Short-term carbon cycling at a Sphagnum farming site under drought stress

Paludiculture is a new land use option for degraded peatlands, producing biomass under wet and peat preserving conditions. While previous studies indicate a significant greenhouse gas mitigation potential, the impact of bryophyte and vascular plant species on carbon cycling is not yet fully understood, especially under drought stress and climate warming conditions. In July 2018, we conducted a pulse labelling experiment at a Sphagnum farming area in Northwestern Germany to trace sequestered carbon dioxide (¹³CO₂) in above-ground biomasses of peat mosses (Sphagnum) and dominant vascular plant species purple moor grass (Molinia caerulea), soil microbial biomass (C_mic), dissolved organic carbon (DOC), as well as dissolved and emitted CO₂ and methane under drought stress induced by summer drought and simulated climate warming using Open Top Chambers (OTCs). We observed fast label allocation to all investigated carbon pools with the exception of DOC. Although label uptake was clearly higher in Molinia compared to Sphagnum, most carbon was lost via respiration within a few days and the percentage of stored carbon after 140 days was clearly higher in Sphagnum and C_mic. Differences between warmed and control plots were small, presumably due to the already hot and dry conditions. Our results highlight that carbon uptake and storage processes are maintained even under extreme drought conditions, while further experimental warming using OTCs was less influential. The presented findings confirm the important role of Sphagnum in carbon retention and the risk of methane emissions even at low water levels via plant-mediated transport. Consequently, an elaborate irrigation management and control of vascular plants are the key to successful Sphagnum farming and GHG mitigation.