1 Introduction

Stable isotopes of water (2H and180) are valuable natural tracers to study soil water movement and mixing processes in the vadose zone. Precise measurements of soil water content and its isotopic composition at different depths are key to reliably quantify plant water uptake as well as the partitioning of evapotranspiration into evaporation and transpiration (e.g. Mahindawansha et al., 2018; Rothfuss et al., 2010). The storage and interaction of different soil water compartments (mobile vs. tightly bound water) in the vadose zone is affected by a variety of soil properties (Gaj et al., 2019); for instance, the interactions between minerals, organic matter, and microorganisms (Pronk et al., 2017) or the presence of macropores (Sprenger et al., 2019). For isotopic applications, these different properties can affect the reliable determination of the isotopic composition of soil waters.
It has been known for many years that bound soil water has a distinct isotopic identity compared to that of mobile water (Araguás-Araguás et al., 2000). Many studies have compared the various techniques available for sampling soil water held at different tensions. For example, mobile water sampled by suction cups, has a different isotopic composition than soil water extracted by cryogenic vacuum extraction, which is known to be a “brute force technique” (Brooks et al., 2010; Figueroa-Johnson et al., 2007; Orlowski et al., 2016a; Zhao et al., 2013). Further, the various existing soil water extraction methods for isotope analysis can be affected by soil water content (Hendry et al., 2015; Newberry et al., 2017; Wassenaar et al., 2008), texture (Koeniger et al., 2011; Orlowski et al., 2016b; West et al., 2006), clay mineral composition (Adams et al., 2019; Gaj et al., 2017; Oerter et al., 2014), carbonate content (Meißner et al., 2014), organic matter (Orlowski et al., 2016a) and the different pore spaces that may or may not be extracted via the different approaches (Orlowski et al., 2019, Kübert et al., 2020). Recently, Bowers et al. (2020) stressed the fact that mechanisms controlling the isotopic composition and exchange between the mobile and more tightly bound soil water pools in natural ecosystems are largely unexplored. This incomplete understanding leads to complications when interpreting soil water contributions to plant water uptake under different moisture conditions as well as an accurate partitioning of evapotranspiration.