Hidden carbon in mangrove soils may play a larger role in climate regulation than previously thought

(Press-News.org) Mangrove forests are widely known as powerful natural carbon sinks, but new research shows that a lesser-known form of carbon stored in their soils could significantly influence how coastal ecosystems store and transport carbon.

A new study investigating mangrove soils in the Zhangjiang Estuary of China reveals 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 findings also highlight the importance of dissolved black carbon, a mobile form that can travel through water and link land-based carbon storage to ocean systems.

Black carbon is unusually resistant to decomposition because of its condensed aromatic structure. This stability allows it to remain in soils for centuries, making it a potentially important but underrepresented component of the global carbon cycle. In mangrove ecosystems, which already trap large amounts of organic carbon, this recalcitrant material could 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 sampled soils along land-to-sea gradients and at multiple depths within the mangrove forest. They found 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 tended to decrease with increasing distance from land and with soil depth, indicating that environmental conditions strongly influence their distribution.

The study also identified the key environmental factors controlling 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. Meanwhile, soil nitrogen and moisture were major controls on dissolved black carbon, suggesting that microbial activity and hydrological processes regulate how carbon moves through mangrove sediments.

Interestingly, although total black carbon decreased with depth, the fraction of highly condensed and stable carbon increased in deeper layers. This pattern indicates that the most resistant carbon structures may persist in subsurface soils and offshore areas, potentially serving as long-term carbon reservoirs.

These findings suggest that mangrove soils may function not only as carbon storage sites but also as dynamic regulators of carbon transport between land and ocean. Because dissolved black carbon can be exported to coastal waters, mangroves may influence marine carbon cycles as well as terrestrial ones.

The results could 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.

“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 nations increasingly look to coastal ecosystems to offset carbon emissions, studies like this one provide a deeper understanding of how mangroves function as long-term carbon sinks and how their protection could contribute to climate mitigation efforts.

 

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Journal reference: Cao C, Hu W, Yin S, Zhou T, Zeng L, et al. 2026. Soil black carbon distribution in a mangrove blue carbon ecosystem. Environmental and Biogeochemical Processes 2: e006 doi: 10.48130/ebp-0026-0001  

https://www.maxapress.com/article/doi/10.48130/ebp-0026-0001  

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Environmental and Biogeochemical Processes (e-ISSN 3070-1708) is a multidisciplinary platform for communicating advances in fundamental and applied research on the interactions and processes involving the cycling of elements and compounds between the biological, geological, and chemical components of the environment. 

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