INPUT-OUTPUT RELATIONS OF MAJOR IONS IN EUROPEAN FOREST ECOSYSTEMS

A framework for the assessment of the state and trend of forest ecosystems from ion budget data is presented. Atmospheric deposition is regarded as an input and seepage water as a system output. A simple compartment model, subdividing the forest ecosystem, is used to calculate ion balances and the corresponding internal H+ production/consumption processes. The 'buffer range' of the forest soil substantially determines the reaction of a system to input and internal generation of acidity. Forest soils in the carbonate buffer range are characterised by high seepage water pH (6.2-8.6). High concentration levels of HCO3 are balanced by alkali and alkali earth ions (M(b) or 'base' cations). In the cation exchange buffer range, pH values remain above pH 4.2 and concentrations of dissolved aluminium are low. With increasing proton load the chemical characteristics of the soil solid phase are changing. The consequences are a decrease in acid neutralisation capacity (ANC) and base saturation (BS), and the formation of interlayer Al. At BS values below 10-15%, H+ is buffered almost solely by the dissolution and exchange of aluminium (and/or iron) compounds. Acidity, mainly as Al3+ and H+, is transported with seepage water and thus influences deeper soil and bedrock layers. Examples of the mentioned budget types are given, utilizing data from long-term case studies in Germany.