Whether small and shrubby or tall and majestic, mangroves have an unusual ability – they are specially adapted to grow in brackish water, and can tolerate ocean waves lapping at their stilt-like roots. As stands mature, soil and decaying plant matter becomes captured in the intricate web of their roots.
“The soil in mangrove ecosystems stores huge amounts of carbon,” says U.S. Forest Service scientist Carl Trettin. “Mangrove stands can contain higher carbon density than any terrestrial ecosystem.”
Trettin and Christina Stringer, the study’s lead author, had estimated the amount of carbon stored in the mangrove forests in an earlier study. For this study, which was published in the journal Wetlands Ecology and Management, the scientists collected and analyzed soil samples from 52 plots in the Zambezi River Delta, Mozambique.
The Delta covers 4,600 acres, and the scientists compared mangrove soil from four habitats – river, creek, interior, and the seaward fringe. The seaward fringe sites were closest to the ocean, and mangrove stands there were mixed with sandy areas where palm trees and grasses grew.
“We assessed chemical and physical soil properties across these four types of habitat,” says Trettin. “Our goal was to determine whether soil properties and carbon storage ability were associated with specific habitats.” The scientists also identified the sources of organic material enmeshed in the mangrove roots.
“We expected to find differences in soil properties,” says Trettin. “In particular, we expected that soil from sites on the Zambezi River Delta would be very different than soils near the ocean.” The study confirmed that differences in soil properties were greatest when comparing the seaward fringe and the interior areas.
However, soil properties across the study area were more similar than the scientists were expecting, despite the large study area.
“Soils in the Zambezi River Delta had low variation,” says Trettin. The similarities could be explained by the local geography, as the area is an active delta, which means that the Zambezi River is constantly carrying new sediments downstream and land is constantly being re-shaped.
“The low variation among soil properties probably reflects this structureless nature,” says Trettin. “The delta is constantly accumulating new sediments and organic matter is produced very quickly.”
Trettin and his colleagues were able to determine that in the seaward fringe, most of the organic matter had originated in the ocean, while in the interior sites, decaying plant matter was an important contributor.
Most soil carbon and nitrogen was stored about a foot below the surface of the soil, and the concentrations gradually decreased at deeper levels. In the interior sites, deeper levels of the soil stored much less carbon than similar soil depths at other sites.
The study also showed that mean carbon concentrations in the soil were between 1.38 and 2.38 percent, which is similar to what other studies have found.
“We expected variations in the hydrologic connectivity and sediment accumulation of our sites to affect the amount of carbon stored,” says Trettin. “We found a number of statistically significant differences between the sites, but understanding mangroves’ role in the global carbon cycle will take more time and research.”
Forest Service scientists from the Pacific Southwest Research Station are also studying mangroves and have found that after 10 years, restored mangrove plantations store as much carbon as intact forests. Learn more about mangrove restoration.
For more information, email Carl Trettin at email@example.com.