Magnitude, spatial patterns and drivers of methane fluxes from coastal salt marshes and mangrove forests in China

Methane（CH4 ） is a highly potent greenhouse gas due to its high radiative forcing and global warming potential.Understanding the magnitude, spatial patterns, and drivers of CH4 emission from wetlands at the regional scale is crucial for formulating suitable strategies to mitigate future climate change. In this study, we synthesized the CH4 flux data reported for major plant communities in the coastal salt marshes and mangrove forests in China and produced a spatial map of CH4 fluxes from the mangrove sediments and salt marshes in China based on the results of machine learning models. Moreover, we examined the main drivers of CH4 flux from these coastal wetlands and estimated the annual CH4 emissions from the salt marshes and mangrove forests in different coastal provinces of China using a bottom-up approach. Our results showed that the mean CH4 fluxes from the salt marsh and mangrove ecosystems in China were 2.22±0.29 and 2.26±0.55 mg m-2h-1, respectively. Across China, the total annual CH4 emission from salt marshes was 31,179.75 Mg CH4 a-1, while that from mangroves was 5,533.97 Mg CH4 a-1with the sediments contributing to 12.7%(701.64 Mg CH4 a-1) of the total efflux. The salt marshes and mangroves in China accounted for 2.75% and 0.14%–0.15% of the global annual CH4 emissions from these two ecosystems,respectively. Moreover, we found that CH4 emissions from the salt marshes and mangroves could offset the cooling impact arising from net carbon dioxide uptake by 88% and 53%, respectively, over a 100-year time scale. Mean annual temperature, soil moisture, and soil organic carbon content were the main factors controlling CH4 emissions from these coastal wetlands.Furthermore, we found that CH4 fluxes from the salt marshes in China decreased with latitude, which implied a possible increase in marsh CH4 emission in response to global warming that should be considered carefully when developing nature-based climate solutions to achieve national goals for carbon neutrality.