Deep Seepage Estimation Under Forests in the North German Lowlands—A Deuterium Oxide Labeling Experiment in a Scots Pine and Douglas Fir Stand

Climate resilient tree species are increasingly used in the process of forest conversion toward climate-change adapted stands. Data to quantify the impact of this forest conversion on deep seepage is scarce. This study aims to estimate deep seepage in a native Scots pine (Pinus sylvestris) stand and in a nonnative Douglas fir (Pseudotsuga menziesii) stand applying the peak shift method, based on the analysis of a tracer peak movement in the vadose zone. Deep seepage was estimated by determining the water volume between the lower root zone limit and the tracer peak below the root zone. Deuterium oxide (²H₂O) was used as a conservative tracer using small amounts of irrigation water (5.1 mm) to remain close to the natural water contents but high ²H₂O-concentrations (calculated δ²H mixture concentration ≈111 000‰ VSMOW) compared to the natural ²H₂O background concentrations. After applying the ²H₂O-tracer, soil samples were taken five times at each location over a period of 15 months with sampling depths up to 4.6 m, and 0.1 m sampling intervals. Soil water was extracted using cryogenic vacuum distillation, and stable water isotope ratios were measured with cavity ring-down laser spectrometry. From April 2023 to April 2024, deep seepage in the Scots pine stand was 0–23 mm/a (0%–2.8% of gross precipitation) while in the Douglas fir stand it amounted to 0–14 mm/a (0%–1.7% of gross precipitation). The experiment was combined with the analysis of throughfall, tracer mass recovery, grain size, bulk density and tree rooting depths as well as with soil probe measurements of volumetric water contents and matric potentials. The combination of the methods was essential to define error ranges of the peak shift method and to understand soil water dynamics between the tracer sampling dates.