The element carbon is the foundation for all life. Many compounds formed from carbon are essential to life on earth. Carbon is an invaluable resource and it is continually cycled through Earth’s ecosystems—taken from the atmosphere by plants, used by animals that consume those plants, deposited in the soil, and returned to the atmosphere by plants and animals.

While there is concern over rising levels of carbon (in the form of carbon dioxide) in the atmosphere, understanding the carbon cycle helps us recognize that soils and plants can store carbon in the biosphere In fact, there is approximately three times as much carbon stored in the Earth’s soils than is contained in the atmosphere.

Knowing where current carbon stocks in the United States are higher and where they are lower can be used to guide wise decision making about land use and management, helping us choose practices that favor carbon storage in soils and plants.

Carbon and the Carbon Cycle
 

Plants use carbon from the atmosphere to capture energy from the sunlight via photosynthesis, converting carbon dioxide into more complex organic compounds. These organic molecules have two fates: they are either used by plants and returned to the atmosphere via respiration or they are stored by the plant and eventually added to the soil in the form of plant litter. In the soil, carbon is digested by microbes and returned to the atmosphere via respiration.

Carbon Fluxes
 

Imbalances in the carbon cycle can be attributed to the burning of fossil fuels, deforestation, and the importation of food products. Fossil fuels contain carbon storage from ancient sunlight. The burning of fossil fuels releases an excess of carbon dioxide, increasing the levels of this greenhouse gas in the atmosphere. Carbon can be sequestered, or stored, in the soil by plants. Soils are a major sink for carbon. In fact, soils store about twice as much carbon as the world’s plants and atmosphere combined. About 1/3 of all the carbon stored in the soil is found at depths over 1 meter. Appropriately managing the land can help reduce greenhouse gas in the atmosphere and help preserve this essential element.

Managing Carbon in the Biosphere
 

Land use and management practices play a significant role in the carbon cycles in different ecosystems. In undisturbed forested systems, the amount of plant litter at the surface is much higher. Because this litter is not incorporated into the soil, it decomposes slowly. This results in high amounts of organic carbon in the surface of forested systems. In croplands, plants are harvested yearly, causing the carbon that accumulated in the plants to be removed from the local ecosystem. Finally in wetlands, high levels of carbon are stored due to the restricted soil microbe activity. Wetland soils are so saturated that they lack the oxygen necessary for microbes.

Understanding the Numbers
 

Carbon Stock

Carbon stocks are currently reported in USDA CarbonScapes in two pools: shallow soil (0-30 cm) and deep soil (0-100 cm). Both estimates are derived from published data from the USDA Natural Resources Conservation Service’s Soil Survey Geographic (SSURGO) database. The text and graphic at the top of the summary reporting box indicate which pool is being reported.

Carbon stock is a measure of concentration of carbon in a particular carbon pool across a particular geographic extent. It is reported as a mass of carbon divided by the area. Examples of carbon stock units include: megagrams of carbon per hectare or any of the other Carbon Units under the Carbon Units pull down menu. You may also choose to express the carbon values in units of carbon dioxide equivalents (CO2-e). These equivalent values indicate the mass of carbon dioxide that the carbon in that pool would generate if completely converted to CO2.

Minimum and maximum values represent the lowest and highest reported values among all counties, watersheds, or ecological regions in the same carbon pool of the selected area.

Percentile represents the ranking of the selected area compared to other counties, watersheds, or ecological regions in that same carbon pool. Interpret the percentile value as the number of counties, watersheds, or ecological regions with less carbon than the selected area. For example, the county with maximum carbon stock in this data set is Dare County, NC, with 1,018 megagrams of carbon per hectare. It is, therefore, in the 100th percentile, indicating that 100% of the counties have less carbon than Dare County.

Carbon Mass

Carbon mass is a measure of the total mass of carbon in a particular carbon pool across a particular geographic extent. Unlike the carbon stock, it is not divided by the area. Examples of carbon mass units include: million metric tonnes of carbon (the default), grams of carbon, gigatonnes of carbon, US tons of carbon, or rail cars of coal. You may also choose to express the carbon values in units of carbon dioxide equivalents (CO2-e). These equivalent values indicate the mass of carbon dioxide that the carbon in that pool would generate if completely converted to CO2.

Additionally, the estimated carbon mass is also translated into three equivalence values : (i) the equivalent number of passenger vehicles’ annual carbon emissions, (ii) the number of homes’ annual energy use, and (iii) the number of acres of forest needed to capture that same amount of carbon in one year. Calculated mass equivalence numbers are derived from published equations provided by the U.S. Environmental Protection Agency (http://www.epa.gov/cleanenergy/energy-resources/refs.html).

Note that, because the carbon mass values are not divided by the area of the geographical unit, it is generally true that larger counties or larger ecological regions have more carbon mass than smaller counties or smaller ecological regions.