High N2O and CO2 emissions from bare peat dams reduce the climate mitigation potential of bog rewetting practices

The rewetting of drained peatlands is currently a common practice for re-establishing near-natural hydrological conditions and for reducing peatland greenhouse gas (GHG) emissions, especially of carbon dioxide (CO 2 ) and nitrous oxide (N 2 O), which are enhanced under peatland drainage and extraction. In the originally bare and drained peatland Himmelmoor (Quickborn, Germany), the rewetting process started stepwise in 2004 by blocking drainage ditches with peat as well as by creating polders surrounded by peat dams. In this research we examined differences in CO 2 , methane (CH 4 ) and N 2 O emissions between a flooded (FL) area, a bare peat dam (PD) area and an abandoned (but still drained) extraction (E) area during a period in 2012 and during a period of two years in 2014-2016. The results showed that all study areas were GHG sources, although large differences were identified between the different sites. Winter CO 2 emissions from all sites (FL, PD, E) were within the range previously reported for rewetted peatlands, but summer CO 2 emissions from PD (1-20 µmol m -2 s -1 ) strongly exceeded the reported average range for similar surfaces. Very low and irregular CH 4 fluxes were detected at both PD and FL, ranging from -6 to 24 nmol m -2 s -1 at PD and from -13 to 49 nmol m -2 s -1 at FL. In comparison to other peatlands, the observed N 2 O emissions were high, especially at the PD sites with maximum daily means of 23 nmol m -2 s -1 in the summer of 2012. In general, the flooded excavation sites (FL) showed lower GHG emissions than the not-rewetted excavation area (E). Also, despite the relatively small coverage of the peat dams (PD), these areas showed a larger total GHG emission than the E and FL sites. This negative effect of peat dams during the first years after flooding could be mitigated by stimulating their colonisation by moss or heath vegetation, which reduces the soil N pool and thus can be expected to reduce N2O fluxes from the peat dams. © 2019 International Mire Conservation Group and International Peatland Society