Potential soil C sequestration on U.S. agricultural soils

Abstract

Soil carbon sequestration has been suggested as a means to help mitigate atmospheric CO2 increases, however there is limited knowledge about the magnitude of the mitigation potential. Field studies across the U.S. provide information on soil C stock changes that result from changes in agricultural management. However, data from such studies are not readily extrapolated to changes at a national scale because soils, climate, and management regimes vary locally and regionally. We used a modified version of the Intergovernmental Panel on Climate Change (IPCC) soil organic C inventory method, together with the National Resources Inventory (NRI) and other data, to estimate agricultural soil C sequestration potential in the conterminous U.S. The IPCC method estimates soil C stock changes associated with changes in land use and/or land management practices. In the U.S., the NRI provides a detailed record of land use and management activities on agricultural land that can be used to implement the IPCC method. We analyzed potential soil C storage from increased adoption of no-till, decreased fallow operations, conversion of highly erodible land to grassland, and increased use of cover crops in annual cropping systems. The results represent potentials that do not explicitly consider the economic feasibility of proposed agricultural production changes, but provide an indication of the biophysical potential of soil C sequestration as a guide to policy makers. Our analysis suggests that U.S. cropland soils have the potential to increase sequestered soil C by an additional 60-70 Tg (1012 g) C yr-1, over present rates of 17 Tg C yr-1 (estimated using the IPCC method), with widespread adoption of soil C sequestering management practices. Adoption of no-till on all currently annually cropped area (129 Mha) would increase soil C sequestration by 47 Tg C yr-1. Alternatively, use of no-till on 50% of annual cropland, with reduced tillage practices on the other 50%, would sequester less - about 37 Tg C yr-1. Elimination of summer fallow practices and conversion of highly erodible cropland to perennial grass cover could sequester around 20 and 28 Tg C yr-1, respectively. The soil C sequestration potential from including a winter cover crop on annual cropping systems was estimated at 40 Tg C yr-1. All rates were estimated for a fifteen-year projection period, and annual rates of soil C accumulations would be expected to decrease substantially over longer time periods. The total sequestration potential we have estimated for the projection period (83 Tg C yr-1) represents about 5% of 1999 total U.S. CO2 emissions or nearly double estimated CO2 emissions from agricultural production (43 Tg C yr-1). For purposes of stabilizing or reducing CO2 emissions, e.g., by 7% of 1990 levels as originally called for in the Kyoto Protocol, total potential soil C sequestration would represent 15% of that reduction level from projected 2008 emissions (2008 total greenhouse gas emissions less 93% of 1990 greenhouse gas emissions). Thus, our analysis suggests that agricultural soil C sequestration could play a meaningful, but not predominant, role in helping mitigate greenhouse gas increases.