Mangrove soils store a hidden form of carbon that resists decay for centuries

Mangrove forests have long been credited as powerful carbon sinks. Their dense root systems trap organic matter, their waterlogged soils slow decomposition, and their canopies sequester carbon at rates that rival tropical rainforests per unit area. But a study published in Environmental and Biogeochemical Processes points to a carbon fraction in mangrove sediments that has received far less attention - one that may be far more durable than the organic carbon typically measured in blue carbon assessments.

The research, conducted at the Zhangjiang Estuary in southeastern China, examined how black carbon distributes itself through mangrove soils and what controls that distribution. Black carbon is the residue of incomplete combustion - the charred remnants left by wildfires, agricultural burning, and fossil fuel use. Its defining characteristic is its condensed aromatic structure, which makes it extraordinarily resistant to microbial breakdown. While organic carbon in soils can cycle back to the atmosphere within years or decades, black carbon can persist for hundreds to thousands of years.

Two forms, one ecosystem

The researchers distinguished between two forms of black carbon in the mangrove sediments. Particulate black carbon, the solid fraction, ranged from roughly 0.95 to 1.67 grams per kilogram of soil. Dissolved black carbon, a mobile form that travels through water, ranged from less than 1 to more than 12 milligrams per kilogram. Both concentrations decreased with increasing distance from land and with soil depth - a pattern suggesting that their distribution is not random but shaped by specific environmental conditions.

Plant biomass emerged as the dominant driver of particulate black carbon accumulation. Areas with greater vegetation density showed higher black carbon concentrations, likely because active plant growth promotes organic matter input and soil stabilization. For dissolved black carbon, soil nitrogen content and moisture levels were the key controls, pointing to microbial activity and hydrology as the main regulators of how mobile carbon behaves in these systems.

One pattern stood out as particularly significant: although total black carbon declined with depth, the proportion composed of highly condensed, structurally complex carbon increased in deeper layers. The most resistant forms of black carbon - those with the tightest aromatic structures and the longest potential residence times - appear to concentrate where they are least accessible to decomposition.

Carbon that travels to the ocean

Dissolved black carbon introduces a dynamic that complicates simple accounting of mangrove carbon stocks. Unlike particulate carbon that stays put in sediments, dissolved black carbon moves with water. It can migrate through pore spaces in soils, enter tidal channels, and eventually reach coastal and open ocean systems. This means mangroves do not just store carbon locally - they may actively export a stable carbon fraction to the ocean, where it can persist in deep water for extended periods.

"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 finding that dissolved black carbon links terrestrial and marine carbon cycles has implications for how scientists model global carbon budgets. Current estimates of blue carbon storage often focus on in-situ organic carbon without accounting for this lateral transport. If dissolved black carbon is exported to the ocean in meaningful quantities, blue carbon assessments may be systematically underestimating the contribution of coastal wetlands.

What this means for conservation policy

Mangroves are disappearing at an alarming rate. Coastal development, aquaculture expansion, and sea-level rise have reduced global mangrove cover by roughly 20 to 35 percent over the past five decades, depending on the region. Conservation programs increasingly invoke blue carbon as a justification for protection, with some governments and carbon credit markets assigning monetary value to the carbon stored in coastal ecosystems.

If black carbon represents a significant and durable component of mangrove carbon storage, those valuations may be incomplete. The study suggests that maintaining plant biomass - through intact vegetation and the prevention of deforestation - is particularly important for sustaining the conditions that allow black carbon to accumulate. Soil moisture and nitrogen levels, which regulate dissolved black carbon mobility, also need to be considered when assessing how land-use changes might alter carbon dynamics.

"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."

The study is limited to a single estuary site in China, and black carbon inputs, soil conditions, and vegetation types differ across the world's mangrove forests, which span tropical and subtropical coastlines across Asia, Africa, the Americas, and Australia. Whether the patterns observed at Zhangjiang apply broadly will require comparable studies in other regions. The relationship between dissolved black carbon export and long-term ocean carbon storage also warrants closer investigation, as the fate of this mobile fraction once it leaves the estuary remains poorly constrained.

What the research does establish clearly is that the carbon story of mangroves extends beyond organic matter. A combustion-derived, chemically stable fraction is present in these sediments, it accumulates in predictable ways tied to vegetation and soil conditions, and it has the potential to travel - connecting one of the world's most threatened ecosystems to carbon cycles that extend far offshore.