Monday, January 30, 2012

Freshwater wetlands are important carbon sinks says scientific study

A new study has emphasised the importance of the carbon sequestration capacity of freshwater wetlands. The researchers suggest that temperate freshwater wetlands may have a significant part to play in offsetting greenhouse gas emissions as carbon sinks.

The study was conduced by William Mitsch, an environment and natural resources professor and Blanca Bernal, a graduate student, both from Ohio State University. The study - Comparing carbon sequestration in temperate freshwater wetland communities (abstract) - compares several wetlands at two Ohio wetland sites: one composed of mostly stagnant water and one characterized by water regularly flowing through it. The study showed that the stagnant wetland had an average carbon storage rate per year that is almost twice as high as the carbon storage rate of the flow-through wetland.

For this study the scientists measuring carbon storage in the stagnant wetland found that carbon measurements exceeded that recorded in recent years in various types of wetlands. Accordingly, they suggest that temperate freshwater wetlands may have a significant role in worldwide strategies to offset greenhouse gas emissions.

The capacity of temperate wetlands to store carbon has been under rated with more attention and research on boreal peatlands. “These numbers are a lot higher than those often used to determine policy about wetlands. All of our numbers are, in general, considerably higher than average rates of carbon sequestration for boreal peatlands, but the boreal peatland numbers rule the roost in climate change,” said William Mitsch. “Wetlands make up 6 to 8 percent of the landscape, but they hold much more than 6 to 8 percent of the world’s carbon. They are the forgotten carbon sink.”

Mitsch explained “In an ecosystem that’s terrestrial, especially a forest, plant life falls to the floor but the forests don’t fill up with leaves because they’re decomposing, returning some of that carbon dioxide back into the atmosphere,” Mitsch said. “Wetlands tend to accumulate this litter over centuries, maybe over thousands of years, and they have plants and are productive systems, but they tend not to decompose everything. So carbon builds up in the soil for a long time.”

The scientists collected soil core samples from a forested wetland in Gahanna, in central Ohio, and from Old Woman Creek, a freshwater wetland near Lake Erie in northern Ohio. The Gahanna wetland is a depressional wetland or swamp that remains saturated year-round. Old Woman Creek experiences pulses of water from both agricultural land and lake entry points. The carbon content of the soil and depth of the sediment that stored carbon over the past 50 years was analyzed using radiometric dating with Cesium 137 and Lead 210 isotopes to determine the age of sediments being studied, and the rate of carbon storage per year.

The depressional wetland community as a whole sequestered an average of 317 grams of carbon per square meter per year (2,750 pounds of carbon per acre per year), compared to the average 140 grams per square meter per year (1,215 pounds per acre per year) stored by the flow-through wetland area. By comparison, boreal peatlands in Canada and Siberia sequester much less, at 15 to 25 grams carbon per square meter per year (130 to 220 pounds of carbon per acre per year).

The key to wetlands working as an active sink is the presence of plants absorbing carbon dioxide from the atmosphere through photosynthesis, while reducing the amount of respiration of that carbon dioxide back into the air due to the presence of standing water.

For the sites researched, the depressional wetland amounted to 59 acres, compared to the flow-through wetland’s coverage of 138 acres, but the total annual carbon storage for each is similar at almost 85 tons of carbon per year. The highest carbon uptake – 473 grams per square meter per year (4,100 pounds per acre per year) – was found in the most heavily forested area of the stagnant wetland.

“Few studies have been done in temperate wetlands other than ours, and even fewer have been done in forested wetlands like our Gahanna Woods wetland, where we measured the highest rates,” Mitsch said.

According to Birol Kayranli et al in Carbon Storage and Fluxes within Freshwater Wetlands: a Critical Review (PDF) (2010) "wetlands can be both sources and sinks of carbon, depending on their age, operation, and the environmental boundary conditions such as location and climate." While flooded wetlands generally sequester carbon dioxide, they can also become net carbon sources releasing greenhouse gases such as nitrous oxide and methane.

The Kayranli paper concludes "Wetland protection and restoration measures can improve the carbon sequestration potential of wetlands. However, it takes several decades for the carbon sequestration ability of restored wetlands to reach levels comparable to those of natural wetlands such as peatlands and forested wetlands." Predicting how the carbon balance of wetlands will respond to climate change will depend on many environmental factors connected to wetland hydology and temperature.