Isotopic signals across the forest water cycle

Forests greatly impact the water cycle by redistributing water between the atmosphere and the subsurface, via processes including interception, infiltration, and transpiration. In order to understand which water is redistributed among different ultimate fates, and thereby better understand the role of vegetation in that process, we use stable isotopes to quantify transport processes across the different compartments of the forest water cycle. At our Waldlabor hillslope laboratory (Zurich, Switzerland) we have frequently measured and sampled water fluxes across the forest water cycle since April 2020. Specifically, we measure the isotopic composition in precipitation, throughfall, stemflow, bulk and mobile soil waters in depths of 10, 20, 40 and 80cm as well as in deep mobile water (from boreholes in the unsaturated zone up to 7m depth), groundwater and surface discharge at the outlet of the catchment. We also assess soil water uptake by beech and spruce trees from destructive sampling of twig xylem every three weeks.

The isotopic composition in precipitation was similar to what we found in throughfall and stemflow, so canopy interception processes did not substantially alter the isotopic signal. The seasonal variation in precipitation isotope composition was strongly dampened with depth in subsurface storages (through the soil layers, deep mobile water, and groundwater to streamflow). The assessment of the new water fractions in soil waters of different depths and the deeper soil drip water showed that the fractions of new precipitation in each layer decreased with depth. Although precipitation is almost equally distributed throughout the year, we found that soil new water fractions were generally smaller in summer compared than in winter. Water in spruce and beech xylem has a similar isotope signal throughout the year, potentially suggesting use of deep sources that contain a relatively stable mixture of summer and winter water. Together, these data provide opportunities for new perspectives on how subsurface flows and vegetation interact.