Development of cryogenic extraction system for δ18O and δ2H measurement of water in soils and plants.

The stable isotope composition of water (δ¹⁸O and δ²H) represents a useful tool to distinguish among different water pools along the soil-plant-atmosphere continuum. Using δ²H and δ¹⁸O as tracers helps gain a better understanding of plant root water uptake and dominant ecohydrological processes. To determine which pools of water are used for plant physiologic functions and returned to the atmosphere by transpiration, a common approach is to analyze the isotopic composition of water in both soil and plant. Cryogenic water extraction (CWE; Orlowski et al., 2016) is the most widely used laboratory-based technique to extract water from soil samples for isotopic analysis. However, recent studies have shown that the extraction conditions (time, temperature, and vacuum) and soil physical and chemical properties may affect the extracted soil-water isotope composition even significantly.

We have developed an efficient and cost-effective cryogenic vacuum equipment to extract water from soil or vegetation and this presentation aims at discussing some preliminary results. The equipment has been specifically designed to meet the following requirements: i) enable to quantify the accuracy of a CWE continuous flow extraction line, and ii) identify a specific extraction standard protocol for soil and vegetation samples. Two experiments have been carried out to evaluate the isotope fractionation induced by the system and how different operational parameters (i.e. times and temperature of extraction) can affect the results. Firstly, a known water isotopic ratio was processed by the vacuum system to determine the measurement accuracy and reproducibility by comparing pre- and post-processed water isotopic signatures. The likely causes of observed biases induced by sample processing are assessed and a relevant correction procedure is suggested. Subsequently, measurements were carried out on replicated samples taken from two differently-textured soils that, after being dried, were saturated in the laboratory up to different water content values with water of known isotopic composition. Also, plant samples were collected from plants grown in a greenhouse and irrigated with water of known isotopic composition.

Water from all samples was extracted by our CWE system and then analyzed using an isotope ratio mass spectrometer in Gas Bench mode for analyses and in temperature conversion elemental analysis (TC/EA) mode for. Preliminary results have quantified the isotope fractions on average of -1.6 ‰ for δ¹⁸O and 14.2 ‰ for δ²H. Normalization of stable isotopes from unknown samples according to observed fractionation has enabled the observed bias to become virtually zero, leading to a replicate reproducibility of δ¹⁸O and δ²H for soil water of 0.6 ‰ and 3 ‰, respectively. The analyses carried out up to now did not find statistical evidence that the soil types and soil-water contents may affect the extraction method and the accuracy of our protocol.