Mangroves are among the most carbon-dense ecosystems in the world, making them strong allies in the fight against climate change. Their so-called ‘blue carbon’ is mostly found in soil. New international research reveals that the nature of this soil carbon varies significantly depending on the coastal landscape where mangroves grow. These findings challenge the "one-size-fits-all" approach to mangrove conservation and restoration, with major implications for climate policy and mangrove restoration projects.
This study was conducted by the MangC network, coordinated by Dr Marie Arnaud (IEES, IRD, Sorbonne Université and Birmingham University), and notably including Dr Cornelia Rumpel (IEES, CNRS). This group had dig deep into the mangrove mud to reveal the secret of the mangrove blue carbon. The study was published in Communications Earth and Environment in Springer Nature.
Not all mangrove carbon is the same
The study shows that in deltaic mangroves, much of the carbon is locked away — bound to minerals and therefore probably stable at longer timescales. The carbon there sticks tightly to minerals, forming what scientists call mineral-associated organic matter. The carbon contained in those associations are hidden from microbial reach, resulting in its protection most likely for decades, even centuries. Analysis of lignin, the chemical that wood is rich in, showed that the sources of this carbon are probably partly located upstream.
But in a mangrove on a karstic (limestone) open coast, the story is different. There, the carbon originates principally from the mangrove trees and is less decomposed being present mostly as root fragments woven into the dark, spongy soil. This is particulate organic matter. It’s wobbly, unbound, and could be freely available to microbes if the environmental conditions change. A rise in temperature, a shift in water flow, or land-use changes could tip the balance, accelerating its decomposition and releasing this carbon back to the atmosphere as green-house gases.
Estuarine and open-coast mangroves told a more mixed story — their soils contained a blend of the above two carbon pools, and the stage of decomposition of the organic matter sat somewhere between the extremes. By using the chemistry of these coastal soils, this study revealed not just how much carbon is stored in mangroves — but showed that its nature can be different, depending on the surrounding landscape.
“We found that the type of coastline geomorphology dramatically influences the nature of mangrove carbon” explains Marie Arnaud, researcher at the Institute for Sustainable Development, France and lead author of the study. “This means we need to tailor our restoration and conservation strategies to the geomorphic settings — one size doesn't fit all.”
A new lens for blue carbon policy
The study urges policymakers and restoration practitioners to move beyond simple carbon stock assessments and instead consider the “form” of blue carbon. Why? Because different forms respond differently to climate change, disturbances or restoration practices.
“If we want blue carbon to be part of the climate solution, we need to protect not just how much carbon is stored — but in which form it is stored,” adds Cornelia Rumpel co-author of the study and researcher at CNRS, France.
The MangC team
The interdisciplinary MangC team was created in 2021 and includes a dozen researchers. The success of the team relies on the intentional inclusion of scientists with different backgrounds (ecologists, biogeochemists, chemists), career stages, and nationalities, which allowed for original perspectives, ideas, and points of view. These findings are just the beginning of the MangC team, which continues to grow and will keep shedding light on the processes beneath our feet in the mangroves! This initiative happens thanks to this motivated team and their funder, notably the European Union's Horizon 2020 research and innovation grant No 101062040 and the PEPR FairCarboN TROPECOS grant ANR-22-PEXF-012.
The authors of the study are Catherine E. Lovelock (The University of Queensland, Australia), Marion Maceiras (Sorbonne Université, France), Dang Thuong-Huyen and Thanh-Phong Le (Ho Chi Minh City University of Technology, Vietnam), Sarah Robin and Cyril Marchand (Université de la Nouvelle-Calédonie, New Caledonia), Samuel Abiven (École Normale Supérieure, France), Amrit Kumar Mishra (James Cook University, Australia), Syed Hilal Farooq (IIT Bhubaneswar, India), Tuhin Bhadra (Adamas University, India), Cornelia Rumpel and Axel Felbacq (CNRS,Sorbonne Université, France), Ahmad Aldrie Amir (Universiti Kebangsaan Malaysia), Nicolas Bottinelli and Marie Arnaud (IEES, Institute of sustainable development, Sorbonne Université, France / University of Birmingham, UK).