Influence of soil organic matter on stable water isotope analysis of soil pore water using the H2O(liquid)–H2O(vapor) vapor equilibration method

• δ 18 O and δ 2 H were analyzed using the direct liquid–vapor equilibration method. • Influence of soil texture, volumetric water content and organic matter were shown. • Mineral-water interactions significantly influenced δ 18 O and δ 2 H measurements. • A regression using volumetric water content and organic matter is presented. The direct liquid–vapor equilibration (DLVE) method is commonly used to analyze the stable isotope composition of soil pore water ( δ 18 O, δ 2 H) by laser spectroscopy. The influence of soil texture, soil volumetric water content (VWC), and the presence of organic matter impacting reliable results is not yet fully understood. We conducted two different experiments: in the first experiment, natural silty topsoil with 4 % organic matter, chemically washed soil with 1 % organic matter, and two non-organic sediments (sand and kaolinite) were used to study the impact of soil organic matter on the DLVE method for a gradient of volumetric porewater contents. In the second experiment, different concentrations of humic acids were mixed with sandy sediment and porewater of known isotopic composition to investigate whether organic matter alone or its interactions with the sediment impacted the DLVE measurements. Results showed an influence of VWC on measured vapor isotope compositions, especially during drier conditions, with an unexpected lower accuracy (larger bias from the known isotope ratio) for sandy sediment compared to kaolinite when VWC was below 0.15 cm 3 cm −3 . Besides VWC and soil texture, organic matter additionally altered the expected vapor isotope composition, as natural silty topsoil with 4 % organic matter showed lower accuracy at all volumetric water contents than the silty soil with only 1 % organic matter. Results of the second experiment showed that humic acids were significantly (p < 0.05) influencing DLVE outcomes regardless of the humic acid concentrations for the soil–water mixture samples, with a threshold behavior of markedly decreased accuracy above 2 % humic acid concentration for both δ 18 O and δ 2 H. As pure mixtures of water and humic acid generally had higher accuracy, the interaction of humic acid with the soil might be a factor. A multivariate regression analysis was carried out using volumetric water content and soil organic matter as independent variables to correct for biases. While for oxygen isotopes the results could be improved, hydrogen isotopes still showed low accuracy especially under dry conditions and larger percentages of humic acid. Our findings indicate that in addition to soil texture and volumetric water content, the interaction between the soil matrix and organic matter probably affects the DLVE method measurements, particularly at concentrations exceeding 0.1 g of humic acid per ml of water. In certain instances, this interaction may be corrected, thus addressing existing methodological gaps.