Why are water retention characteristics important?
The soil water retention curve (SWRC) describes the relationship between soil water content and soil water potential (Vogel and Cislerova, 1988). The SWRC not only provides detailed knowledge on the physical and hydraulic properties of soil, but also affects root water uptake and evaporation (Ciocca et al., 2014; Pan et al., 2019; Quade et al., 2018). The SWRC is considered one of the most important soil hydraulic properties (Hillel, 2004; Rawls and Brakensiek, 1989).
We know that soil physical properties are closely linked to the pore size distribution expressed by soil texture. However, we do not know much about how interactions between the soil matrix, water vapor and liquid water exchange within the subsurface are affecting the soil water isotopic composition. It is not necessarily correct to assume that soil water and vapor in vadose zones have the same isotopic compositions as equilibrated bulk water and vapor, respectively, which is underlined by a study of Lin et al. (2018). But evaporation from the soil surface, and thus the underlying soil water vapor, play an important role in the hydrologic cycle and affect the soil water (vapor) isotopic composition (Brooks, 2015; Soderberg et al., 2012). Most often, the Craig-Gordon model is applied to estimate equilibrium and kinetic isotopic fractionation during evaporation (Craig et al., 1965; Horita et al., 2008). However, significant deviations between measured and modelled values (from Craig-Gordon) of soil evaporate isotopic composition can occur (Braud et al., 2009; Haverd et al., 2011; Rothfuss et al., 2010). Soderberg et al. (2012) recommended to include the soil water potential effect on kinetic fractionation during soil water evaporation in the Craig-Gordon model and Quade et al. (2018) call for further investigations of the temporal dynamics of kinetic fractionation factors. This parameter can also be calculated from soil water content using an appropriate SWRC. It is however difficult to determine the exact behavior of the SWRC above (or below) the residual water content, as measurements are time consuming and data are scarce (Ciocca et al., 2014). Moreover, for dry conditions occurring preferentially at shallow soil depths where evaporation into the atmosphere takes place, the application of stable water isotope techniques with regard to the dry end of the water retention curve (especially around the wilting point) is largely unknown but nevertheless important (Gaj et al., 2019). Particularly under unsaturated conditions and when clay contents are high, the tightly bound soil water pool becomes more relevant (Adams et al., 2019; Bowers et al., 2020). Gaj et al. (2019) pointed out that not the water content but the soil tension is the dominant controlling factor on the isotopic equilibrium fractionation factor. Hence, a texture with high clay fraction and a water content of 10% will show a similar effect as a sandy texture at 1% water content. The authors further showed that the wettability of soil grains expressed by the contact angle between the water drop and the soil grain affects the equilibrium condition of bound water and water vapor.
Few studies have looked into the relevance of soil water held across different sized pores and water adsorbed on various soil materials with respect to their isotopic composition. Thus, our current knowledge on potential isotope fractionation effects is very limited and inconclusive as highlighted by Lin et al. (2018). This is problematic since much research is linking the soil water isotopic composition to that of plant water sources or atmospheric water vapor, which is further used for modeling processes within the plant-water-atmosphere continuum. A holistic assessment of soil water isotopes across various pore sizes and tensions is therefore needed to explore whether plants take up matrix water that is incompletely mixed with isotopically distinct mobile soil water. Thus, the objective of our lab experiments was to investigate the effect of water retention characteristics on the water isotopic composition of soil pore water. The null hypotheses guiding our work was that soil water sampled along the pF curve (experiment 1) and sequentially over a period of 7 days under a 15 bar pressure (experiment 2) shows the same isotopic composition among each other and does not differ isotopically from the introduced isotopic label. Additionally, we checked whether an isotopic exchange between the ceramic plate water of the pressure extractors and the sequentially extracted soil water occurred (experiment 2).