Nutrient release and flux dynamics of CO₂, CH₄, and N₂O in a coastal peatland driven by actively induced rewetting with brackish water from the Baltic Sea

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.