DO FORESTED WETLANDS CONTRIBUTE TO GLOBAL WARMING?

Since the Industrial Revolution, greenhouse gases (GHG) concentrations in the atmosphere have increased significantly: carbon dioxide (CO2) by 30%, methane (CH4) has doubled and nitrous oxide (N2O) by 16%. Wetlands contribute to global methane release by 25% but they also sequester carbon. Most studies on wetlands have focused on Northern peatlands but half of the wetlands in the United States are forested wetlands. Can we assume that all wetlands behave the same way?

As conditions in the submerged soils turn from aerobic to anaerobic, oxygen is depleted and no longer acts as the primary electron acceptor; other ions come into play such as nitrate and organic matter in the most reduced conditions. As a result, nitrous oxide, methane and carbon dioxide are generated.

The objective of this research is to determine how changes in water table levels affect GHG emissions in forested wetlands. To do so, soil samples were collected from Francis Marion National Forest (SC) into 15 gallons tanks and brought back into the laboratory to allow for better control over the water table levels. Three soil types were selected with different organic matter content (OMC in %): peat (32%), sandy (7.4%) and clay (4.9%). Soils were submerged for 6 weeks and aerated for 2 weeks. Redox potential and GHG emissions were measured regularly.

All soils got reduced after 6 weeks submerged. CO2 and CH4 were detected only from the soils with a high OMC: peat and sandy soils. We concluded that a threshold might exits around 5% OMC that determines whether or not methane will be released. N2O was not detected; this confirms previous studies that suggested that high fluxes of N2O usually occur in pulse-flooded areas rather than permanently-flooded areas.

To conclude, not all wetlands are alike and their contribution to global warming is determined by water table levels and organic matter content of the soil. It is crucial to make that distinction and consider each wetland type specifically to make the right management decision.