SOIL ACIDITY AND ADSORBED ANIONS IN SWEDISH FOREST SOILS - LONG-TERM CHANGES

To assess the effects of acid deposition on the soil acidity of Swedis h forests, six sets of forest soils were analysed for pH, and the valu es obtained compared with measurements made during 1951-1963. In one o f the sites in southwest Sweden, the contents of extractable anions we re also compared. It was found that the forest soils of southern Swede n exhibited large pH decreases of around one pH unit throughout the so il profile, with forest soils in northern Sweden relatively unaffected . For most sites, including the northernmost site, the soil pH values seem to have approached a steady-state value. Acid deposition was the major cause of the observed pH decreases in southern Sweden. Elevated atmospheric inputs of sulphur and selenium, along with the recorded pH decreases would, in theory, have increased the adsorption of sulphate and selenite. It was found, however, that phosphate-extractable sulph ate and selenite have not increased significantly between 1951 and 198 9 in the spodic horizons of southwest Swedish forest soils. Instead, t he pool of adsorbed organic anions, measured as NH4F/HCl-extractable C , increased significantly in the forest. Increased adsorption of organ ic anions probably explains the fact that sulphate and selenite have n ot increased to any considerable degree, because organic anions exert a negative influence on the adsorption of the other anions. These find ings have implications for the prediction of recovery times in respons e to decreased acidic inputs to watersheds, as reversibly adsorbing su lphate would act to delay the recovery.