Atmospheric deposition and soil acidification in five coniferous forest ecosystems: a comparison of the control plots of the EXMAN sites☆

Abstract The five sites of the EXMAN project conducted in Ireland, Denmark, Netherlands, Northern and Southern Germany were compared regarding (1) the marine and anthropogenic components of deposition, (2) the acidification of soil and consequences for Al status, and (3) the nitrate load of seepage. The marine deposition decreases with decreasing rates from the coast inland. It vanishes at a distance of more than 600 km of the sea. The most part of sea salt input in conifer forests is due to dry deposition. Sea salt Mg 2+ in throughfall near the coast by far exceeds the demand of trees. The anthropogenic deposition of N and S at the Irish site is about 20%; at the Danish site, about 60% of that at the inland sites. At the Irish site, the anthropogenic deposition is the two- to threefold of the preindustrial deposition. NH 4 + prevails at all sites as acid component in throughfall, controlling the pH values that vary between 3.9 and 5.3. In drainage water leaving the root zone, the mean pH values vary only between 4.1 and 4.4. Proton budgets for the forest floor have shown that N turnover dominates as a proton source at the inland sites, whereas at the coastal sites the dominant source results from the production of organic acids. The main proton sink is due to H + output. Proton budgets for the total root zone indicate that an important proton gain is caused at the most sites by proton excretion of the roots in connection with base cation uptake. In addition, at some sites, the release and output of SO 4 2− appears to be a considerable proton source. At all sites, buffering and output of Al represent the main proton sink. The Al solubility of each layer of the EXMAN sites was compared with the solubility of a synthetic gibbsite. Al saturation exists only at the lower boundary of the main root zone. In the soil layers above, there is an undersaturation that is largest at the humus layer efflux. The relationship between Al dissolved and Al adsorbed, both expressed in cation percentages, is rather weak for the coastal sites in contrast to the inland sites. The importance of the ionic strength effect of sea salt input is discussed with respect to the deep reaching Al saturation and acidification of the soils. Moderate to strong Al stress is indicated at all sites in the mineral soil. The nitrate load of the seepage water depends on the N status of the ecosystems rather than N deposition when the throughfall exceeds 20 kg N ha −1 yr −1 . An attempt was made to classify the EXMAN sites with respect to the N status with the aid of the N flux gradient by depth.