Brackish Water Rewetting of a Temperate Coastal Peatland: Effects on Biogeochemistry, Microorganisms and Greenhouse Gas Emissions

Abstract 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 CO 2 emissions, while keeping post-rewetting methane (CH 4 ) emissions low. Sulfate-containing brackish water should favor sulfate reduction and therefore limit CH 4 production and/or lead to increased CH 4 consumption. Here, we compared CO 2 and CH 4 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 CH 4 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 CH 4 and CO 2 concentrations along with δ 13 C records indicated that both methanogenesis and CH 4 oxidation increased post-rewetting. Although brackish water rewetting raised average net CH 4 emissions from 2 to 25 mg CH 4 m − 2 d − 1 at previously drained locations, these fluxes were lower than CH 4 emissions reported from most freshwater peatlands. Net CO 2 emissions remained high with levels around 4 g CO 2 m − 2 d − 1 , but ecosystem respiration strongly decreased from on average 19 to 6 g CO 2 m − 2 d − 1 . The remaining net CO 2 emissions were likely associated with a lower uptake of CO 2 compared to its release after extensive vegetation die-back. Hence, the re-establishment of site-specific vegetation is important to sustain the net CO 2 uptake besides low CH 4 emissions.