Soil radiocarbon measurements show that mineral soil carbon under a recovering temperate forest in South Carolina turns over twice as fast as carbon in undisturbed soil. The observed 12-year turnover time influences the design and interpretation of CO2 fertilization experiments. Experiments conducted on formerly disturbed sites will show a soil carbon fertilization response considerably faster than experiments conducted on native sites. Calculating the soil carbon CO2 fertilization factor from observed increases in soil carbon requires values for the turnover time and inventory of active soil carbon. We also use the observed turnover time to estimate the rate of atmospheric carbon dioxide sequestration in soil following agricultural abandonment. Although using the observed turnover rate increases estimates of soil carbon uptake an abandoned land, the amount of carbon sequestered globally is minimal because the net area of land being abandoned is small. ¿ American Geophysical Union 1995