While a number of studies have investigated adsorption and microbial immobilization as sulfate (SO4-2) retention mechanisms, few have investigated these processes under field-like conditions on a regional, ecosystem basis. Adsorption and microbial immobilization of SO4-2 were studied in four northern hardwood stands that span an atmospheric deposition gradient in the Lake States region (5 to 10 kg S/ha/yr). Soil cores collected in spring, summer, and autumn were labeled with 35SO4-2 to trace the flux of S between physical and biological sinks, and to investigate seasonal variation in sink strength. Intact soil cores were injected with Na235SO4 and incubated for 8 d in the laboratory at field temperature to study rates of adsorption and microbial immobilization. The amount of 35S recovered within these pools was significantly different between surface and subsurface soil horizons. Microbial immobilization was the dominant S sink in the A+E horizon, whereas adsorption was the most important S sink in the B horizon. During the 8-d incubation, the proportion of 35S that was immobilized in the A horizon (49% of applied 35S) was equivalent to the proportion of 35S adsorbed in the B horizon (47% of applied 35S). Microbial immobilization sequestered an additional 25% of the applied 35S in the B horizon. Adsorption and microbial immobilization were not significantly different among sampling dates. Sulfur retention in forested ecosystems should be viewed as a combination of geochemical and microbially-mediated processes. However, given current levels of S deposition at these sites, neither process seems to represent a significant mechanisms for long-term S retention.