Sulfate Reduction in a Forested Catchment as Indicated by δ34S Values of Sulfate in Soil Solutions and Runoff

Abstract

In a forested catchment in the Fichtelgebirge mountains (NE-Bavaria, Germany) the long term SO₄ ^2- budget (average 1988–1994) indicated that about 40% of the input with throughfall (16.8 kg SO₄ ^2- S·ha⁻¹·yr⁻¹) was retained in the catchment. In order to identify processes acting as potential SO₄ ^2- sinks, δ³⁴S values of SO₄ ^2- in soil solutions and runoff were measured between May and November 1994. δ³⁴S values of the runoff and the fen were higher (5.8‰) than the δ³⁴S values of the soil solution of the oxic soils in the terrestrial area (3.9‰). Because there is no lithogenic S source within the catchment, it can be assumed that SO₄ ^2- deposition is the only S source in the catchment. Thus the results were interpreted as a result of SO₄ ^2- reduction within the catchment, because the uptake of ³²S is favoured during the dissimilatory SO₄ ^2- reduction and ³⁴S is consequently enriched in the soil solution. To estimate the amount of SO₄ ^2- reduced isotopic fractionation factors between − 9‰ and −46‰ were considered, resulting in SO₄ ^2- reduction rates of 1.8–9.3 kg SO₄ ²-S·ha⁻¹yr⁻¹. It was concluded that besides dissimilatory SO₄ ^2- reduction another sink exists in the catchment (e.g. SO₄ ^2- sorption in deep soil layers).