Brackish water rewetting of a temperate coastal peatland: Effects on biogeochemistry, microorganisms and greenhouse gas emissions

Around 4 % of global greenhouse gas (GHG) emissions originate from drained peatlands. Unlike rewetting drained peatlands with freshwater, brackish water rewetting of coastal peatlands might not only reduce CO₂ emissions, but also keep methane (CH₄) emissions low. The re-establishment of the natural brackish water regime of coastal peatlands with high sulfate levels should favor sulfate reducing bacteria as well as sulfate-driven anaerobic methane oxidizers 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 fen along a moisture gradient before, and a water level gradient after rewetting.

Brackish water rewetting increased the abundances of CH₄ producing archaea (methanogens) as well as the abundances of sulfate reducing bacteria (SRB) in most of the study site, except at former ditch areas, where methanogenic and SRB abundances had been high before. At the same time, the aerobic methanotroph community was less present, indicating lower aerobic CH₄ oxidation potentials after rewetting. Pore water CH₄ and CO₂ concentrations along with δ¹³C records suggest that both, methanogenesis and CH₄ oxidation, increased after rewetting. Brackish water rewetting raised average CH₄ emissions from 2 to 25 mg CH₄ m^-2 d^-1 at locations that were previously drained, which is lower than CH₄ emissions reported from most freshwater peatlands. Net CO₂ emissions remained high after rewetting with values around 4 g CO₂ m^-2 d^-1. However, since ecosystem respiration strongly decreased from on average 19 to 6 g CO₂ m^-2 d^-1, the remaining net CO₂ emissions were mostly associated with low CO₂ uptake due to extensive die-back of the vegetation. Hence, brackish water rewetting can keep CH₄ emissions relatively low, but, as in freshwater peatlands, hydrological management must allow for the re-establishment of site-specific vegetation to sustain net CO₂ uptake.