Quality of agricultural soils is largely a function of soil organic matter. Tillage and crop management impact soil organic matter dynamics by modification of the soil environment and quantity and quality of C input. We investigated changes in pools and fluxes of soil organic C (SOC) during the ninth and tenth year of cropping with various intensities under conventional disk-and-bed tillage (CT) and no tillage (NT). Soil organic C to a depth of 0.2m increased with cropping intensity as a result of greater C input and was 10% to 30% greater under NT than under CT. Sequestration of crop-derived C input into SOC was 22+/-2% under NT and 9+/-4% under CT (mean of cropping intensities +/- standard deviation of cropping systems). Greater sequestration of SOC under NT was due to a lower rate of in situ soil CO 2 evolution than under CT (0.22+/-0.03 vs. 0.27+/-0.06g CO 2 -C g - 1 SOC yr - 1 ). Despite a similar labile pool of SOC under NT than under CT (1.1+/-0.1 vs. 1.0+/-0.1g mineralizable C kg - 1 SOC d - 1 ), the ratio of in situ to potential CO 2 evolution was less under NT (0.56+/-0.03) than under CT (0.73+/-0.08), suggesting strong environmental controls on SOC turnover, such as temperature, moisture, and residue placement. Both increased C sequestration and a greater labile SOC pool were achieved in this low-SOC soil using NT and high-intensity cropping.