Xylem water in riparian willow trees (Salix alba) reveals shallow sources of root water uptake by in situ monitoring of stable water isotopes

Root water uptake (RWU) is an important critical zone process, as plants can tap various water sources and transpire these back into the atmosphere. However, knowledge about the spatial and temporal dynamics of RWU and associated water sources at both high temporal resolution (e.g. daily) and over longer time periods (e.g. seasonal) is still limited. We used cavity ring-down spectroscopy (CRDS) for continuous in situ monitoring of stable water isotopes in soil and xylem water for two riparian willow (Salix alba) trees over the growing season (May to October) of 2020. This was complemented by isotopic sampling of local precipitation, groundwater, and stream water in order to help constrain the potential sources of RWU. A local eddy flux tower, together with sap flow monitoring, soil moisture measurements, and dendrometry, was also used to provide the hydroclimatic and ecohydrological contexts for in situ isotope monitoring. In addition, respective bulk and twig samples of soil water and xylem water were collected to corroborate the continuous in situ data. The monitoring period was characterised by frequent inputs of precipitation, interspersed by warm dry periods, which resulted in variable moisture storage in the upper 20 cm of the soil profile and dynamic isotope signatures. This variability was greatly damped at 40 cm, and the isotopic composition of the subsoil and groundwater was relatively stable. The isotopic composition and dynamics of xylem water were very similar to those of the upper soil, and analysis using a Bayesian mixing model inferred that overall ∼90 % of RWU was derived from the upper soil profile. However, while for the soil water signatures, the direct equilibrium method showed good comparability with in situ results, for xylem water, the cryogenic extractions signatures were only moderately or not at all comparable. Sap flow and dendrometry data indicated that soil water availability did not seriously limit transpiration during the study period, though it seemed that deeper (>40 cm) soil water provided a higher proportion of RWU (∼30 %) in a drier period in the late summer. The study demonstrates the utility of prolonged real-time monitoring of natural stable isotope abundance in soil-vegetation systems, which has great potential for the further understanding of ecohydrological partitioning under changing hydroclimatic conditions.