Intercomparison of soil pore water extraction methods for stable isotope analysis

Abstract

Measurements of δ 2 H and δ 18 O composition of pore waters in saturated and unsaturated soil samples are routinely performed in hydrological studies. A variety of in-situ and lab-based pore water extraction methods for the analysis of the stable isotopes of water now exist.While some have been used for decades (e.g. cryogenic vacuum extraction) others are relatively new, such as direct vapor equilibration or the microwave extraction technique.Despite their broad range of application, a formal and comprehensive intercomparison of soil water extraction methods for stable isotope analysis is lacking and long overdue. Here we present an intercomparison among five commonly used lab-based pore water extraction techniques (high pressure mechanical squeezing, centrifugation, direct vapor equilibration, microwave extraction, and cryogenic extraction). We applied these extraction methods to two physicochemically different soil types that were dried and rewetted with water of known isotopic composition at three different water contents. Our results showed that the extraction approach can have a significant effect on pore water isotopic composition as all methods exhibited significant deviations from the spiked reference water, depending secondarily on the soil type and soil water content. Most pronounced, cryogenic water extraction showed large deviations from the spiked reference water, whereas mechanical squeezing and centrifugation provided results closest to the spiked water for both soil types. We also compared results for each extraction method -where liquid water was obtained -on both an OA-ICOS and IRMS. Differences between these two analytical instruments were negligible for these organic compound-free waters. We suggest that users of soil water extraction approaches carefully choose an extraction technique that is suitable for the specific research question, adapted to the dominant soil type and water content of the study.This article is protected by copyright. All rights reserved.

Membrane inlet laser spectroscopy to measure H and O stable isotope compositions of soil and sediment pore water with high sample throughput

“…The ‘true’ δ 2 H liq and δ 18 O liq values of soil water are difficult to determine. For example, Orlowski et al . showed that for standards made of oven‐dried soil with added water, vacuum extraction does not reliably yield extracted water with the same isotope composition as the added water.…”

“…Distillation of soil water with heat under vacuum with cryogenic collection and subsequent liquid water analysis has been considered to be the best approach, despite evidence that it is prone to unreliable results and artefacts induced during extraction . Mechanical squeezing and centrifugation have also been used as alternative methods for physical extraction of soil water …”

The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations

“…The isotope difference is thus significant for δ 18 O, less significant for 17 O-excess, according to the analytical error. Although evaporative kinetic fractionation of the top soil water suctioned by the porous cup under vacuum cannot be ruled out, isotopic exchanges between the soil water and oxygenbearing phases of the rhizosphere may also have impacted the soil water isotopic composition (Bowling et al, 2017;Chen et al, 2016;Oerter et al, 2014;Orlowski et al, 2016). Hereinafter, we consider the isotope signatures of the water absorbed by the roots of F. arundinacea to be equivalent to the irrigation water that fed the saturation level at the base of the tank.…”

Soil water migration in the unsaturated zone of semiarid region in China from isotope evidence

“…Stable isotopes provide evidence for assessing plant water sources according to their variations caused by equilibrium and kinetic isotopic fractionation mechanisms (Orlowski et al, 2016a;Haverd et al, 2011;Zhao et al, 2013;Bhatia et al, 2011). Several studies (Liu et al, 2013;Mathieu and Bariac, 1996;Allison and Barnes, 1985) have shown that transpiration do not cause isotopic fractionation of soil water.…”