Afforestation-Related Fertilisation Quickly Turns Barren Cutaway Peatland Into a Carbon Dioxide Sink

Energy peat extraction has declined rapidly in Europe in recent years, leaving thousands of hectares of land requiring after-use management and planning. A popular after-use option, afforestation, is understudied and there is a limited understanding of its overall effect on greenhouse gas (GHG) and energy exchange. In this study, we present a multi-year record of eddy covariance fluxes of carbon dioxide (CO₂), energy fluxes and surface albedo, chamber measurements of methane (CH₄) and (N₂O), and estimates of lateral carbon (C) losses from dissolved organic carbon (DOC) measurements from a cutaway peatland in Finland during the first 3 years of afforestation. The site was fertilised with wood ash and 2-year-old Scots pine (Pinus sylvestris) saplings were planted. Wild vegetation emerged at the site in the first summer after fertilisation. Satellite-derived leaf area index data showed clear year-on-year increases, and there was good agreement with changes in CO₂ fluxes over the full study period. The albedo of the site increased with plant cover annually, resulting in negative radiative forcings and net CO₂-equivalent (CO₂-eq) removals. After being a source of CO₂ in the first year (144 ± 20 g CO₂-C m^-2 year^-1), the site transitioned to a sink for the next two study years (-36 ± 12 and -19 ± 19 g CO₂-C m^-2 year^-1). Annual fluxes of CH₄ and N₂O were small but not negligible. Net annual C losses stopped after 1 year, where DOC losses offset CO₂ uptake, and the afforestation resulted in a mean change in the annual net ecosystem C balance of 172 g C m^-2 year^-1 and radiative balance of -688 g CO₂-eq m^-2 year^-1 over a 100-year time horizon. Cutaway peatlands typically remain long-term sources of C if abandoned, and our results indicate that the afforestation process can rapidly revegetate barren peatlands and halt net C losses.