4 Conclusions
Recent literature has called for a discrete isotopic sampling along the
pF curve (e.g. Gaj et al. (2019); Gaj and McDonnell (2019); McDonnell
(2014)). We have investigated the effect of water retention
characteristics on the water isotopic composition of soil pore water
sampled along the pF curve for two different soil types. The null
hypothesis guiding our work was that water held at different tensions
has the same isotopic composition. We tested this in two different
experiments where soils where spiked with a known isotopic label. We
collected soil water along the pF curve (Exp. 1) and sequentially over a
period of 7 days under a 15 bar pressure (Exp. 2). Our work has shown
that the sampled soil water differed isotopically from the introduced
isotopic label over time and sequentially along the pF curve.
Our results provide valuable insight into how soil water retention
characteristics affect the soil water isotopic composition sampled along
the pF curve. This has implications for interpreting the water isotopic
composition of mobile and bulk water in soils (with respect to the
tension water is held in the soil) and further modeling of the fast and
slow flow domain in the vadose zone (Sprenger et al., 2018).
Clearly, more research needs to be done. Future studies should consider
testing retention characteristics on a variety of different soil types,
so that retention curve approach parameters as in Gaj et al. (2019) can
be applied to predict soil water fractionation effects under natural and
non-stationary conditions.
Studies using soil water extraction techniques (e.g., suction cups vs.
cryogenic vacuum extraction) that apply different pressure levels for
extraction should consider the various effects pressure can have on the
soil water isotopic composition. This is particularly important when
different soil water pools are compared.
Our work and previous studies (e.g., Adams et al. (2019); Gaj et al.
(2019)) implies caution in interpreting isotope results of extracted
soil water and a need to better characterize processes that govern soil
water fractionation with respect to soil water retention
characteristics. We hope that with our study we contribute to stimulate
much needed new research in these areas.
Acknowledgements. We thank student assistant Julian Drescher for
his support during the lab experiment and Marcel Gaj for fruitful
discussions along the way.
Competing interests. The authors declare that they have no
conflict of interest.
Author contributions. NO and LB designed the experiment, NO
performed the experiment and data analysis. Both authors prepared the
manuscript.
Data Availability Statement: Data is available upon request.