Mangrove forests on the Yucatan Peninsula store record amounts of carbon

Coastal mangrove forests are carbon storage powerhouses, tucking away vast amounts of organic matter among their submerged, tangled root webs.

But even for mangroves, there is a “remarkable” amount of carbon stored in small pockets of forest growing around sinkholes on Mexico’s Yucatan Peninsula, researchers report May 5 in Biology Letters. These forests can stock away more than five times as much carbon per hectare as most other terrestrial forests.

There are dozens of mangrove-lined sinkholes, or cenotes, on the peninsula. Such carbon storage hot spots could help nations or companies achieve carbon neutrality — in which the volume of greenhouse gas emissions released into the atmosphere is balanced by the amount of carbon sequestered away (SN: 1/31/20).

At three cenotes, researchers led by Fernanda Adame, a wetland scientist at Griffith University in Brisbane, Australia, collected samples of soil at depths down to 6 meters, and used carbon-14 dating to estimate how fast the soil had accumulated at each site. The three cenotes each had “massive” amounts of soil organic carbon, the researchers report, averaging about 1,500 metric tons per hectare. One site, Casa Cenote, stored as much as 2,792 metric tons per hectare.

Mangrove roots make ideal traps for organic material. The submerged soils also help preserve carbon. As sea levels have slowly risen over the last 8,000 years, mangroves have kept pace, climbing atop sediment ported in from rivers or migrating inland. In the cave-riddled limestone terrain of the Yucatan Peninsula, there are no rivers to supply sediment. Instead, “the mangroves produce more roots to avoid drowning,” which also helps the trees climb upward more quickly, offering more space for organic matter to accumulate, Adame says.

As global temperatures increase, sea levels may eventually rise too quickly for mangroves to keep up (SN: 6/4/20). Other, more immediate threats to the peninsula’s carbon-rich cenotes include groundwater pollution, expanding infrastructure, urbanization and tourism.