A new laboratory approach to extract soil water for stable isotope analysis from large soil samples

The reliability of soil water stable isotope analysis is -among other things- based on a correct soil water extraction. Currently used extraction methods are prone to isotope fractionation (especially with clay samples) and exhibit shortcomings limiting and/or complicating their usage. A newly developed soil water extraction method –Circulating Air Soil Water Extraction– is based on the principle of complete evaporation and condensation of the soil water in a closed circuit. Owing to its simple design, there is no need for any chemicals, gases, high pressure or high-temperature regimes. On the other hand, at present, the proposed apparatus with four independent extraction slots can be used at most twice a day.

The experimental tests proved no significant isotope fractionation effects leading to erroneous results caused by the extraction. In all experiments, the δ¹⁸O and δ²H did not exceed the limits ± 0.2 ‰ and ± 2 ‰, respectively, which is fully acceptable for hydrologic studies. Extraction of pure water samples shifts the isotope composition by 0.04±0.06 ‰ and 0.06±0.35 ‰ for δ¹⁸O and δ²H, respectively.

Soil water extraction tests were conducted with five distinct soil types (loamy sand, sandy loam, sandy clay, silt loam, and clay) using 40-150 grams of pre-oven-dried soil, which was subsequently rehydrated to 10 and 20 % water content. The shift in the isotopic composition ranged from -0.04 and 0.07 ‰ for δ¹⁸O and from 0.4 to 1.3 ‰ for δ²H with the corresponding standard deviations ± (0.08 – 0.25) ‰ and ± (0.34 – 0.58) ‰. The results exhibit high accuracy which predetermines this method for high-precision studies where unambiguous specification of the water origin is required. The accuracy is adversely counterbalanced by a reduced number of processed samples per day: at present eight (2 x 4 simultaneously measured samples at four extraction slots).

The proposed extraction method has proven versatility in handling various soil types with different soil textures and water contents. The main advantages are the high accuracy of the results, simple design of the apparatus setup, low operating costs, time reduction in operating the device, easy maintenance, and the ability to process large soil samples providing large and representative quantities of soil water.