Sulfate reduction and S-oxidation in a moorland pool sediment

Abstract

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ²⁻ reduction rates in the sediment, (2) depletion of SO₄ ²⁻ in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ²⁻ in the overlying water.

Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ²⁻ injection. Rates of SO₄ ²⁻ consumption in the overlying water were estimated by changes in SO₄ ²⁻ concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m⁻² d⁻¹. Rates of SO₄ ²⁻ uptake from the enclosed water column varied from −0.5, −0.3 mmol m⁻² d⁻¹ (November) to 0.43–1.81 mmol m⁻² d⁻¹ (July, August and April). Maximum rates of oxidation to SO₄ ²⁻ in July 1990 estimated by combination of SO₄ ²⁻ reduction rates and rates of in situ SO₄ ²⁻ uptake in the enclosed water column were 10.3 and 10.5 mmol m⁻² d⁻¹ at an organic rich and at a sandy site respectively.

Experiments with³⁵S²⁻ and³⁵SO₄ ²⁻ tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ²⁻ and (2) the presence of mainly non SO₄ ²⁻-S derived from reduced S transported from the sediment into the overlying water. A³⁵S²⁻ tracer experiment showed that about 7% of³⁵S²⁻ injected at 1 cm depth in a sediment core was recovered in the overlying water column.

Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.