Changes of greenhouse gas fluxes and corresponding microbial communities upon rewetting of a coastal peatland with brackish seawater

Rewetting of drained peatlands reduces the emissions of carbon dioxide (CO₂) and nitrous oxide (N₂O) substantially. However, elevated methane (CH₄) emissions can occur, at least in the short-term. The impact of rewetting coastal peatlands with brackish water remains yet unclear, although beneficial effects such as lower CH₄ emissions seem likely, due to high sulfate availability. Here, we compare pre- and post-rewetting greenhouse gas fluxes, biogeochemical parameters and the abundance of specific microbial groups in a coastal peatland at the German Baltic Sea coast that was formerly drained and used as an agricultural grassland and recently rewetted with brackish water. 

We hypothesized that flooding with brackish seawater reduces CO₂ emissions despite favoring sulfate-reducers. It should also limit CH₄ production and favor anaerobic methane and thus keep CH₄ emissions low although aerobic methane oxidation may decrease. We measured CH₄ and CO₂ fluxes along a soil wetness gradient before rewetting and along a water level gradient after rewetting with brackish seawater and estimated cumulative CH₄, CO₂ net ecosystem exchange (NEE), and ecosystem respiration (R_eco). Soil cores for biogeochemical and microbial analyses were taken at seven locations along the transect pre- and post-rewetting. We used quantitative polymerase chain reaction (qPCR) on 16S rRNA, mcrA, pmoA and dsrB genes to quantify the abundances of total prokaryotes, methanogens, aerobic methanotrophs and sulfate-reducing bacteria.

After rewetting, cumulative CH₄ net fluxes and NEE increased at locations that were previously dry, while R_eco halved compared to before rewetting. This correlated with the absolute abundances of specific microbial groups and the surface/pore water biogeochemistry. Under the newly created water-logged conditions, the abundances of methanogenic as well as of sulfate-reducing bacteria (SRB) increased at previously dry sampling locations, but remained constant at the former ditch location. At the same time, the abundance of the aerobic methanotroph community on previously dry locations decreased, which indicates lower aerobic methane oxidation potentials. Pore water CH₄ and CO₂ concentrations suggest that gas production most likely increased at the former terrestrial locations and stable carbon isotope measurements support an increase of methanogenesis in the peat at some locations. Isotopic analyses also provide some support for persistent methane oxidation either through anaerobic or aerobic taxa at one location.

Brackish water rewetting strongly modified the dominant methane-cycling processes but resulted in higher greenhouse gas emissions of both CO₂ and CH₄ in the first year after rewetting. As expected, CH₄ emissions after rewetting were lower than in freshwater rewetted fens, while NEE was unexpectedly high. Since R_eco strongly decreased, we assume that peat mineralization was successfully prevented and that ongoing CO₂ emissions rather derived from strongly reduced CO₂ uptake, supply of terminal electron acceptors (especially sulfate), and excess substrate availability from decaying vegetation. There is great potential for reduction of both, CH₄ and CO₂ emissions after the initial boost when readily available substrate is depleted. However, our study also reveals the complexity of peatland restoration and the possibility of transient effects upon rewetting, and therefore the value of undrained, pristine peatlands as well as their importance in sequestering carbon.