Technical note: Simple, exact and reliable way to extract soil water for stable isotope analysis

Water stable isotope analysis in ecohydrological studies often requires soil water extraction. Here, we present a new soil water extraction method based on the principle of complete evaporation and condensation of the soil water in a lose circuit. We have developed an apparatus that has four extraction slots and can be used multiple times a day. Thanks to its simple design, there is no need for any chemicals, gases, high pressure or high-temperature regimes. A set of system functionality tests confirmed that the extraction method has high accuracy and high precision and does not cause any isotope fractionation effects leading to erroneous results. When extracting pure water samples, the accuracy is 0.04 ‰ and 0.06 ‰ for δ18O and δ2H, respectively, with a precision of ± 0.06 ‰ and ± 0.35 ‰ respectively. Soil water extraction tests were conducted with three soil types (loamy sand, sandy loam and sandy clay) using 50–80 grams of soil and water content of 20 %. The accuracy for the extraction of oven-dried and rehydrated soils ranged between -0.04 and 0.03 ‰ for δ18O and 0.06 and 0.68 ‰ for δ2H with precision of ± 0.06 to 0.13 ‰ and ± 0.34 to 0.58 ‰ for δ18O and δ2H, respectively in individual tests. These results are more accurate than results achieved by cryogenic vacuum extraction, which is the most widely used extraction method for soils. So far, our method was only tested for soil water extractions.