Carbon sequestration is the removal of carbon (as CO2) from the atmosphere through photosynthesis, and the storage of carbon in plants or soil (as organic matter). The amount of carbon (OM) in the soil depends on the balance between inputs of organic matter from plants and outputs from respiration by microbes and other organisms.
How important is soil for storing carbon? On a global scale, the carbon in soil organic matter is about twice that in the atmosphere and 3 times that in vegetation. The oceans contain >10 times the amount in soil, vegetation, and air combined.
Atmospheric CO2 levels have increased from 280 parts per million (ppm) in 1850 to 365 ppm in 1996. During this period, cultivated U.S. soils have lost from 20-70 percent of their native OM levels. It is believed that soil degradation worldwide has contributed significantly to the increase in atmospheric CO2.
Agriculture, forestry, and rangeland management are the only major sectors of the U.S. economy that could sequester more carbon through photosynthesis than they emit through other processes. It is estimated that through implementation of best management strategies, U.S. agricultural lands could sequester between 7 and 10 percent of all U.S. carbon emissions until new soil carbon equilibrium levels are reached.
NRCS has the technical expertise to help make this happen through programs that rehabilitate degraded lands, conserve soil, and restore wetlands. By restoring organic matter levels in our soils, we help offset CO2 emissions, and we achieve other environmental and economic benefits that NRCS has always worked toward. These benefits include reduced erosion and compaction, greater water and nutrient holding capacity, and better tilth and rooting environment.
Soil organic matter levels can be increased through reduced tillage, more intensive cropping systems, establishment and improvement of perennial vegetation, and erosion control. On cropland, no-till compared to plowing reduces the amount of CO2 that escapes from the soil by nearly 10 times. In many places, no-till in combination with high-residue crop rotations and cover crops will sequester the most CO2. Establishing wetlands and converting cropland to trees and grass will also sequester carbon.
An excellent, easy-to-read source of further information is "The Potential of U.S. Cropland to Sequester Carbon and Mitigate the Greenhouse Effect," by R. Lal, J.M. Kimble, R.F. Follett and C.V. Cole. 1998. Ann Arbor Press, Chelsea, Michigan.
Reprinted with permission from "NRCS This Week", 3/10/2000. National Resource Conservation Service (NRCS), United States Department of Agriculture (USDA).