• greenhouse gas;
  • agriculture;
  • carbon;
  • soil

Soils account for the largest fraction of terrestrial carbon (C) and thus are critically important in determining global C cycle dynamics. In North America, conversion of native prairies to agriculture over the past 150 years released 30–50% of soil organic carbon (SOC) stores [Mann, 1986]. Improved agricultural practices could recover much of this SOC, storing it in biomass and soil and thereby sequestering billions of tons of atmospheric carbon dioxide (CO2). These practices involve increasing C inputs to soil (e.g., through crop rotation, higher biomass crops, and perennial crops) and decreasing losses (e.g., through reduced tillage intensity) [Janzen et al., 1998; Lal et al., 2003; Smith et al., 2007].

Managing agricultural soils to increase SOC storage is an immediately available, cost-competitive option for mitigating CO2 emissions [Smith et al., 2007; Thomson et al., 2008], with a technical potential to offset as much as 800 teragrams of CO2 per year in the United States (∼13% of U.S. CO2 emissions) [Lal et al., 2003] and 5000 teragrams per year globally (∼17% of global CO2 emissions) [Smith et al., 2007]. Managing soils to sequester organic C also brings attendant benefits such as increased and sustainable crop productivity; improved quality of soil, water, and air; reduced energy inputs; and reduced vulnerability to climate change [Lal, 2004].