Effects of sulfate concentration in the overlying water on sulfate reduction and sulfur storage in lake sediments

We investigated the effects of sulfate concentration on sulfate reduction and net S storage in lake sediments using³⁴S as a tracer. The water overlying intact sediment cores from the hypolimnion of Mares Pond, MA, was replaced with two Na₂³⁴SO₄ solutions at either ambient (70 M) or elevated (260 M) sulfate concentrations. The ³⁴S of the added sulfate was 4974 . Over two months, the net sulfate reduction rate in the ambient sulfate treatment was zero, while the net rate for the high sulfate treatment was 140 moles/m²/d. The water overlying the cores was kept under oxic conditions and the sediment received no fresh carbon inputs, thus the net rate reported may underestimate the in situ rate. Gross sulfate reduction rates calculated by isotope dilution were approximately 350 moles/m²/d for both treatments. While the calculation of gross sulfate reduction rates in intact sediment cores can be complicated by differential diffusion of³⁴S and³²S, isotopic fractionation, and the possible formation of ester sulfates, we believe these effects to be small. The results suggest that sulfate reduction is not strongly sulfate-limited in Mares Pond. The difference in net sulfate reduction rates between treatments resulted from a decrease in sulfide oxidation and suggests the importance of reoxidation in controlling net S storage in lake sediments. In both treatments the CRS and organic S fractions were measurably labelled in³⁴S. Below the sediment surface, the CRS fraction was the more heavily labelled storage product for reduced sulfides.