Mangroves are recognized as crucial blue carbon ecosystems in mitigating climate change. However, methane (CH₄) emissions may partially counterbalance their C sequestration capacity. While recent studies indicated that methane oxidation minimized emissions in mangroves, the contribution of plant-mediated methane release remains underexplored. Emerging evidence suggested that wetland (including mangroves) trees can emit significant amounts of soil-derived methane through their stem, a factor that has not been well quantified in mangrove blue C sequestration. Here, we examined in situ mangrove stem CH₄ emissions along a latitudinal gradient and built a global mangrove CH₄ dataset. We found that stem emissions, previously overlooked, can partially offset the blue carbon function of mangroves. Mangrove stem CH₄ flux rates were well correlated with wood density, soil organic content and salinity, with lower wood density salinity and higher wood water content facilitating higher emissions. Strong correlation between stem CH₄ emissions and soil CH₄ flux, suggesting that the majority of stem CH₄ emissions originate from mangrove sediments. Through a synthesis of mangrove CH₄ ion emission data, we estimate that global mangrove stem CH₄ emissions amount to 730.60 (95%CI: 586.09– 876.93) Gg yr^-1, which can partially offset 16.9 % blue carbon burial in mangroves. If we included the soil-mediated methane (10.6%), the total methane emissions in mangroves can offset 27.5% blue C burial. Therefore, mangrove methane emission is greater than previously estimated. Our findings emphasize the importance of incorporating stem CH₄ emissions into future blue carbon budgets for mangroves.
 

Blue Carbon, Methane, Stem, Mangrove, Coastal wetlands, GHGs,