Conservation tillage impacts on national soil and atmospheric carbon levels

Soil organic matter is the largest global terrestrial C pool and is a source of CO2, CH4, and other greenhouse gases. Changes in soil organic C (SOC) content and fossil fuel C emissions in response to conversion of conventional tillage to conservation tillage in the contiguous USA for field crop production by the year 2020 were projected by developing a model based on published data, and geographic databases of current conservation tillage usage and agricultural SOC. Three scenarios of conservation tillage use, 27% (current usage), 57% (Scenario 2), and 76% (Scenario 3) of field cropland planted, were considered. The SOC content for major field crops to 30-cm depth was 5304 to 8654 Tg C (Tg = 10(12) g), with 1710 to 2831 Tg C at 0- to 8-cm depth, and 1383 to 2240 Tg C at 8- to 15-cm depth. Maintaining current levels of conventional tillage until 2020 would result in 31 to 52 Tg SOC loss. Scenario 2 conventional tillage resulted in 18 to 30 Tg C SOC loss, and Scenario 3 yielded 9 to 16 Tg SOC loss, which were C savings of 21 to 36 Tg C over maintaining current levels of tillage. Conversion of conventional tillage to no-till resulted in 80 to 129 Tg C gain in soil for Scenario 2, and 286 to 468 Tg C for Scenario 3. No-till and conventional tillage had similar SOC contents below the 15-cm depth. Minimum tillage conserved current levels of SOC but did not consistently increase SOC above levels of conventional tillage. Fossil fuel emissions from field manipulations and herbicide production for conventional tillage are 53 kg C ha-1 yr-1, minimum tillage is 45 kg C ha-1 yr-1, and 29 kg C ha-1 yr-1 for no-till. Fuel emissions for maintaining current levels of tillage practices are 157 Tg C, 149 Tg C for Scenario 2, and 146 Tg C for Scenario 3 for 30 yr. Increasing the amount of conservation tillage to Scenario 3 levels will change these agricultural systems from sources of C (188-209 Tg C) to C sinks (131-306 Tg C). The SOC benefit of Scenario 3 (277-452 Tg C) is equivalent to 0.7 to 1.1% of the total projected U.S. fossil fuel C emissions for the next 30 yr.