Site-dependent carbon and greenhouse gas balances of five fen and bog soils after rewetting and establishment of Phalaris arundinacea paludiculture

Peatlands cover 3 % of the Danish land area, but drainage of these areas contributes to approximately 25 % of the total agricultural greenhouse gas (GHG) emissions. Paludiculture, defined as agriculture on wet or rewetted peatlands, has been proposed as a strategy to mitigate GHG emissions while keeping up production. However, little is known about the net GHG effects during establishment and how it is influenced by soil biogeochemical conditions. In this study, we determined annual carbon balances of five Danish peatlands, three fens and two bogs, for the first year of cultivation with reed canary grass (RCG) with two annual cuts in a mesocosm set-up under controlled conditions. Biomass yields were highly variable, ranging between 0.8 and 7.4 t dry matter (DM) ha^-1 yr^-1, and significantly higher on fen peat soils. Ecosystem respiration (R_eco) fluxes of CO₂ were naturally highest from sites with high biomass establishment. Methane emissions were site-specific, ranging between 0.03 and 1.85 t CH₄ (CO₂eq ha^-1 yr^-1), and affected by biomass growth, as well as bulk density and the iron content within soil. Nitrous oxide fluxes were negligible, despite nitrogen (N) fertilisation with 200 kg N ha^-1 yr^-1. Driven by net primary production (NPP) we found that the fen sites were GHG sinks, with a global warming potential (GWP) of -1.3 to -11.5 t CO₂eq ha^-1 yr^-1 during the first year of rewetting and RCG establishment. The bogs remained sources of carbon (5.3 t CO₂eq ha^-1 yr^-1). Our results highlighted that the nutrient-rich Danish fen peatlands showed a potential for GHG mitigation by paludiculture under extensive agricultural management, while this was not the case for the bog sites due to poor biomass establishment. In conclusion, we found the highest GHG mitigation potential by rewetting and RCG paludiculture on nutrient-rich fen peatlands.