Mangrove ecosystems rank among the most efficient "blue carbon" systems on Earth, capable of absorbing and storing vast quantities of atmospheric carbon dioxide (CO2). However, mangroves also release methane (CH4), a potent greenhouse gas, potentially offsetting a portion of their climate mitigation benefits. While prior research has focused primarily on methane emissions from mangrove soils and water surfaces, the role of tree stems as an emission pathway and its significance for global blue carbon accounting have remained largely unexamined.
In a new study, researchers from the South China Botanical Garden of the Chinese Academy of Sciences conducted a global-scale assessment of methane emissions from mangrove tree stems. They also developed a comprehensive database of these emissions to date. The team's findings were published in Nature Geoscience on November 14.
The researchers integrated long-term in situ monitoring from multiple mangrove sites across China, global literature datasets, and machine learning models. This approach allowed them to systematically evaluate the drivers of stem methane emissions and quantify how these emissions offset mangroves' carbon sequestration capabilities.
The results revealed that mangrove tree stems represent a significant yet previously underestimated source of methane. Additionally, stem emissions showed a strong correlation with soil methane fluxes, indicating that methane produced by anaerobic microbial activity in mangrove soils is transported upward through specialized aerenchyma tissues within the trees.
Field observations and isotopic analysis further supported these findings, uncovering a clear gradient in methane flux along stem height: Emissions were highest near the stem base and decreased steadily as height increased.
At the global level, the study estimates that mangrove tree stems release approximately 730.6 gigagrams (Gg) of methane annually. This offsets roughly 16.9% of the carbon buried in mangrove sediments each year. When soil methane emissions are included, total methane losses could offset up to 27.5% of the blue carbon sequestered by mangroves.
These findings suggest that assessing blue carbon benefits solely based on sediment carbon burial may substantially overestimate the climate mitigation potential of mangrove ecosystems.
This work was supported by the National Natural Science Foundation of China and the National Key R&D Program of China, among other sources.
