Assessing the isotopic biases of soil water from cryogenic water extraction in different soil types in China

The utilization of deuterium (δ²H) and oxygen (δ¹⁸O) isotope ratios in cryogenically extracted water from soil samples is a widely employed method in hydrological and ecological research. Nevertheless, an increasing body of research indicates that cryogenic water extraction (CWE) leads to δ²H depletion in soil water. To investigate the widespread existence of this phenomenon, samples from eight physicochemically distinct soils in China underwent rehydration with a reference water at five different water contents and were subsequently extracted using CWE. In comparison to the reference water, significant and inconsistent δ²H depletion was observed in all eight soil samples. The δ¹⁸O bias also exhibited variation, ranging from enrichment to depletion. Generally, Z score assessments indicated unacceptable results for all soils. Water content emerged as the most influential variable affecting both δ²H and δ¹⁸O biases, while soil properties had different impacts on these biases. Source water, as calculated by a linear regression model, revealed that the isotopic composition of extracted soil water differed from that of the reference water. The cryogenic extraction error in soil water could not solely attributed to fractionation processes during the extraction but resulted from the release of tightly bound soil water into the reference water. Using the influencing factors, correction models for δ²H and δ¹⁸O biases by CWE were developed. By these models, the δ²H and δ¹⁸O biases were mostly successful corrected. High soil water extraction efficiency (e.g., 99%) was recommended to minimize isotopic biases. These efforts necessitate further testing, particularly in ecohydrological studies involving isotope measurements of soil water through CWE.