Nutrient release and flux dynamics of CO 2 , CH 4 , and N 2 O in a coastal peatland driven by actively induced rewetting with brackish water from the Baltic Sea

. The rewetting of drained peatlands supports long-term nutrient removal in addition to reducing emissions of carbon dioxide (CO 2 ) and nitrous oxide (N 2 O). However, rewetting may lead to short-term nutrient leaching into adjacent water and high methane (CH 4 ) emissions. The consequences of rewetting with brackish water on nutrient and greenhouse gas (GHG) fluxes remain unclear, although beneficial effects such as lower CH 4 emissions seem likely. Therefore, we studied 15 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 of GHG fluxes (CO 2 , CH 4 , N 2 O) accompanying the change from drained to inundated conditions were analyzed based on measurements of the surface water concentrations of nutrients (dissolved inorganic nitrogen (DIN), phosphate (PO 43- )), oxygen (O 2 ), components of the CO 2 20 system, CH 4 , and N 2 O 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 decreased to 8.5 ± 26.9 mg m −2 h −1 . These comparatively low CH 4 fluxes can likely be attributed to the suppression of methanogenesis by the available O 2 and sulfate, which serve as alternative electron acceptors. The post-rewetting N 2 O 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 35 into adjacent coastal water, at least on a short time scale 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 CH 4 emissions. carrier gas stream (99.999 % N 2 ) transfers the gaseous components to a Peltier device and a Nafion ® drying tube ® to remove water residues. The produced CO 2(g) is detected by an infrared detector (LICOR 7000; Germany). Certified reference materials (CRM; Scripps of Oceanography, University of San Diego, USA) were used for calibration. Triplicate measurements were conducted for each sample, and a precision of ± 5 µmol kg −1 was achieved. 270 A T was measured by potentiometric titration (glass electrode type LL Electrode plus 6.0262.100, Metrohm, Filderstadt, Germany) in the open-cell configuration, after Dickson et al. (2007). The system was calibrated with the same CRM used for C T and resulted in the same precision. 320 because the wind speed over 24 h was lowest at night and highest at midday and because sampling was usually conducted within the selected time interval. The Schmidt number was approximated by a linear interpolation between the freshwater and seawater values. Atmospheric-equilibrium conditions (C a ) were calculated using the atmospheric data for CO 2 and CH 4 obtained from the ICOS station “Utö” (Finnish Meteorological Institute, Helsinki). Due to the seasonal changes in the atmospheric dry molar fraction of CO 2 and CH 4 , mean values for each season were computed. For N 2 O, the atmospheric dry 325 mole fraction from station Mace Head was selected (National University of Ireland, Galway; data from the NOAA GML carbon cycle cooperative global air sampling network (Dlugokencky et al., 2019a, 2019b)). A mean value of the atmospheric N 2 O concentration during the investigation period was calculated due to its minor seasonality. Equilibrium concentrations were then calculated using the solubility coefficient (K 0 ) from Weiss and Price (1980). leach high amounts of nutrients. Occasional measurements of porewater nutrient concentrations in the peat of your study area revealed 520 DIN and PO 43- concentrations up to 1 order of magnitude higher than those in the surface water (Anne Breznikar, unpublished data), providing further support for the leaching of nutrients out of the peatland and into the inner bay. The estimated annual nutrient exports from the peatland of 12.5 ± 19.5 t yr −1 DIN-N and 0.2 ± 0.5 t yr −1 PO 4 -P (peatland/inner bay) and 36.5 ± 10.9 t yr −1 DIN-N and 0.2 ± 0.3 t yr −1 PO 4 -P (peatland/central bay) were high, given the small size of the flooded peatland (~0.5 km² at 0 masl). For comparison, the Warnow, a small river that flows into the Baltic Sea near the city of Rostock, Mecklenburg-Vorpommern, and drains an area of ~3300 km², had a mean annual DIN-N and PO 4 -P export of 1200 ± 500 t yr −1 and 19.9 ± 7.6 t yr −1 , respectively, over the last 25 years (HELCOM, 2019). When normalized to the same dimensions, our study area exported 25.0–73.0 t DIN-N km −2 yr −1 and 0.4 t PO 4 -P km −2 yr −1 , whereas the Warnow exported only 0.36 t DIN-N km −2 yr −1 and 0.01 t PO 4 -P km −2 yr −1 . Overall, the total nutrient export from the flooded peatland to the inner bay and to the central bay in the first year after rewetting accounted for ~1 and ~3 %, respectively, of the annual DIN-N and PO 4 -P loads of the Warnow. These results highlight the importance of currently still unmonitored and small, independently draining areas along the coastline of the Baltic Sea, in particular those that become intentionally flooded (HELCOM, approaches that include a high temporal resolution, such as achieved with sensor-based or eddy-supported measurements.