What stable water isotopes may tell us about belowground processes

Processes in the rooting zone and root water uptake decisively affect ecosystem resilience to stressors such as drought or deposition of air pollutants. However, the rooting zone is literally a black box. Water stable isotopes may shed light on some parts of this black box and tell us about water residence times, mixing of different water pools, plant water sources, preferential flow, soil evaporation, and many more. While this flexibility to use stable water isotopes is advantageous, it also entails many degrees of freedom. The additional collinearity of the hydrogen and oxygen isotopes leads to a largely underdetermined problem to solve. Bayesian mixing models help quantify the resulting uncertainty and add some constraints to the system by prior knowledge. Particularly in the case of disentangling plant water sources, additional data such as soil water content, matric potential or sap flow are needed since i) the isotope gradient smoothes with soil depth and ii) isotope-derived relative changes in root water uptake cannot be translated to absolute ones. Framing this ancillary data in a physically based water balance model may refine our predictions and help to trace water and related nutrient fluxes through the belowground. This contribution will summarize some of the experience gained during an in-situ monitoring study conducted in the drought summer of 2018 and a one-year sampling campaign at ten long-term ecosystem monitoring sites across Switzerland.