A novel system design to obtain daily in-situ soil and xylem water stable water isotope data in the Rosalia Research Forest (Austria)

The climate change induced increased frequency and intensity of drought and rainfall events impacts the interaction of trees with components of the hydrological cycle, e.g., rainfall, soil water, or groundwater. To study this, more often in-situ measurement systems are applied capable of high-resolution measurement of the stable water isotopes (d¹⁸O, d²H) of soil and xylem water. These systems often use gas-permeable probes to sample water vapor in isotopic equilibrium with the liquid xylem or soil water, which are connected to transport tubing that guide the vapor sample to the gas-inlet of a field-deployed isotope analyzer. Previous system designs used N₂ gas cylinders and mass flow controllers to provide a carrier gas for the water vapor sample and additionally to flush the transport lines in between sampling to avoid condensation that can lead to erroneous measurements. Here, we present a simplified version of an in-situ isotope measurement system that avoids using gas cylinders and mass flow controllers, showing first results obtained within the first six months of operation.

Daily sampling started in July 2025 for two soil water profiles (at depths of 10, 20, 30, and 60 cm) and xylem water of four beech (Fagus sylvatica S.) trees. Soil probes consist of a 10-cm long gas-permeable tube with a 1/8-inch tube inserted into it that transports water vapor samples to a Picarro laser spectrometer solely using a vacuum pump, i.e., pulling instead of pushing the sample. A second 1/8-inch tube inside the probe is connected to the atmosphere and allows for pressure equilibration using ambient air. For trees, contrary to previous systems that installed gas-permeable tubes in tree boreholes, we inserted two 1/8-inch tubes into boreholes with the same functions as for soil probes: transport and pressure equilibration using ambient air. To prevent condensation, we heated and additionally flushed the transport lines each night by pulling air through the tubes using a vacuum pump instead of pushing dry air through, thus avoiding gas cylinders and mass flow controllers.

First results of collecting daily data showed no major issues with our system. Comparing our measurements to collected precipitation isotopes of the same period, our data indicated a fast reaction of soil and xylem water to precipitation events. The obtained isotope ratios of soil and xylem water plot close to the local meteoric water line. Therefore, it is unlikely that the ambient air that is used to equilibrate the pressure significantly altered the water vapor isotope ratios. Approximately 60% of all samples had a relative humidity larger than 90% which is necessary for a reliable measurement. Cases of lower relative humidity could be explained by a drought experiment and large summer temperatures that naturally dry out soil. Future work will focus on improving the system after further analyses by, e.g., adapting the flushing period during the night, and by comparing its results to manually taken control samples.