It is not immediately evident that the observed changes in soil chemistry (Fig. 5.1) are in fact related to atmospheric inputs (Fig. 5.2). The emission data indicate that half of the accumulated total acidity has been deposited after 1950, and one-third after 1970.

   A very general estimate of a stand proton balance (Ulrich, 1989) indicates that during a 100-year rotation period a forest will accumulate 5-7 mol m^-2 cation equivalents in its biomass and timber, which is harvested and removed from that site. The system could eventually lose up to 10 mol cation equivalents m^-2 ground area from natural leaching of nitrate, and it has received a cumulative acid deposition between 6 and 34 mol H⁺ m^-2 from the atmosphere depending upon tree species and exposure. The total loss of 10-40 mol H⁺ m^-2 (removal by timber harvest and by leaching to groundwater) must be balanced by two processes, namely silicate weathering (2 to 10 mol H⁺ m^-2) and leaching of exchangeable cations from cation exchanges. This indicates that the input from acid deposition and timber harvest requires large amounts of cations which indeed could induce the changes in base saturation of soils. Harvesting of timber was previously balanced by silicate weathering. However, any cation loss in addition to forest harvesting could lead to a decrease in exchangeable base saturation (Ca, Mg, K) and the observed changes in forest soils. If the initial state of the forest soil was at 50% base saturation before industrialization (see above) a 0.5 m deep profile of sandy soil would store about 2 mol m^-2 of exchangeable basicity and a loamy soil between 6 and 30 mol m^-2. If the cation requirement for timber growth and soil leaching removed 10-40 mol cation equivalents m^-2 then low base saturation may be reached in forest soils depending on the rate of weathering.

   The acid/base reactions of soils depend on the chemical nature of the substrate, the incoming acid, and the pH in the soil solution. Only if the pK of the acid is below the pH of the soil solution it will contribute to the acidification process. In this case the acid reacts with the soil matrix, which buffers the soil solution against changes in pH. In a humid climate, soils exhibit a characteristic pH response to an accumulated deposition of strong acids (Fig. 5.3; Ulrich, 1981; Van Breemen et al. , 1983). Between pH 8.6 and 6.2, buffering is primarily by calcium carbonate, which becomes dissolved during the buffering process. Once this reservoir becomes exhausted, soils enter an 'exchange-buffering range', which extends to pH 4.2 and in which mainly clay minerals progressively exchange cations for hydrogen ions which release aluminum ions from the clay lattice. Both cation exchange capacity and pH decrease with continued leaching of base cations, and exchangeable Al ions finally become the dominant reactive cation species. In case of an input of sulfuric acid, this acid may be immobilized by forming aluminum alone, such as nitrate or organic acids (which may also occur naturally in soil) can continue to promote the acidification process below pH 4.2 and allow Al ions to enter the soil solution. Thus, sulfates are accumulated in the soil profile during the acidification process at higher base saturation, but may be mobilized again when soils acidify below pH 4.2 (Ulrich, 1987). This has been demonstrated by input/output budgets (Ulrich, 1989, Forschungsbeirat Waldschäden,1989).

   In a soil-chemical model Kaupenjohann(1989) demonstrated that sulfate deposition alone will not acidify soils below pH 4.2 because of binding with Al. However, as soon as other acids are leached, such as nitric acid, the acidification process is strongly enhanced. Since the uptake of nitrate by roots reverses the acidification which took place during nitrification it is important to understand why nitrate is not used by the plant cover. If plants consumed nitrate by uptake, the increase in acid strength would be terminated by this plant process.

   It becomes clear that the 'ecosystem experiment' of acid rain is quite complicated, since we are dealing not only with soils but also with a plant cover, and because sulfate and nitrate have been deposited in a changing ratio (Fig. 5.2). In addition, ammonium was added in changing amounts to the total deposition which, in contrast to nitrate, will enhance the acidification of soils if used by plants as a nitrogen source.