Sampling soil water along the pF curve for δ2H and
δ18O analysis
Natalie Orlowski1,2 and Lutz
Breuer1,3
1Institute for Landscape Ecology and Resources
Management (ILR), Research Centre for BioSystems, Land Use and Nutrition
(iFZ), Justus Liebig University Giessen, Giessen, 35392, Germany
2now at: University of Freiburg, Hydrology,
Friedrichstrasse 39, Freiburg im Breisgau, 79098, Germany
3Centre for International Development and
Environmental Research (ZEU), Justus Liebig University Giessen,
Senckenbergstrasse 3, Giessen, 35390, Germany
Correspondence to : Natalie Orlowski
(natalie.orlowski@hydrology.uni-freiburg.de)
Abstract. Soil water stable isotopes are widely used across
disciplines (e.g. hydrology, ecology, soil science, and
biogeochemistry). However, the full potential of stables isotopes as a
tool for characterizing the origin, flow path, transport processes and
residence times of water in different eco-, hydro-, and geological
compartments has not yet been exploited. This is mainly due to the large
variety of different methods for pore water extraction. While recent
work has shown that matric potential affects the equilibrium
fractionation, little work has examined how different water retention
characteristics might affect the sampled water isotopic composition.
Here, we present a simple laboratory experiment with two well-studied
standard soils differing in their physico-chemical properties (e.g.,
clayey loam and silty sand). Samples were sieved, oven-dried and spiked
with water of known isotopic composition to full saturation. For
investigating the effect of water retention characteristics on the
extracted water isotopic composition, we used pressure extractors to
sample isotopically labelled soil water along the pF curve. After
pressure extraction, we further extracted the soil samples via cryogenic
vacuum extraction. The null hypothesis guiding our work was that water
held at different tensions shows the same isotopic composition. Our
results showed that the sampled soil water differed isotopically from
the introduced isotopic label over time and sequentially along the pF
curve. Our and previous studies suggest caution in interpreting isotope
results of extracted soil water and a need to better characterize
processes that govern isotope fractionation with respect to soil water
retention characteristics. In the future, knowledge about soil water
retention characteristics could be applied to predict soil water
fractionation effects under natural and non-stationary conditions.