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

Abstract

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 δ 34 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 δ 18 O-values of sulfate within the uppermost 30 cm of the investigated soils. Mineralization of carbon-bonded sulfur to SO 4 2− in the forest floor and humic mineral soil horizons is the only known process which can explain the observed shifts in δ 18 O sulfate. The fact that this 18O-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. Since the δ 34 S-values of precipitation sulfate were barely altered during percolation through the soils, sulfate mobilities were inferred from a lysimeter experiment with undisturbed soil cores from the same sites, using the stable isotope composition of the irrigation sulfate as a tracer. Fifteen cores of each of the five forest soils, were repeatedly irrigated over 20 months with 34S- and 18O-enriched sulfate in three different treatments (35, 63, and 131 kg S ha −1 respectively). Despite the fact that the mean residence time of the seepage water was of the order of only a few months, the throughput of irrigation sulfate did not exceed 34% for all soils and irrigation treatments during the experiment. The low recovery of irrigation sulfate in the seepage water implies mean residence times for sulfur in the uppermost 60 cm of the forest soils of the order of decades, much longer than previously suggested.