Abstract. The rewetting of drained peatlands supports long-term nutrient removal in addition to reducing emissions of carbon dioxide (CO2) and nitrous oxide (N2O). However, rewetting may lead to short-term nutrient leaching into adjacent water and high methane (CH4) emissions. The consequences of rewetting with brackish water on nutrient and greenhouse gas (GHG) fluxes remain unclear, although beneficial effects such as lower CH4 emissions seem likely. Therefore, we studied the actively induced rewetting of a coastal peatland with brackish water, by comparing pre- and post-rewetting data from the peatland and the adjacent bay. Both the potential transport of nutrients into adjacent coastal water and the shift in GHG fluxes (CO2, CH4, and N2O) accompanying the change from drained to inundated conditions were analyzed based on measurements of the surface water concentrations of nutrients (dissolved inorganic nitrogen, DIN, and phosphate, PO43-), oxygen (O2), components of the CO2 system, CH4, and N2O together with manual closed-chamber measurements of GHG fluxes. Our results revealed higher nutrient concentrations in the rewetted peatland than in the adjacent bay, indicating that nutrients leached out of the peat and were exported to the bay. A comparison of DIN concentrations of the bay with those of an unaffected reference station showed a significant increase after rewetting. The maximum estimated nutrient export (mean ± 95 % confidence level) out of the peatland was calculated to be 33.8 ± 9.6 t yr−1 for DIN-N and 0.24 ± 0.29 t yr−1 for PO4-P, depending on the endmember (bay vs. reference station). The peatland was also a source of GHG in the first year after rewetting. However, the spatial and temporal variability decreased, and high CH4 emissions, as reported for freshwater rewetting, did not occur. CO2 fluxes (mean ± SD) decreased slightly from 0.29 ± 0.82 g m−2 h−1 (pre-rewetting) to 0.26 ± 0.29 g m−2 h−1 (post-rewetting). The availability of organic matter (OM) and dissolved nutrients were likely the most important drivers of continued CO2 production. Pre-rewetting CH4 fluxes ranged from 0.13 ± 1.01 mg m−2 h−1 (drained land site) to 11.4 ± 37.5 mg m−2 h−1 (ditch). After rewetting, CH4 fluxes on the formerly dry land increased by 1 order of magnitude (1.74 ± 7.59 mg m−2 h−1), whereas fluxes from the former ditch decreased to 8.5 ± 26.9 mg m−2 h−1. These comparatively low CH4 fluxes can likely be attributed to the suppression of methanogenesis and oxidation of CH4 by the available O2 and sulfate in the rewetted peatland, which serve as alternative electron acceptors. The post-rewetting N2O flux was low, with an annual mean of 0.02 ± 0.07 mg m−2 h−1. Our results suggest that rewetted coastal peatlands could account for high, currently unmonitored, nutrient inputs into adjacent coastal water, at least on a short timescale such as a few years. However, rewetting with brackish water may decrease GHG emissions and might be favored over freshwater rewetting in order to reduce CH4 emissions.

<p>Rewetting of drained peatlands reduces the emissions of carbon dioxide (CO<sub>2</sub>) and nitrous oxide (N<sub>2</sub>O) substantially. However, elevated methane (CH<sub>4</sub>) 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 CH<sub>4</sub> 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.<span> </span></p><p>We hypothesized that flooding with brackish seawater reduces CO<sub>2</sub> emissions despite favoring sulfate-reducers. It should also limit CH<sub>4</sub> production and favor anaerobic methane and thus keep CH<sub>4</sub> emissions low although aerobic methane oxidation may decrease. We measured CH<sub>4</sub> and CO<sub>2</sub> fluxes along a soil wetness gradient before rewetting and along a water level gradient after rewetting with brackish seawater and estimated cumulative CH<sub>4</sub>, CO<sub>2</sub> net ecosystem exchange (NEE), and ecosystem respiration (R<sub>eco</sub>). 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.</p><p>After rewetting, cumulative CH<sub>4</sub> net fluxes and NEE increased at locations that were previously dry, while R<sub>eco</sub> 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 CH<sub>4</sub> and CO<sub>2 </sub>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.</p><p>Brackish water rewetting strongly modified the dominant methane-cycling processes but resulted in higher greenhouse gas emissions of both CO<sub>2</sub> and CH<sub>4</sub> in the first year after rewetting. As expected, CH<sub>4</sub> emissions after rewetting were lower than in freshwater rewetted fens, while NEE was unexpectedly high. Since R<sub>eco</sub> strongly decreased, we assume that peat mineralization was successfully prevented and that ongoing CO<sub>2</sub> emissions rather derived from strongly reduced CO<sub>2</sub> uptake, supply of terminal electron acceptors (especially sulfate), and excess substrate availability from decaying vegetation. There is great potential for reduction of both, CH<sub>4</sub> and CO<sub>2</sub> 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.</p>

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