Challenges in N2O isotope measurements using CRDS analysers

Nitrous oxide (N2O) has a significant global warming potential of about 300 times that of CO2 and a steadily rising atmospheric concentration. Therefore, understanding N2O production and consumption pathways in major source ecosystems, such as agricultural soils, coupled with accurate quantification of associated N2O emissions, is critically important in the context of climate change.The relative abundance of 15N and 18O substituted N2O isotopocules (e.g.,14N15N16O, 15N14N16O, 14N14N18O) to the predominant isotopic form (14N14N16O) serves as valuable tracers for the distinction between important biogeochemical soil processes, such as nitrification and denitrification, which in turn enhances our understanding of N2O emissions. In this regard, advances in cavity-ring-down-spectroscopy (CRDS) have enabled precise measurement of isotopic species in ambient N2O, which holds key advantages over isotope ratio mass spectrometry in its ability to measure online, on-site and site-specific with respect to 15N substitution in the N2O-molecule.Despite the CRDS technique's ease in measuring the isotopic composition of N2O-isotopocules, the apparent isotope data requires significant post-processing, since spectral fitting is controlled by a complex interplay between fundamental physical parameters, instrumental parameters, gas matrix composition, instrumental drift, and fitting algorithms, some of which also depend on the absolute N2O concentration. Therefore, to retrieve accurate and comparable results, it is necessary to apply appropriate reference gases with minimal differences in gas composition to the sample in the measurement sequence. Remaining deviations between sample and reference have to be post-corrected using predefined, analyser-specific correction functions.This work provides a comprehensive and detailed correction and calibration protocol, exemplified by the reduction of N2O isotopic data obtained from a Picarro G5131-i isotopic and gas concentration analyser. This protocol outlines the theoretical and mathematical framework for the necessary corrections and suggests a logical order for applying these corrections. Moreover, the protocol provides a standalone MATLAB code for streamlined and automatic data reduction that can be employed once the required analyser-specific correction functions are established. The developed algorithms were validated on a suite of target gases, which accounts for concentration-based interferences from various species.With this protocol, we aim to enable researchers to accurately and efficiently acquire high-quality N2O isotope data from CRDS instruments and similar devices and contribute to standardized community guidelines for post-processing N2O isotope data. In a prototype application, we analysed N2O from automated flux chambers to track biogeochemical processes in agricultural soils. Subsequently, these insights will be integrated into soil biogeochemical models, to enable upscaling of emission data.