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Mangrove degradation threatens carbon storage in China's Pearl Bay

Carbon sinks are important in the fight against climate change as they draw down atmospheric carbon dioxide levels by storing organic carbon in the oceans and soil, for example. Within tropical and subtropical intertidal locations, wetland plant communities, known as mangrove forests, are key carbon sinks of organic matter.

However, their ability to sequester carbon is under threat from both natural and anthropogenic activities, enhanced by climate change, including erosion, land subsidence and sea level rise, as well as rising temperatures leading to water vapor deficit and higher salinity.

The magnitude of this reduction in carbon storage according to the level of mangrove degradation is uncertain, though new research in Frontiers in Marine Science has attempted to answer this important question.

Researchers Chuanyi Guo and Dr. Loh Pei Sun of Zhejiang University, China, and global colleagues investigated the mangrove reserve of Pearl Bay, China, to address these concerns.

Here, the mangrove trees (comprised of Aegiceras corniculatum, Bruguiera gymnorhiza, Avicennia marina and Kandelia obovate) have an average age of 50 years and the top sediments (1 m depth) are estimated to contain an average of 144.5 Mg of organic carbon per hectare, which is half the estimated national average for China's mangrove forests and approximately a quarter of the global average.

Using sediment cores dated between 1963 and 2020, the scientists quantified the grain size, bulk elemental composition, stable carbon isotopes and abundance of lead, lignin and organic matter from bottom to top, representing a transect through time.

As such, they identified significant changes along the two cores, including:

• Reduction in mean sedimentation rate from ~80 mm y^-1 between 1977 and 1980 to ~17 mm y^-1 across 1980 to 2021.

• Decline in fine sediment fractions containing organic carbon with coarser sand content consequently increasing and ineffectively capturing organic matter, being ~39% sand, ~47% silt and ~13% clay through the period 1963 to 2001, changing to ~50% sand, ~36% silt and ~12% clay from 2001 to 2020.

• The ratio of total organic carbon to total nitrogen decreased from 25 to 15 across the study interval, with soil organic carbon declining from 2.57 to 1.97 Mg of organic carbon per hectare per year preceding and after 2001 respectively.

• Stable carbon isotopes (δ¹³C) increased in both cores by ~0.85 ‰.

• Organic matter from mangroves decreased with respect to that derived from river and marine sources (~65%, ~23% and ~10% respectively up to 2001 and ~57%, ~31% and ~11% respectively thereafter).

• Reduced lignin content through both cores, a primary component of woody biomass possessing high carbon content.

Notably, the reduction in organic matter and lignin content since 2001, linked to degradation of the mangrove landscape by sea level rise and enhanced tidal action, as well as human activities like wastewater discharge and conversion to shrimp farms by the fishing industry, decreased carbon storage in the bay. The data also indicated lower tidal ranges and slower sedimentation rates exacerbating organic matter diminishment.

Despite mangrove forests only comprising 0.1% of Earth's land area, this research is important to inform the sustainable management of mangroves to lessen further loss of organic matter inputs, as well as the degradation of existing carbon stored in the soils.

In the latter case, carbon would be released back to the atmosphere and make mangroves carbon sources instead of sinks. Such efforts will help to conserve mangrove forests both as a natural habitat for wildlife, as well as for their key role in helping the planet mitigate against climate change.