Terrestrial Ecosystem Carbon Storage and Greenhouse Gas Fluxes
Terrestrial ecosystems in the United States are one of the largest and globally significant carbon sinks (U.S. Climate Change Science Program (CCSP), 2007). In the conterminous United States, forests cover about 246 million hectares, with an additional 52 million hectares in Alaska. Forests are a considerable carbon sink in the U.S., but can be highly variable in their annual carbon-storage capacity due to natural disturbances and land use decisions (Goodale and others, 2002)). Fire, for example, is a disturbance that affects a forest?s carbon storage and has effects of both releasing CO2 and CH4 back into the atmosphere and strengthening forest ecosystem?s ability to increase sequestration over long-term.
Croplands are also significant carbon pools, but store carbon differently than forests (Potter and others, 2007). Because of annual harvest, the accumulation of carbon in crops is short-lived, so much of the carbon is returned to the soil. Management practices, such as no-till agriculture, can affect the amount of carbon sequestered back into the soil by leaving a substantial amount of crop residue to slowly decompose and become part of the soil organic matter. Other factors to determine sequestration rates, as well as fluctuations in N2O emissions, are fertilization, crop type, and soil-drainage capacity (Del Grosso and others, 2005).
Grasslands/shrublands are very similar to croplands in that most of the carbon stock is stored in the soil. While grasslands/shrublands can be net sinks for carbon, the capacity to store carbon varies across the landscape and is subject to grazing intensity and woody encroachment in landscape (Reeder and others, 2000).
Wetlands are transitional areas between uplands and aquatic ecosystems and are generally inundated periodically or permanently with water, or have saturated soils and support vegetation adapted to anaerobic conditions. Most of the carbon is stored in the soil, but both woody and nonwoody vegetation and sediments contribute to the sequestration of carbon in wetlands. Carbon is lost from wetlands through methanogenesis (the formation of methane by microbes known as methanogens), in anaerobic soils and through oxidation of organic matter when wetlands are drained. Only about 48% of the original wetland area in the United States still exists, with about 70 million acres in the conterminous U.S. and 43 million more acres in Alaska (CCSP, 2007, Bridgham and others, 2006).
For this assessment, the above ecosystems will be defined primarily using the USGS National Land Cover Dataset (NLCD) thematic classes (Homer and others, 2004). For example, boundaries of forest ecosystems are defined as those of NLCD forest classes. Shrub and grassland ecosystems consist of NLCD shrub and grass classes for temperate climate, and tundra, moss, lichen classes in boreal climate. For croplands, NLCD cultivated crop classes and pasture class make up the category. For wetlands, both NLCD and NOAA C-CAP (Coastal Services Center, 2009)) datasets form the primary foundation of the definition. The wetland boundaries are further processed following Bridgham and others (2006) using NRCS SSURGO soil database wetland soil modifiers.
Bridgham S.D., Megonigal J.P., Keller J.K., Bliss N.B., Trettin C. 2006. The carbon balance of North American wetlands. Wetlands 26: 889-916.
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Potter, Christopher, Klooster, Steven, Hiatt, Seth, Fladeland, Matthew, Genovese, Vanessa, and Gross, Peggy, 2007, Satellite-derived estimates of potential carbon sequestration through afforestation of agricultural lands in the United States: Climatic Change, v. 80, no. 3-4, doi: 10.1007/s10584-006-9109-3.
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