Mangroves and black carbon: a hidden carbon store

Mangroves are widely recognised for their ability to store carbon, yet new research indicates that they may do so through more pathways than conventional estimates capture.

Scientists have found that a durable, persistent form of carbon associated with fires and fossil-fuel combustion can accumulate in mangrove soils. A dissolved form may also be carried from land into coastal waters.

The research examines mangrove soils in China’s Zhangjiang Estuary, with particular attention to black carbon. This soot-like material is created through incomplete combustion and is exceptionally resistant to degradation.

According to the study, black carbon may be an understated but significant component of long-term carbon storage in coastal wetlands, as well as of carbon transfer into the ocean.

What makes black carbon different

Most conversations about mangrove “blue carbon” centre on organic carbon from dead roots, leaves and other plant matter that becomes buried in low-oxygen sediment.

Black carbon has a different origin. It is produced when biomass or fossil fuels do not burn completely, and its molecular structure makes it remarkably stable.

Its condensed aromatic structure means black carbon resists decomposition. It can remain in soil for centuries, potentially serving as a long-term carbon store in ecosystems such as mangroves, which already retain substantial amounts of material.

“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 Junjian Wang from the Sudan University of Science and Technology.

“Understanding how this carbon behaves is crucial for accurately evaluating the climate benefits of mangrove conservation and restoration.”

Sampling mangrove soils from land to sea

To establish where this carbon ends up, the scientists collected soil samples along a land-to-sea gradient and at several depths within the mangrove forest.

This approach is important because mangroves are not uniform settings. Conditions nearer the land may differ greatly from those closer to open water, while surface soils can function differently from deeper layers, where oxygen is limited and decomposition is slower.

The team analysed two types of black carbon. The first was solid black carbon held in the soil, while the second was dissolved black carbon: a mobile form that can travel with water and may be exported to coastal seas.

Black carbon concentrations in the soil ranged from approximately 0.95 to 1.67 grams per kilogram of soil.

Dissolved black carbon levels ranged from under 1 to above 12 milligrams per kilogram. In general, both types declined farther from land and at greater soil depths.

This distribution indicates that landscape position and local environmental conditions strongly influence the storage and transport of black carbon, rather than it being evenly dispersed throughout the system.

Why black carbon builds up

The study also examined the factors governing these carbon pools. For solid black carbon, plant biomass emerged as the most influential factor.

The researchers propose that denser vegetation supplies more organic matter to the soil and creates greater opportunities for carbon to become stabilised and retained.

Soil nitrogen and moisture were the principal influences on dissolved black carbon. This suggests an interaction between biological and hydrological processes: microbial activity may shape carbon processing, while water movement affects how much becomes dissolved and mobile.

The key point is that black carbon is not simply present without change. It responds to mangrove forest productivity, soil wetness and the way nutrients influence the microbial environment.

A surprising pattern at depth

One of the study’s more notable findings emerged when the researchers examined deeper soil layers.

Overall black carbon levels fell with depth, yet the share that was highly condensed and stable rose. Put another way, deeper soils may contain less black carbon, but the carbon that remains is more durable and resistant to breakdown.

This implies that the most stable forms could persist in subsurface layers, and perhaps in offshore areas where material is transported and buried.

Over time, this may form enduring carbon reservoirs that can be overlooked when attention is limited to surface soils.

Mangroves as carbon banks

Rather than depicting mangroves as static carbon stores, the research also highlights carbon movement.

Dissolved black carbon can move through water, meaning mangroves may both retain carbon and help control its flow between land and sea.

As dissolved black carbon can be exported into coastal waters, mangroves may affect marine carbon cycles as well as those on land.

This matters because it links local soil processes with wider ocean systems. From this perspective, a mangrove forest functions as both a carbon bank and a carbon transfer station.

Why this matters for climate policy

Mangrove conservation is frequently presented as a natural climate solution, and with good reason. However, the study suggests that climate accounting may overlook part of the story when black carbon and its dissolved form are excluded.

“Our work highlights the importance of considering different carbon forms when assessing blue carbon ecosystems,” Wang said.

“Improved understanding of these processes will help refine global carbon budgets and guide strategies to strengthen natural climate solutions.”

In practical terms, conservation and restoration policy may need to consider not only the extent of mangroves under protection, but also the soil conditions that allow carbon to remain stable.

If black carbon does contribute to long-term sequestration, preventing disturbance to mangrove soils could be as important as safeguarding the trees themselves.

The complex role of mangroves

The findings form part of an expanding effort to understand coastal ecosystems more closely as countries seek ways to offset emissions.

Mangroves already have a well-established reputation as carbon sinks, but this work indicates that their contribution may be more complex than previously understood.

They retain conventional organic carbon, preserve black carbon that is centuries old, and influence the movement of carbon from land to sea.

If so, mangroves are doing more than simply helping: they are carrying out the planet’s slow, unseen climate work in ways that scientists are only beginning to quantify.