Acid rain, due to wet and dry deposition of S and N compounds, is an increasing environmental problem in China. A considerable deposition of alkaline dust serves to mitigate the acidifying effect to varying extent. Data from 3 years, a monitoring of water chemistry in 10 water compartments (i.e., two qualities of deposition, two types of throughfall, solution in five genetic soil horizons, and runoff) at five well documented sub-tropical forested catchments, have been interpreted in order to identify key processes governing the water chemistry in catchments suffering acid rain. This study of water chemistry in regions with sub-tropical climate supplements similar monitoring studies conducted in temperate regions with different types of soils and compositions of deposition. Natural organic acids as well as nutrient cycling of K+ have strong influence on the water chemistry in throughfall and upper soil horizons at the relatively pristine sites. At sites receiving elevated S and N deposition an accelerated cycling of K+ removes much of the mineral acidity in throughfall. The soil uptake of this K+ results in release of H+. Nitrification and/or assimilation of a substantial deposition of reduced N contributes at some sites also significantly to the acidity in the soils. During the study period, Ca2+ in solution was exchanged for Al3+ in the soils with an effective base saturation less than 20%. In deeper soil horizons most of this mobilized Al is readsorbed
along with SO42-.