Aluminum output fluxes from forest ecosystems in Europe: a regional assessment

Data from 89 forested catchments and plots across Europe were used to define empirical relationships between aluminum leaching and input fluxes of major ions, output fluxes of major ions, ecosystem parameters such as soil pH, and combinations of these. Forests that release dissolved Al to seepage or surface waters are located primarily in areas receiving the highest loading of acid rain, and the output flux of Al shows the highest correlations to the throughfall flux of inorganic nitrogen, the output fluxes of NO3-, H+, and SO4(2-), and the mineral soil pH. If the speciation of Al is taken to be Al3+ (an overestimate), Al is released in a nearly 1:1 molar charge ratio with the sum of NO3- and SO4(2-) in runoff or seepage water over a wide range of basepoor bedrock types and acid deposition across Europe. The empirical data point to a threshold range of N deposition of 80 to 150 mmolc N m(-2) yr(-1) and a (less clearly defined) range of S deposition of 100 to 200 mmolc SO4(-2) m(-2) yr(-1) above which Al released from forests exceeds 100 mmolc Al m(-2) yr(-1). Within this threshold range, the sites that release little or no dissolved Al are those that continue to assimilate input N and/or have high soil pH (>4.5).