No influence of CO(2) on stable isotope analyses of soil waters with off‐axis integrated cavity output spectroscopy (OA‐ICOS)

RATIONALE: It was recently shown that the presence of CO(2) affects the stable isotope (δ(2)H and δ(18)O values) analysis of water vapor via Wavelength‐Scanned Cavity Ring‐Down Spectroscopy. Here, we test how much CO(2) is emitted from soil samples and if the CO(2) in the headspace influences the isotope analysis with the direct equilibration method by Off‐Axis Integrated Cavity Output Spectroscopy (OA‐ICOS). METHODS: The headspace above different amounts of sparkling water was sampled, and its stable isotopic composition (δ(2)H and δ(18)O values) and CO(2) concentration were measured by direct equilibration and by gas chromatography, respectively. In addition, the headspace above soil samples was analyzed in the same way. Furthermore, the gravimetric water content and the loss on ignition were measured for the soil samples. RESULTS: The experiment with the sparkling water showed that CO(2) does not influence the stable isotope analysis by OA‐ICOS. CO(2) was emitted from the soil samples and correlated with the isotopic fractionation signal, but no causal relationship between the two was determined. Instead, the fractionation signal in pore water isotopes can be explained by soil evaporation and the CO(2) can be related to soil moisture and organic matter which both enhance microbial activity. CONCLUSIONS: We found, despite the high CO(2) emissions from soil samples, no need for a post‐correction of the pore water stable isotope analysis results, since there is no relation between CO(2) concentrations and the stable isotope results of vapor samples obtained with OA‐ICOS. © 2016 The Authors. Rapid Communications in Mass Spectrometry Published by John Wiley & Sons Ltd.