Around 4% of global greenhouse gas (GHG) emissions originate from drained peatlands. Unlike rewetting drained peatlands with freshwater, brackish water rewetting is expected to reduce CO2 emissions, while keeping post-rewetting methane (CH4) emissions low. Sulfate-containing brackish water should favor sulfate reduction and therefore limit CH4 production and/or lead to increased CH4 consumption. Here, we compared CO2 and CH4 fluxes, pore water geochemistry, and associated microbial communities of a coastal peatland along a transect one year before and after rewetting (Fig. 1) to evaluate the effect of brackish water rewetting. Brackish water rewetting increased the abundance of both CH4 producing archaea (methanogens) as well as sulfate reducing bacteria (SRB) in most sub-sites along the transect. At the same time, the aerobic methanotroph community was overall less present after rewetting. Pore water CH4 and CO2 concentrations along with δ13C records indicated that both methanogenesis and CH4 oxidation increased post-rewetting. Although brackish water rewetting raised average net CH4 emissions from 2 to 25 mg CH4 m− 2 d− 1 at previously drained locations, these fluxes were lower than CH4 emissions reported from most freshwater peatlands. Net CO2 emissions remained high with levels around 4 g CO2 m− 2 d− 1, but ecosystem respiration strongly decreased from on average 19 to 6 g CO2 m− 2 d− 1. The remaining net CO2 emissions were likely associated with a lower uptake of CO2 compared to its release after extensive vegetation die-back. Hence, the re-establishment of site-specific vegetation is important to sustain the net CO2 uptake besides low CH4 emissions.
