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CARBON SEQUESTRATION

Concern over rising global temperatures and climate change has directed attention to wetlands since they are recognized as important carbon sinks. Drainage of wetlands releases carbon to the atmosphere in the form of carbon dioxide, one of several greenhouse gases influencing global temperatures. In wetlands, organic matter (carbon) accumulates in the soils as well as in vegetation. Woody plants, thereby, store carbon for longer periods than annual herbaceous plants. While the above-ground biomass of perennial herbs is released back into the aquatic ecosystem seasonally, the below-ground biomass remains in the substrate and contributes to longer-term storage. Temperate and subtropical wetlands are recognized as important for attenuating global warming (Whiting and Chanton 2001).

Interestingly, tidal salt marshes sequester up to fifty times more carbon per acre than is sequestered by tropical forests (Pidgeon 2009). Salt marshes, unlike freshwater wetlands, do not release significant quantities of methane (a recognized greenhouse gas contributing to global warming) to the atmosphere (Chmura 2009). Studies in Georgia have found that among tidal wetlands, the tidal freshwater wetlands and brackish marshes sequester more carbon and retain more nutrients than salt marshes (Loomis and Craft 2010). In fact, tidal fresh and brackish marshes sequestered 66 percent of the carbon and 69 percent of the nitrogen stored in all tidal wetlands in the three-river system studied (Ogeechee, Altamaha, and Satilla) even though they represent only 41 percent of the marsh area. Anaerobic conditions resulting from prolonged flooding or soil saturation typically lead to an accumulation of organic matter. Therefore, wetlands that experience longer duration of soil saturation should accumulate more organic matter. Northern bogs that are nearly continuous saturated in boreal to arctic climates where low evapotranspiration rates occur are recognized as major global carbon sinks. Consequently, wetlands with the wetter water regimes (i.e., seasonally flooded and wetter) should store more carbon than wetlands in the same region with drier water regimes that promote more oxidation and decomposition of organic matter. Seasonally flooded and wetter vegetated wetlands are rated as high for the carbon sequestration function, while drier wetlands (temporarily flooded and seasonally saturated) are assigned a moderate rating. Tidal flats (unconsolidated shores, mudflats in particular, except sandy beaches and sand flats) are listed as moderate because they sequester carbon at lower rates than vegetated coastal wetlands (Duarte et al. 2005). Ponds were also designated as moderate because recent studies have indicated the cumulative importance of small ponds in sequestering carbon through sedimentation processes (Downing 2010). Several types of ponds that are not likely to be capture organic-enriched sediment from local watersheds are excluded from this function: aquaculture, commercial, industrial, residential-stormwater, sewage treatment, and isolated diked ponds (impoundments).

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