Modelling acidification effects on coniferous forest soils

Two submodels for simulating the leaching of forest soils are described. SOILORG is used for O, E, and top B layers where Al(OH)(3) is absent and organic matter is the major base cation storage. SOILMIN cares for the rest of the profile where Al(OH)(3) control of Al is assumed and goethite provides most of the sulphate storage, clay mineral surfaces providing base cation storage. Results are presented from a test run for the period 1911 to 2030, based on data from a 260 cm deep soil profile in the SW of Sweden investigated 1990 and on a likely deposition scenario. Considering that the deposition of base cations exceeded the removal by stemwood in 1911 when the simulation started, the biologic acidification of the soil profile had reached a steady state before 1911 so that no additional acidification took place before 1930 and very little before 1950. After 1950 it was strongly enhanced by the increased acid deposition. In the mineral soil a considerable resistance against acidification is offered both by base cation exchange and sulphate adsorption, creating an acidification front which moved slowly down the B-horizon then accelerated, reaching the bottom of the profile in 1990. A deposition reduction by 2/3 during 1990-2010 will cause a partial recovery of pH, particularly in the deeper parts of the profile.