Abstract: Wetlands serve as atmospheric carbon dioxide (CO₂) sinks, as well as atmospheric methane (CH₄) source due to the anaerobic soil environment. Although some studies report that the CH₄ emission from wetlands partially offset their net CO₂ uptake, there is no global data analysis on the offset of net ecosystem exchange of CO₂ (NEE) by CH₄ emission in wetland ecosystems. In this study, we collected the data sets of NEE and CH₄ flux which were simultaneously measured in the inland wetlands (peatland and non-peatland wetland) and coastal wetlands (seagrass beds, salt marshes and mangroves) around the world. The results showed that all types of wetlands were atmospheric CO₂ sink, with the NEE values ranking as follows: mangrove (-2011.0 g CO₂·m^-2·a^-1) < salt marsh (-1636.6 g CO₂·m^-2·a^-1) < non-peatland wetland (-870.8 g CO₂·m^-2·a^-1) < peatland (-510.7 g CO₂·m^-2·a^-1) < seagrass bed (-61.6 g CO₂·m^-2·a^-1). When CH₄ flux being converted into CO₂-equivalent flux (CO₂-eq flux) based on the 100-year scale global warming potentials, we found that the CH₄ emissions partially offset 19.4%, 14.0%, 36.1%, 64.9% and 60.1% of the net CO₂ uptake in seagrass beds, salt marshes, mangroves, non-peatland wetland and peatland, respectively. Over the 20-year scale, CH₄ emissions partially offset 57.3%, 41.4%, 107.0%, 192.0% and 177.3% of the net CO₂ uptake, respectively. Some mangroves, peatlands, and non-peatland wetlands acted as net CO₂ equivalent source. Over the 100-year scale, the net greenhouse gas balance of each wetland ecosystem was negative value, which indicated that even accounting CH₄ emission, wetland ecosystem was still an atmospheric carbon sink. Our results indicated that clarifying the main regulation mechanism of CH₄ emission from wetland ecosystems and proposing reasonable CH₄ reduction measures are crucial to maintain the carbon sink function in wetland ecosystems, and to mitigate the trend of climate warming.

Key words: net ecosystem exchange of CO₂, CH₄ emission, offset, net greenhouse gas balance, coastal wetland, inland wetland