Topsoil removal reduced in-situ methane emissions in a temperate rewetted bog grassland by a hundredfold

Peatland restoration is seen as an effective contribution to help achieve the aims of the Paris Agreement because currently huge amounts of peatlands in Northern Central Europe are under unsustainable drainage-based land use. If net zero greenhouse gas emissions from peatlands shall be reached by 2050, restoration measures have to be done as soon as possible. However, rewetting drained peatlands that were under intensive grassland use frequently results in high methane (CH₄) emissions, which is often seen as a counter-argument against rewetting. To find the source of high CH₄ emissions after rewetting and to explore the best possible way of peatland restoration (i.e., low CH₄ emissions after rewetting) under near-natural conditions, we installed a field trial in a drained bog in north-western Germany. The trial consists of seven plots (~8 × 24 m²) representing the status quo-intensive grassland use- and six restoration approaches with combinations of rewetting either on the original surface or after topsoil removal (TSR), biomass harvesting or spreading Sphagnum spp. to initiate vegetation succession. On all seven plots we measured CH₄ fluxes using closed chambers. In addition, we investigated CH₄ production potential by incubating soil samples and determining methanogen abundance by quantitative PCR. Compared to rewetting on the original surface, CH₄ emissions were reduced on TSR plots by factor 30 to 400. Spreading of Sphagnum spp. had only little effect on CH₄ emissions during the first year of establishment. TSR also reduced CH₄ production potential and methanogen abundance. Further, the response of CH₄ fluxes to methanogen abundance was lower after TSR. This suggests that both reduction in labile substrate and in methanogen abundance contribute to near-zero CH₄ emissions after TSR. These are the first field-scale results that demonstrate the efficiency of removing degraded topsoil to avoid high CH₄ emissions after rewetting.