Sulfate Reduction in a Forested Catchment as Indicated by δ(34)S Values of Sulfate in Soil Solutions and Runoff

Abstract

Abstract In a forested catchment in the Fichtelgebirge mountains (NE-Bavaria, Germany) the long term SO(4) (2-) budget (average 1988-1994) indicated that about 40% of the input with throughfall (16.8 kg SO(4) (2-) S·ha(-1)·yr(-1)) was retained in the catchment. In order to identify processes acting as potential SO(4) (2-) sinks, δ(34)S values of SO(4) (2-) in soil solutions and runoff were measured between May and November 1994. δ(34)S values of the runoff and the fen were higher (5.8‰) than the δ(34)S 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 SO(4) (2-) deposition is the only S source in the catchment. Thus the results were interpreted as a result of SO(4) (2-) reduction within the catchment, because the uptake of (32)S is favoured during the dissimilatory SO(4) (2-) reduction and (34)S is consequently enriched in the soil solution. To estimate the amount of SO(4) (2-) reduced isotopic fractionation factors between - 9‰ and -46‰ were considered, resulting in SO(4) (2-) reduction rates of 1.8-9.3 kg SO(4) (2)-S·ha(-1)yr(-1). It was concluded that besides dissimilatory SO(4) (2-) reduction another sink exists in the catchment (e.g. SO(4) (2-) sorption in deep soil layers).