Climate Emergency News - How Mangrove Swamps Capture Atmospheric Carbon

Mexican mangroves have been capturing carbon for 5,000 years | News

Researcher entering BCS mangroves

Credit: Ramiro Arcos Aguilar

At the moment, we, and the planet we're floating in space on, need all the friends we can get, and few are more useful to us that the world's mangrove swamps. With humans releasing carbon into the atmosphere from fossils fuels as quickly as we can dig the stuff up or extract it from the geology, our friendly mangrove swamps are locking it back into the soil as quickly as they can bury the organic matter some of it is turned into. It can't count for all of it, of course, but every little helps.

And a team of scientists from the University of California Riverside (UCR) and the University of California San Diego (UCSD), have shown that it can stay locked up for 5,000 years, provided we keep the swamps alive and processing it. If not, it'll be released back into the atmosphere, with possibly catastrophic results.

Mangroves are higly specialised plants that can live and thrive in salt and brackish water subject to tidal changes, and with their roots in anoxic mud that would prevent most other plants from obtaining nutrients. Mangroves can quickly colonise these estuarine and coastal mudflats with a simple trick - their relatively large seed start to germinate and produce a long, pointed root while they are still attached to the parent plant. When they are released, they fall straight down to the soft mud and the root becomes embedded, in effect planting itself. It then produces the leaf-bearing stems which send down ariel roots, with air tubes like snorkels, into the mud to provide the oxygen the feeding roots need in the anoxic mud.

Mangrove forests - Wikipedia.

The presence of these dense 'forests' of mangroves slows down the flow of water and causes more mud and floating plant matter to be deposited, so the swap deepens and extends over time, if left undisturbed. But it's what gets locked up in the mud that is the important thing from the point of view of Earth's carbon cycle. Because of the lack of oxygen, bacterial activity is low, giving rise to peat-like layers of plant matter, which decays at almost negligible rate.

The scientists have shown that the carbon locked up in this peat remains there for about 5000 years, as the University of California - Riverside news release explains:

Mangroves in Baja California.

Credit: Matthew Costa/UCSD

The team expected that carbon would be found in the layer of peat beneath the forest, but they did not expect that carbon to be 5,000 years old. This result, along with a description of the microbes they identified, is now published in the journal Marine Ecology Progress Series.

What’s special about these mangrove sites isn’t that they’re the fastest at carbon storage, but that they have kept the carbon for so long. It is orders of magnitude more carbon storage than most other ecosystems in the region.

Emma Aronson, senior co-author
Environmental microbiologist
University of California Riverside,
Riverside, California, USA

Peat underlying the mangrove trees is a combination of submerged sediment and partially decayed organic matter. In some areas sampled for this study, the peat layer extended roughly 10 feet below the coastal water line.

Unusual roots of the mangroves.

Credit: Matthew Costa/UCSD

Little oxygen makes it to the deepest peat layer, which is likely why the team did not find any fungi living in it; normally fungi are found in nearly every environment on Earth. However, oxygen is a requirement for most fungi that specialize in breaking down carbon compounds. The team may explore the absence of fungi further in future mangrove peat studies.

There are more than 1,100 types of bacteria living beneath the mangroves that consume and excrete a variety of chemical elements. Many of them function in extreme environments with low or no oxygen. However, these bacteria are not efficient at breaking down carbon.

These sites are protecting carbon that has been there for millennia. Disturbing them would cause a carbon emission that we wouldn’t be able to repair any time soon.

If we let these forests keep functioning, they can retain the carbon they’ve sequestered out of our atmosphere, essentially permanently. These mangroves have an important role in mitigating climate change.

Matthew Costa, first author
Coastal ecologist
University of California San Diego
San Diego, California, USA.

The deeper you go into the peat soils, the fewer microorganisms you find. Not much can break down the carbon down there, or the peat itself, for that matter. Because it persists for so long, it’s not easy to make more of it or replicate the communities of microbes within it.

Mia Maltz, co-author
Microbial ecologist
University of California Riverside,
Riverside, California, USA

There are other ecosystems on Earth known to have similarly aged or even older carbon. Arctic or Antarctic permafrost, where the ice hasn’t yet thawed allowing a release of gases, are examples. Potentially, other mangrove forests as well. The researchers are now scouting mangrove research sites in Hawaii, Florida and Mexico’s Yucatan Peninsula as well.

Carbon dioxide increases the greenhouse effect that is causing the planet to heat up. Costa believes that one way to keep this issue from worsening is to leave mangroves undisturbed.

The abstract to the team's open access paper in the journal Marine Ecology Progress Series, gives more technical details:

ABSTRACT: Mangroves provide important ecosystem services, including storing carbon belowground for millennia. Mangrove carbon storage relies in part on high primary productivity, but essential to the long-lived nature of this storage is the slow rate of microbial decomposition of peat. In this study, we (1) examined how carbon and nitrogen densities and microbial community composition vary with peat age and (2) describe the formation of peat deposits over time. At 4 mangrove sites near La Paz, Baja California Sur, Mexico, we cored the sediments until rejection and obtained 5 cm samples at 20 cm intervals. In these samples, we measured organic carbon (C_org), total nitrogen, δ¹³C, δ¹⁵N, and radiocarbon (¹⁴C) age. We observed peat carbon densities of 3.4 × 10-2 ± 0.2 × 10-2 g cm-3, C_org:N ratios of 42 ± 3, and inter-site variation in C_org:N that reflects differing preservation conditions. Recalcitrant organic matter sources and anaerobic conditions leave a strong imprint on peat microbial communities. Microbial community composition and diversity were driven by depth and sediment characteristics, including C_org:N ratio and ¹⁴C age. Carbon dating allowed us to reconstruct the accumulation of organic matter over the last 5029 ± 85 yr. Even over this long time scale, though microbes have evidently continuously cycled the peat nitrogen pool, peat carbon density remains effectively unchanged.

Costa MT, Ezcurra E, Aburto-Oropeza O, Maltz M, Arogyaswamy K, Botthoff J, Aronson E (2022)
Baja California Sur mangrove deep peat microbial communities cycle nitrogen but do not affect old carbon pool.
Mar Ecol Prog Ser 695:15-31. DOI: 10.3354/meps14117

Copyright: © 2022 The authors. Published by Inter-Research Science Publishers. Open access
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

Although mangrove forests alone can't solve the mess we've made of the planet, they help, and destroying them would be catastrophic, so we have to do all we can to protect and encourage them, as though our lives depend on it, because they do, and so do the lives of our children, grandchildren and future generations.

There is no planet B!

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