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 climate mitigation strategy, yet its net effect remains uncertain, particularly for shallow-drained peatlands during early transition years. We present seven full flux-years and three half flux-years of carbon dioxide (CO 2 ) and methane (CH 4 ) measured by eddy covariance for the first time in Denmark across two peatland fields undergone rewetting with different vegetation types and harvesting regimes, and an adjacent shallow-drained control field. Rewetting in 2022 raised the mean annual water table level (WTL) by ∼7 cm relative to the control, but interannual variability exceeded the differences between areas. The annual greenhouse gas (GHG) balance (g CO 2 -eq m −2 year −1 ) at a rewetting area decreased from 1593 in 2022 to 195 in 2023, primarily reflecting a shift in CO 2 exchange from a net source to sink, with additional CO 2 losses from biomass harvesting in 2022. Comparatively, the control continued as a GHG source (1105) in 2023 dominated by net CO 2 emissions. In 2024, WTL at all areas was ∼9 cm higher than in 2023, causing shallow flooding (∼+2 cm) and higher GHG emissions (379 to 499) at the rewetting areas than at the control (102) because of increased CH 4 emissions (>400 g CO 2 -eq m −2 year −1 ). Nevertheless, GHG emissions at the rewetting areas in 2024 remained less than half of the emissions observed during the early transition period of 2022. Our results emphasise that interannual variability dominates rewetting effects in shallow-drained peatlands, suggesting that such sites may gain limited climate benefit from rewetting and may require GHG accounting approaches beyond simplified emission factors to account for WTL dynamics. • Rewetting shifted peatland CO 2 balance from net source to near-neutral within 3 years. • Elevated CH 4 emissions in wet years limited the climate benefit despite CO 2 uptake. • Moderate water table rise reversed net carbon loss at the shallow-drained control. • Productive grasses ( Phalaris arundinacea ) likely raise post-rewetting carbon uptake. • Rewetting shallow-drained peatlands may raise CH 4 with minor immediate CO 2 benefits.