Spotting zones of dissimilatory sulfate reduction in a forested catchment : the S-34-S-35 approach

The localization of sulfate reducing sites in forested catchments is of major importance, because dissimilatory sulfate reduction can be a considerable sink for deposited sulfate. To localize dissimilatory sulfate reduction sites in a forested catchment (northeastern Bavaria, Germany), three sites within the catchment (upland site, intermittent seep, fen) were investigated for delta S-34 depth profiles of soil sulfur and potential sulfate reduction rates were measured with S-35 radiolabeling. Stable sulfur isotopes indicate that aerobic metabolism is the dominant process on the upland site and the intermittent seep (delta S-34 Of soil sulfur between + 1.6 and + 9.0 parts per thousand) and dissimilatory reduction is not a significant sink for sulfate. However, results of the S-35 radiolabeling indicated for the upland site that the soil has potentially high sulfate reduction rates under laboratory conditions. Soil sulfur of the fen was markedly depleted in S-34 (delta S-34 between -6 and +2.6 parts per thousand). Both, S-34 and S-35 data indicated that dissimilatory sulfate reduction is an ongoing process on this site. The S-34 and S-35 approaches are complementary. While measurements using S-35 can show momentary potential for dissimilatory bacterial sulfate reduction, delta S-34 data reflect long-term predominance of either assimilatory or dissimilatory S metabolism at a particular site. (C) 2001 Elsevier Science Ltd. All rights reserved.