A study of mangrove soils in the Zhangjiang Estuary of China finds that black carbon, a highly stable product of incomplete combustion from sources such as wildfires and fossil fuel burning, persists in mangrove sediments and may contribute to long term carbon sequestration in coastal environments. The work also underscores the importance of dissolved black carbon, a more mobile form that travels through water and can connect land based carbon storage with ocean systems.
Black carbon is unusually resistant to decomposition because of its condensed aromatic structure, allowing it to remain in soils for centuries and making it a potentially important but underrepresented component of the global carbon cycle. In mangrove ecosystems, which already trap large quantities of organic carbon, this recalcitrant material could further enhance the climate mitigation potential of coastal wetlands.
"Our study shows that mangrove soils store not only large quantities of organic carbon, but also a persistent fraction of black carbon that may remain stable over very long timescales," said the study's corresponding author. "Understanding how this carbon behaves is crucial for accurately evaluating the climate benefits of mangrove conservation and restoration."
The researchers collected soil samples along land to sea gradients and at multiple depths within the mangrove forest. They report that black carbon concentrations ranged from roughly 0.95 to 1.67 grams per kilogram of soil, while dissolved black carbon ranged from less than 1 to more than 12 milligrams per kilogram. Both forms generally decreased with increasing distance from land and with soil depth, indicating that local environmental conditions strongly influence their distribution.
The team then examined the factors that control these carbon pools. Plant biomass emerged as the dominant driver of black carbon accumulation, likely because higher vegetation productivity leads to greater organic matter input and stabilization in soils. In contrast, soil nitrogen and moisture exerted major control on dissolved black carbon, suggesting that microbial activity and hydrological processes regulate how carbon moves through mangrove sediments.
Although total black carbon declined with depth, the fraction of highly condensed and stable carbon increased in deeper layers. This pattern suggests that the most resistant carbon structures can persist in subsurface soils and offshore zones, potentially serving as long term carbon reservoirs in coastal systems.
Taken together, the results indicate that mangrove soils function not only as carbon storage sites but also as dynamic regulators of carbon transport between terrestrial and marine environments. Because dissolved black carbon can be exported to coastal waters, mangrove stands may influence carbon cycling in adjacent marine ecosystems as well as on land.
The findings have implications for climate policy and ecosystem management. Protecting mangrove forests and maintaining soil conditions that favor carbon stability could enhance the long term storage of both organic carbon and black carbon, strengthening the role of blue carbon ecosystems in climate mitigation strategies.
"Our work highlights the importance of considering different carbon forms when assessing blue carbon ecosystems," the author added. "Improved understanding of these processes will help refine global carbon budgets and guide strategies to strengthen natural climate solutions."
As governments and institutions increasingly look to coastal ecosystems to offset carbon emissions, this study provides a deeper perspective on how mangroves operate as long term carbon sinks and how their protection and restoration could contribute more effectively to climate regulation.
Research Report:Soil black carbon distribution in a mangrove blue carbon ecosystem
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Shenyang Agricultural University
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