Nitrogen deposition in the range of 10–40 kg N ha−1 a−1 is common in large parts of Europe. Substancial amounts are deposited as NH4+ having fertilization and acidification effects in ecosystems. In a long term experiment the reactions of different compartments of a forest ecosystem were studied when the system became N saturated by continuously applying (NH4)2SO4. The experiment was conducted in a beech forest and the application of 10 kmolc N ha−1 a−1 lasted 11 yr from 1983 till 1993.
The results revealed that despite the high soil acidity, the applied NH4+ was quickly oxidized to NO3− in the surface 10 cm soil layer and leached to deeper depths. The amount of NO3− leached from the surface soil increased during the initial three years and remained constant on a high level for the rest of the experimental period. Nitrification was associated with acidification of the soil solution, causing high concentrations of Al and Mn2+ in soil solutions. More than 50% of total Al in solution occurred in non-phytotoxic form (Al–SO4 complexes). Moreover, concentration of base cations and dissolved organic carbon increased. Concentrations of SO42− in soil solutions increased during the first few years approaching more or less constant values in the surface 40 cm depth, whereas in 40–100 cm depth it took about 10 yr to reach those levels of sulphate concentrations in soils, indicating its retention in the deeper soil layers. No significant change in the chemistry of throughfall water and leaves was observed, indicating to N-saturation of the trees.