Intensive monitoring of forest ecosystems in Europe; 2: atmospheric deposition and its impacts on soil solution chemistry

In order to gain a better understanding of the effects of air pollution and other stress factors on forests, a Pan-European programme for intensive and continuous monitoring of forest ecosystems has been implemented in 1994. Results of this intensive monitoring programme presented in this paper are related to the surveys on atmospheric deposition and soil solution chemistry, focusing on data for 1997. Both bulk deposition and throughfall of N was higher than that of S at nearly all the plots, especially in Western Europe. The average calculated total N deposition was approximately twice as large as S. Approximately, 55% of the considered plots received an N input above 1000 molc ha-1 yr-1 (14 kg ha-1 yr-1), being a deposition level above which the species diversity of the ground vegetation may decrease. The total input of acidity ranged mostly between 200 and 4000 molc ha-1 yr-1. The deposition of sulphate, nitrate and calcium was significantly higher in the central/eastern part of Europe, but NH4 was slightly higher in western Europe. A highly significant positive correlation of atmospheric deposition and rainfall was observed for most ions. The concentrations of nitrate in soil solution exceeded the EU ground water quality criterion of 800 mmolcm-3 in the subsoil at 9% of the plots. Al/(Ca Mg K) ratios exceeded a critical ratio of 1.0 in approximately 30-39% of the plots, depending on the layer considered. The concentration of potentially toxic Al in the subsoil was strongly related to the concentration of sulphate and nitrate in soils with a base saturation below 25% or a pH below 4.5, indicating that Al release is the dominant buffering process in acid soils. Above a base saturation of 25% and a pH level of 4.5, there was a strong relationship between Ca and strong acid anions, indicating that the acidity is mainly neutralised by the release of base cations in less acidified soils. The variation in concentrations of major ions in the soil solution could be, to a large extent, explained by differences in atmospheric deposition and meteorological conditions.