Greenhouse gas balance of Sphagnum farming on highly decomposed peat at former peat extraction sites

For two years, we quantified the exchange of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) at two different large-scale Sphagnum farming sites. At both, peat extraction left a shallow layer of highly decomposed peat and low hydraulic conductivities. One site was characterized by preceding multi-annual inundation and irrigated by ditches, while the other one was inoculated directly after peat extraction and irrigated by ditches and drip irrigation. Further, GHG emissions from an irrigation polder and the effect of harvesting Sphagnum donor material at a near-natural reference site were determined. GHG mitigation potentials lag behind the results of less decomposed sites, although our results were also affected by the extraordinary hot and dry summer 2018. CO₂ exchanges ranged between -0.6 and 2.2 t CO₂-C ha⁻¹ y⁻¹ and were mainly influenced by low water table depths. CH₄ emissions were low with the exception of plots with higher Eriophorum covers, while fluctuating water tables and poorly developing plant covers led to considerable N₂O emissions at the ditch irrigation site. The removal of the upper vegetation at the near-natural site resulted in increased CH₄ emissions and, on average, lowered CO₂ emissions. Overall, best plant growth and lowest GHG emissions were measured at the previously inundated site. At the other site, drip irrigation provided more favourable conditions than ditch irrigation. The size of the area needed for water management (ditches, polders) strongly affected the areal GHG balances. We conclude that Sphagnum farming on highly decomposed peat is possible but requires elaborate water management.