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.