The effects of human activities on atmospheric concentrations of carbon dioxide (CO2) and other greenhouse gases (GHGs) are under intensive study in the United States and worldwide. Since conversion to cropland during the 17th and 18th centuries, the vegetation and soils of the U.S. forests, grasslands, and wetlands have undergone extensive change. Clearing, tilling, and draining of these soils for long-term cropland use released large amounts of CO2, a GHG, to the atmosphere from the soils' fertile soil organic matter (SOM). The SOM in topsoil often was depleted by up to half of its soil organic carbon (SOC) (Cambardella and Elliott 1992). Now, improved farming technologies, increased farmland productivity, and government programs to return highly erodible lands to permanent vegetation are producing unanticipated benefits by letting soils become major sinks for atmospheric CO2 that is stored in them as increasing levels of SOC.
