To graze or not to graze? Four years greenhouse gas balances and vegetation composition from a drained and a rewetted organic soil under grassland

• CO2 emissions are low in extensive grassland over organic soil with WT above −25 cm.
• Removing grazing reduced the overall climatic footprint in shallow drained and rewetted sites.
• Rewetting and removing grazing on organic soils can return their C sequestering function.
• Management of vegetation succession could strongly affect the efficacy of such mitigation tool.

Nutrient-poor organic soils under maritime grassland are often located in remote wet locations in the landscape. Leaving these soils without drainage maintenance often raise the water table but continuous management (grazing) means they could remain a source of carbon dioxide (CO2) while also turning into a small source of methane (CH4). Due to geographical and socio-economic reasons, removing these sites from agricultural production may be an option to mitigate greenhouse gas (GHG) emissions. To test this hypothesis we measured GHG fluxes over a four year period, at a drained and a rewetted organic soil under grassland, which were both grazed for the first two years and not grazed for the following two years. Statistical response functions estimated for gross primary production (GPP) and ecosystem respiration (Reco) were used to reconstruct annual CO2–C balances using site-specific models driven by soil temperature, solar radiation, soil water table (WT) and leaf area index (LAI). Annually, soil CO2 emissions were comparable when grazed, although the rewetted site had a lower net ecosystem carbon balance (NECB) despite displaying higher CH4 emissions. Both sites have lower CO2 emissions than typical drained organic soils due to management practices: extensive grazing, no fertilisation and mean annual water tables above −25 cm. When grazing stopped, GPP and Reco increased dramatically driven by vigorous growth of vegetation at both sites. The shallow drained site remained a source of CO2 and small source of CH4 while the rewetted site became either neutral or a small sink of CO2 with decreased CH4 emissions compared to the grazing period. Nitrous oxide (N2O) emissions were negligible at either site. Removing grazing significantly reduced the NECB at both sites but in terms of global warming potential (GWP), the greatest GHG mitigation was in the rewetted site which exerted a cooling effect in the second year after the management shift.