In natural coastal wetlands, high supplies of marine sulfate suppress methanogenesis. Coastal wetlands are, however, often subject to disturbance by dyking and drainage for agricultural use and it has been shown that they can turn to potent methane sources when rewetted for remediation, suggesting that the sulfate-related methane suppressing mechanisms were suspended by the preceding land use measures. Here, we unravel the hydrological relocation and biogeochemical S and C transformation processes that induced high methane emissions in a disturbed and rewetted peatland despite former brackish impact. The underlying processes were investigated along a transect of increasing distance to the coastline using a combination of concentration patterns, stable isotope partitioning and analysis of the microbial community structure. We found that dyking and freshwater rewetting caused a distinct freshening and an efficient depletion of the brackish sulfate reservoir by dissimilatory sulfate reduction (DSR). Despite some legacy effects of brackish impact expressed as high amounts of sedimentary S and elevated electrical conductivities, contemporary metabolic processes operated mainly under sulfate-limited conditions. This opened up favorable conditions for the establishment of a prospering methanogenic community in the top 30-40 cm of peat, the structure and physiology of which resembles those of terrestrial organic-rich environments. Locally, high amounts of sulfate persisted in deeper peat layers through the suppression of DSR, probably by competitive electron acceptors of terrestrial origin, for example Fe(III), but did not interfere with high methane emissions on ecosystem scale. Our results indicate that the climate effect of disturbed and remediated coastal wetlands cannot simply be derived by analogy with their natural counterparts. From a greenhouse gas perspective, the re-exposure of dyked wetlands to natural coastal dynamics would literally open up the floodgates for a replenishment of the marine sulfate pool and constitute an efficient measure to reduce methane emissions.
