Limited climate benefits of rewetting a shallow drained peatland when interannual variabilities in CO2 and CH4 fluxes are considered

Rewetting of agricultural peatlands is widely recognised as a key climate mitigation strategy, yet its net effect remains uncertain, particularly during the early transition years. We conducted a multi-year study using continuous eddy covariance measurements to quantify carbon dioxide (CO₂) and methane (CH₄) fluxes across two peatland fields undergone rewetting and an adjacent shallow-drained control field in Denmark. Rewetting raised the mean annual water table level by ~7 cm relative to the control when side-by-side comparisons were possible. Our results demonstrate that rewetting can rapidly shift the ecosystem carbon balance toward net CO₂ uptake, likely due to the successful establishment of productive vegetation such as Phalaris arundinacea prior to rewetting. Early vegetation development may therefore accelerate CO₂ uptake compared with slower trajectories observed in more degraded peatlands. However, this benefit can be partially offset by increased CH₄ emissions, particularly during wet periods, which can rival the CO₂ sink strength and reduce the overall greenhouse gas mitigation potential. Even in shallow-drained control areas, modest increases in water table during wet years were sufficient to temporarily reverse net carbon loss, highlighting the sensitivity of early outcomes to hydrological conditions. Management practices, such as autumn biomass cutting, further influenced CO₂ exchange by enhancing early-season uptake, though biomass removal can partially offset these gains. Collectively, our findings underscore that the net climate effect of peatland rewetting depends strongly on interannual variability, water table dynamics, vegetation establishment, and management interventions. Long-term, ecosystem-scale monitoring is thus essential to capture the full spectrum of environmental variability and optimise restoration strategies for effective climate mitigation.