ALBERT

Description: Abstract Spatiotemporal heterogeneity in soil water content is recognized as a common phenomenon, but heterogeneity in the hydrogen and oxygen isotope composition of soil water, which can reveal processes of water cycling within soils, has not been well studied. New advances are being driven by measurement approaches allowing sampling with high density in both space and time. Using in situ soil water vapor probe techniques, combined with conventional soil and plant water vacuum distillation extraction, we monitored the hydrogen and oxygen stable isotopic composition of soil and plant waters at paired sites dominated by grasses and Gambel's oak (Quercus gambelli) within a semi‐arid montane ecosystem over the course of a growing season. We found that sites spaced only 20 m apart had profoundly different soil water isotopic and volumetric conditions. We document patterns of depth‐ and time‐explicit variation in soil water isotopic conditions at these sites, and consider mechanisms for the observed heterogeneity. We found that soil water content and isotopic variability was damped under Quercus gambelli, perhaps due in part to hydraulic redistribution of deep soil water or groundwater by Quercus gambelli in these soils relative to the grass‐dominated site. We also found some support for H isotope discrimination effects during water uptake by Quercus gambelli. In this ecosystem, the soil water content was higher than that at the neighboring grass site, and thus 25% more water was available for transpiration by Quercus gambelli compared to the grass site. This work highlights the role of plants in governing soil water variation and demonstrates that they can also strongly influence the isotope ratios of soil water. The resulting fine‐scale heterogeneity has implications for the use of isotope tracers to study soil hydrology and evaporation and transpiration fluxes to improve understanding of water cycling through the soil‐plant‐atmosphere continuum.