Methane emissions partially offset carbon sink function in global wetlands: An analysis based on global data.

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