The climate benefits of topsoil removal and Sphagnum introduction in raised bog restoration

Abstract

Many raised bogs in Central Europe are in an unfavorable state: drainage causes high emissions of carbon dioxide (CO2) and nitrous oxide (N2O), while rewetting may result in high methane (CH4) emissions. Also, the establishment of typical bog species is often hampered during restoration. Measures like topsoil removal (TSR) or introduction of target vegetation are known to improve restoration success in other systems, but experiences on bogs after long-term agricultural use are scarce and their climate effects including carbon losses from TSR are unknown. In a field trial in north-western Germany, consisting of seven plots (intensive grassland, IG, and six restoration approaches), we explored the effects of rewetting, TSR and Sphagnum introduction on greenhouse gas (GHG) emissions. We measured GHG fluxes to obtain two-year GHG budgets and applied a radiative forcing model to assess the time-dependent climate effects. Existing uncertainty of decomposition processes in the translocated topsoil has been incorporated by different topsoil accounting scenarios. According to our data, rewetting alone reduced CO2 emissions by approximately 75% compared to IG, but substantially increased CH4 emissions. After TSR and rewetting, on-site CO2 emissions were close to zero or, with Sphagnum introduction, net negative while CH4 emissions remained very low. The climatic warming effect of TSR including C export becomes less climate warming than rewetting nutrient-rich peatlands after a few decades. For raised bog restoration, we therefore recommend a TSR sufficient to achieve nutrient-poor and acidic conditions needed for rapid Sphagnum establishment.