Abstract. In natural coastal wetlands, high supplies of marine
sulfate suppress methanogenesis. Coastal wetlands are, however, often
subject to disturbance by diking and drainage for agricultural use and can
turn to potent methane sources when rewetted for remediation. This suggests
that preceding land use measures can suspend the sulfate-related methane
suppressing mechanisms. 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 diking 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 resemble
those of terrestrial organic-rich environments. Locally, high amounts of
sulfate persisted in deeper peat layers through the inhibition of DSR,
probably by competitive electron acceptors of terrestrial origin, for
example Fe(III). However, as sulfate occurred only in peat layers below
30–40 cm, it did not interfere with high methane emissions on an 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
diked wetlands to natural coastal dynamics would literally open up the
floodgates for a replenishment of the marine sulfate pool and therefore
constitute an efficient measure to reduce methane emissions.
Field-scale simulation of methane emissions from coastal wetlands in China using an improved version of CH4MOD wetland