This project is part of the Swiss Long-term Forest ecosystem Research project (LWF). Objectives: The main objectives and questions of this project are: - to study the long-term trend of the soil solution chemistry in response to atmospheric deposition of acidifying substances. Do nutrient leaching and aluminum represent a potential risk for sensitive plants? - to identify the impact of N-deposition on the chemistry of the soil solution (nitrate leaching below the root zone?). In cooperation with the others teams involved in the LWF project, to identify the potential factors regulating the nitrate production (N deposition, litter quality, soil type, climate). - to estimate the soil water availability to plants to identify potential water stress. Can climatic changes cause a change in the water availability to trees (increasing drought stress for instance during the summer)? Methods The sampling and the analytical methods used for the soil solution monitoring were selected on the basis of recommendations of expert groups from the Forest Soil Coordinating Center of the ICP Forests program (ICP Forests manuals) and experts at the WSL. Soil solution is collected fortnightly in 7 long-term monitoring plots since 1997 or later (see map). The chemistry of the soil solution is monitored by sampling soil solution below the litter layer with gravitation plates and at three lower depths (15, 50 and 80cm) with tension lysimeters (ceramic cups). Vacuum (500 hPa) is applied to the suction cups before each sampling. In each plot, soil solution is collected at 8 sampling points and then pooled in the laboratory for chemical analysis. After filtration, samples are analyzed for pH, electrical conductivity, dissolved organic carbon (DOC), dissolved organic nitrogen (DON), major cations and anions. To assess the acidification status of the soil solution and the associated ecological risks for sensitive plants, the molar ratio of base cations (sum of the molar concentrations of Ca, Mg and K) to aluminum (BC/Al) in the soil solution is calculated. The plant-available soil water has been estimated in 10 long-term monitoring plots (see map) by measuring the soil water suction with water-filled tensiometers at three or four different depths since 1998 or 2000. The tensiometers are read manually fortnightly. At a dry forest plot (Visp), the volumetric soil water content has been measured since 2002 with TDR probes at three different depths. The water balance in the studied forest ecosystems is modeled with different types of models to simulate potential and actual evapotranspiration (PET and AET), drainage and water stress (AET/PET). Measurements of the soil moisture or the soil matric potential at different soil depths are used to calibrate and validate the simulated water fluxes. The simulated water fluxes are used to estimate element fluxes through the soil.