Seasonal isotope distribution in soil profiles and its implications for plant water uptake

The increase in average temperatures and change in precipitation patterns derived from climate change affect forests differently, varying from the species' composition and forest characteristics. Understanding the ecophysiological behavior of trees under climate change and its impacts on the hydrological processes on a catchment scale requires a multidisciplinary approach, with an initial focus on the interactions on the soil-plant-atmosphere continuum. To better characterize soil water availability dynamics and comprehend how it may be affected by climate change, water isotope ratios of conservative tracers (δ¹⁸O, δ²H) can be used as fingerprints of infiltration processes, providing information on the seasonal origin of soil water infiltrated in the vadose zone. This study aims to characterize profiles of water isotopes in soil water to evaluate its seasonal isotope distribution. This information will be essential for further evaluations of seasonal water use by trees, contributing to understanding processes from the plot to the catchment scale.

The study will be conducted in an experimental plot (DRAIN Station) in the Rosalia catchment (950 ha), located on the border between the Austrian states Burgenland and Lower Austria. The catchment elevation ranges from 385 to 725 m, with a mean annual precipitation of 790 mm and a mean annual temperature of 8.2 °C. The soils are predominantly Cambisols, and the main land use comprises forests, predominantly beech (Fagus sylvatica) and Norway spruce (Picea abies). The DRAIN Station is located upstream in a beech stand representative of the forest in the catchment and has an average slope of 16°. This plot is a permanent monitoring station part of the LTER (Long-Term Ecosystem Research), a global network focused on long-term measurements of nitrogen, carbon, and water balance. A variety of environmental variables are measured at plot and catchment scale, adding spatial and temporal heterogeneity in the evaluation of hydrological processes.

At the DRAIN Station, a transect of four soil profiles representative of the plot will be defined and soil samples will be collected every 5-10 cm down to 60 cm using a split spoon sampler. To determine the precipitation water isotope ratios, daily precipitation data collected at the catchment’s climate station will be analyzed. The soil and water samples will be analyzed in the laboratory for stable isotopes (δ¹⁸O, δ²H) using a Picarro laser-spectroscope. The mean monthly water isotope ratios in precipitation will be determined over 12 months and compared with the water isotope profiles of δ¹⁸O and δ²H.

These results will enhance the understanding of the infiltration processes and seasonal distribution of water fluxes in the vadose zone. Moreover, the spatial variability of isotope ratios among soil profiles, such as infiltration depth and velocity, will be assessed. By integrating the seasonal isotope distribution in soil profiles with transit time distribution and hydrological modeling, a deeper understanding of the hydrological processes across different scales can be achieved.