The use of stable sulfur and oxygen isotope ratios for interpreting the mobility of sulfate in aerobic forest soils

The isotope compositions of sulfate in bulk precipitation near Munich (Germany) and of seepage water and soil sulfate in five acid forest soils representative of southern Germany were determined in order to ascertain the sources and dynamics of sulfur. While the δ34 S-values of inorganic sulfate in soil solution and solid phases were found to be nearly identical to those of precipitation sulfate, a depletion of several per mil was observed for the δ18 O-values of sulfate within the uppermost 30 cm of the investigated soils. Mineralization of carbon-bonded sulfur to SO42− in the forest floor and humic mineral soil horizons is the only known process which can explain the observed shifts in δ18Osulfate. The fact that this18O-depleted sulfate recharges the groundwater under forests must be considered, when sulfur and oxygen isotope data of sulfate are used for interpretations of the past geochemistry of groundwater systems.