Assessing the effects of restoration and conservation on gaseous carbon fluxes and climate mitigation capacity across six European coastal wetlands

Coastal wetlands play a substantial role in regulating Earth’s climate through exchanges of greenhouse gases (GHGs). Current European policies promote widespread coastal wetland restoration to reverse historical losses and ongoing pressures. However, substantial uncertainty remains regarding how CO₂ and CH₄ fluxes respond to restoration across different coastal wetland types and whether these responses translate into net climate mitigation in terms of CO₂ equivalents (CO₂-eq). We measured simultaneous CO₂ and CH₄ fluxes using static chambers across four seasons at multiple locations spanning preserved, altered and restored sites within each of six European coastal wetlands of different ecological types. By comparing GHG exchanges and resulting CO₂-eq balances across wetlands, we identified the dominant biogeochemical drivers of CO₂ and CH₄ dynamics and assessed the climate mitigation potential of conservation and restoration actions. CO₂ fluxes were primarily controlled by landscape-scale vegetation cover and inundation, whereas CH₄ emissions responded to more subtle changes in water quality, salinity and wetland hydrodynamics. Comparisons of CO₂-eq balances between altered and restored sites revealed that seagrass replantation and eutrophication reversal generated significant mitigation benefits, driven by enhanced CO₂ uptake and reduced CH₄ emissions, respectively. In contrast, other restoration measures modified CO₂ and CH₄ fluxes in opposing directions, resulting in non-significant net climatic effects of CO2-eq balances. Overall, our results demonstrate that climate mitigation outcomes of coastal wetland restoration are both GHG-specific and wetland-type dependent, underscoring the need for tailored restoration strategies and robust, multi-GHG monitoring to detect and accurately quantify potential climatic benefits.