Soil moisture dynamics of forest soils – magnitude, persistence and implications of soil water repellency

Soil water repellency has been observed across a range of ecosystems, including forests. Among other parameters such as organic matter content and quality, climate, and soil texture, the magnitude and persistence of water repellency is controlled by the initial soil moisture content. With the increasing risk of prolonged and recurrent drought events across Europe causing significant increases in tree mortality, the feedback between soil moisture and soil rewetting is of increasing importance, as delayed soil rewetting may prolong water stress beyond drought events and reduce the plant available water. In this study, we quantified the local contact angle (sessile drop method) and in-situ rewetting dynamics (electrical resistivity tomography), including their relationship with initial soil moisture (sorptivity), of forest soils with contrasting vegetation. The results showed a fundamental difference in the persistence of water repellency and rewetting dynamics between an oak and a spruce stand. Despite prolonged precipitation (> 100 mm) following a dry summer, the sandy loam topsoil under spruce did not rewet after rain events, indicating persistent water repellency and fast water percolation to greater depth via preferential flow paths. In contrast, the sandy loam topsoil under oak rewetted after rainfall and was unaffected by water repellency, despite the similarly high initial contact angle of about 85° of dry soil at both sites. Our results highlight the importance of moisture dependence and persistence of soil water repellency for plant-available soil water in forests. The striking persistence of low soil moisture in topsoil, in combination with shallow-rooted spruce may explain in parts the severe dieback of spruce across Europe. Furthermore, the hydrological response of repellency-affected forest sites is likely to be influenced by the feedback between initial soil moisture, soil wettability and its persistence, and soil rewetting dynamics.