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).