SULFATE ADSORPTION IN FOREST SOILS OF THE GREAT-LAKES REGION

Sulfate adsorption by forest soils modifies the impact of pollutant de position on cation leaching processes. We examined relationships among : SO42- adsorption, soil properties, and seasonal variation in soil so lution chemistry at 13 sites representing deciduous forest ecosystems common in the Great Lakes region. Objectives of the study were to test the validity of previously proposed SO42- adsorption indices, to exam ine within- and among-site variability in SO42- adsorption potential, and to investigate the effects of seasonal changes in soil solution ch emistry on SO42- retention. Mineral soils were sampled by horizon at a ll sites, and soil solutions were sampled at lower E and lower B horiz on boundaries at 10 sites. Proposed indices overpredicted sulfate adso rption in certain SO42- releasing subsurface horizons, seriously limit ing the applicability of the published regression equations. We develo ped improved regression equations using the sum of initial extractable SO42- and additional SO42- adsorbed under laboratory conditions as th e dependent variable. Sulfate retention indices predicted by the impro ved equations were independent of existing levels of extractable SO42- and past history of atmospheric SO42- deposition. Examination of with in- and among-site variability in SO42- adsorption potentials suggeste d that soils need to be grouped tightly on a taxonomic basis for model ling purposes. Seasonal variation in soil solution SO42- concentration s and fluxes was consistent with an annual cycle of SO42- retention an d release. Although seasonal patterns in SO42- concentrations and flux es appeared to be controlled by hydrologic and S-cycling processes, th e magnitude of SO42- fluxes was primarily related to atmospheric SO42- deposition rates.