Comparative assessment of sediment carbon storage and greenhouse gas dynamics in degraded mangroves and saltmarsh ecosystems of the Sundarban, India

The coastlines of the world are lined with wetlands that act as powerful natural engines for climate stability. Among these, mangrove forests and saltmarshes are particularly vital because they trap carbon from the atmosphere and lock it away in the soil beneath them. This process, often called blue carbon, helps slow the warming of the planet. However, these ecosystems are not just storage units; they are also active chemical factories that release gases back into the air. Some of these gases, like carbon dioxide, are part of the natural breathing of the soil, while others, like methane and nitrous oxide, are potent greenhouse gases that can trap heat even more effectively than carbon dioxide. Understanding how these different wetlands store carbon and release gases is crucial, especially when the plants are damaged or degraded by human activity. If we want to protect these landscapes, we need to know exactly how they function under stress, because a wetland that stores a lot of carbon might still be releasing harmful gases in ways we do not fully expect.

In the Sundarban, a vast deltaic region on the eastern coast of India, researchers set out to compare two types of these wetlands: degraded mangrove forests and saltmarshes. They focused on a specific area known as Sagar Island, where human activity has altered the natural landscape to varying degrees. The team selected three distinct sites to study. One site was located far from human settlements and experienced relatively low pressure from people. The other two sites were much closer to villages and towns, facing high levels of human interaction, including agriculture, fishing, and settlement expansion. At each of these locations, the scientists examined both the mangrove areas, where trees like Avicennia and Bruguiera grow, and the adjacent saltmarshes, which are dominated by grasses and low-lying vegetation. Their goal was to look deep into the ground, not just at the surface, to see how much carbon was stored and how the soil was releasing gases into the atmosphere.

To get a complete picture, the researchers dug into the soil down to a depth of one meter. They collected samples in layers, moving from the surface down to the deepest point, to measure the amount of organic carbon trapped in the sediment. They also measured the density of the soil and the levels of salt and nitrogen within it. Simultaneously, they placed sealed chambers over the ground to capture the gases being emitted by the soil. This allowed them to measure the flow of carbon dioxide, methane, and nitrous oxide leaving the ecosystem. By comparing the data from the mangroves and the saltmarshes across the three different sites, they could see how the type of vegetation and the level of human disturbance changed the way these ecosystems behaved.

The results revealed a striking difference in how these two ecosystems store carbon. In the mangrove forests, the amount of carbon in the soil increased as the researchers went deeper. The deepest layers, from sixty to one hundred centimeters down, held the most carbon. This suggests that mangroves are excellent at burying carbon deep underground, where it is protected and preserved for a long time. In contrast, the saltmarshes showed the opposite pattern. The carbon was concentrated near the surface and decreased as they dug deeper. This indicates that saltmarshes store most of their carbon in the top layers of soil, where it is more exposed to the elements and more likely to break down quickly. Because of this deep storage, the mangroves held significantly more total carbon in the top one meter of soil, ranging from nearly one hundred to over one hundred and seven metric tons per hectare. The saltmarshes held less, with totals between sixty-seven and seventy-nine metric tons per hectare.

This difference in storage also influenced how the ecosystems released gases. The saltmarshes, with their carbon concentrated near the surface, released much more carbon dioxide than the mangroves. The measurements showed that saltmarshes emitted between 156.7 and 194.4 milligrams of carbon dioxide per square meter per hour, while mangroves emitted between 104.0 and 134.3 milligrams. This suggests that the carbon in the saltmarshes is being processed and returned to the atmosphere more rapidly. However, the mangroves told a different story regarding methane. Despite holding more carbon overall, the mangroves released more methane, a potent greenhouse gas, than the saltmarshes did. The mangroves emitted between 0.348 and 0.422 milligrams of methane per square meter per hour, whereas the saltmarshes released less, ranging from 0.284 to 0.317 milligrams.

The study also looked at nitrous oxide, another powerful greenhouse gas. Here, the saltmarshes again showed higher activity, releasing more of this gas than the mangroves. The saltmarshes emitted between 89.8 and 95.3 micrograms of nitrous oxide per square meter per hour, compared to 75.4 to 84.5 micrograms from the mangroves. The researchers found that the saltmarshes generally had higher levels of salt and nitrogen in the soil, which seemed to drive these higher emissions of carbon dioxide and nitrous oxide. The mangroves, with their deeper carbon storage and different chemical environment, seemed to favor the production of methane instead.

These findings highlight that not all coastal wetlands function the same way, even when they are located near each other. The mangroves act as deep reservoirs, holding onto carbon securely in the lower soil layers, but they also produce more methane. The saltmarshes, while holding less total carbon, are more active on the surface, releasing more carbon dioxide and nitrous oxide. This means that managing these ecosystems for climate benefits requires a nuanced approach. Protecting the mangroves helps preserve a massive underground carbon bank, but it also means managing the methane they release. Managing the saltmarshes requires attention to the rapid turnover of carbon near the surface and the release of nitrogen-based gases. The study concludes that to truly understand the climate value of these degraded landscapes, we cannot look at carbon storage alone. We must consider the entire picture of how these ecosystems store carbon and exchange gases with the atmosphere, recognizing that different wetlands play different roles in the global climate system.