~d¹⁵N, ~d¹⁵N^SP, and ~d¹⁸O). 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.