High methane emissions dominated annual greenhouse gas balances 30 years after bog rewetting

Abstract. Natural peatlands are important carbon sinks and sources of methane (CH₄). In contrast, drained peatlands turn from a carbon sink to a carbon source and potentially emit nitrous oxide (N₂O). Rewetting of peatlands thus potentially implies climate change mitigation. However, data about the time span that is needed for the re-establishment of the carbon sink function by restoration are scarce. We therefore investigated the annual greenhouse gas (GHG) balances of three differently vegetated sites of a bog ecosystem 30 years after rewetting. All three vegetation communities turned out to be sources of carbon dioxide (CO₂) ranging between 0.6 ± 1.43 t CO₂ ha⁻² yr⁻¹ (Sphagnum-dominated vegetation) and 3.09 ± 3.86 t CO₂ ha⁻² yr⁻¹ (vegetation dominated by heath). While accounting for the different global warming potential (GWP) of CO₂, CH₄ and N₂O, the annual GHG balance was calculated. Emissions ranged between 25 and 53 t CO₂-eq ha⁻¹ yr⁻¹ and were dominated by large emissions of CH₄ (22–51 t CO₂-eq ha⁻¹ yr⁻¹), with highest rates found at purple moor grass (Molinia caerulea) stands. These are to our knowledge the highest CH₄ emissions so far reported for bog ecosystems in temperate Europe. As the restored area was subject to large fluctuations in the water table, we assume that the high CH₄ emission rates were caused by a combination of both the temporal inundation of the easily decomposable plant litter of purple moor grass and the plant-mediated transport through its tissues. In addition, as a result of the land use history, mixed soil material due to peat extraction and refilling can serve as an explanation. With regards to the long time span passed since rewetting, we note that the initial increase in CH₄ emissions due to rewetting as described in the literature is not inevitably limited to a short-term period.