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.