Analysis Of Influence Factors And Calibration Of Direct Vapor Equilibration Method For Measuring Soil Water Isotopes

The Direct Vapor Equilibration-Laser Spectrometer（DVE-LS）method can be used to determine soil water stable isotopes(²H and ¹⁸O).Compared to other physical extraction methods for soil water isotopes,the DVE-LS method directly determines the isotopic values of equilibrated water vapor,which is then converted into soil water isotopic information by standard samples.This method is easy to operate and quick to measure samples,but the measurement results are susceptible to bias due to soil clay content,water content and temperature.Therefore,in this study,soil water isotope was used as the research object to investigate the effects of soil texture,soil water content and equilibrium temperature on soil water isotope values determined by DVE-LS and their causes through indoor soil rehydration and field soil test systems,and to propose corresponding correction methods.The DVE-LS was used to determine and calibrate the field soil water isotopes,and the measurements of soil water by vacuum extraction（CVD）were used as the reference isotope values to compare the consistency of the two measurements and evaluate the predictive ability of the different methods.The main conclusions are as follows:（1）The effect of soil texture and water content on the determination of soil water isotope values by the DVE-LS method was clarified.The isotopic deviation increased significantly when the ratio of clay grain content to water content was greater than 2 and increased with increasing clay grain content and decreasing water content.Correction equations forδ²H andδ¹⁸O deviation were established based on the above two factors（R²=0.93 and R²=0.72）.（2）The effect of different equilibrium temperatures on the determination of soil water isotope values by the DVE-LS method was investigated.Five soil samples with different textures were dried and then wetted to different water contents using reference water samples,and the samples were equilibrated at five temperatures to quantify the deviations between the DVE-LS determination of soil water isotopes and their influencing factors.δ2H was significantly more enriched than the reference water sample,and the enrichment decreased with increasing temperature,whileδ¹⁸O was not significantly different from the reference water sample.The correction equation forδ2H deviation（R²=0.67）was established based on the equilibrium temperature,clay content and water content and the interaction factors.The accuracy of soil water isotopes measured by DVE-LS was improved.different calibration methods were applied for the determination of Changwu and Suide soils by DVE-LS,and the predictive ability between different methods was verified.Using the CVD measurement results as a reference,the RMSE was reduced from 9.29‰to 4.18‰after calibration of water vapor equilibrium3 established at constant temperature for soil isotopes in Suide field,and from 4.07‰to 1.24‰and from 8.99‰to 4.14‰after calibration of water vapor equilibrium4 established at different temperatures for soil isotopes in Changwu and Suide,respectively.This study reveals the effects of equilibrium temperature,clay content and water content on the determination of soil water isotopes by DVE-LS,quantifies the resulting deviations and establishes a correction equation between the deviation values and the influencing factors to improve the accuracy of the method for obtaining soil water isotopes,which is important for promoting the research of DVE-LS in soil water isotopes.