Soil radiocarbon and carbon measurements show that soil carbon has the potential to significantly alter atmospheric carbon dioxide levels. Results from sites worldwide suggest that about half of terrestrial net primary production is sequestered in soil. Eventually, soil microbes oxidize this soil carbon, releasing carbon dioxide back to the atmosphere. Processes such as CO2 fertilization may be increasing the transfer of carbon dioxide from the atmosphere to the soil. CO2 fertilization occurs when elevated atmospheric carbon dioxide levels enhance plant growth. Here we show that soil radiocarbon and carbon measurements from CO2 enrichment experiments indicate that CO2 fertilization is increasing soil carbon storage enough to slow the increase of atmospheric carbon dioxide levels and to help balance the global atmospheric carbon dioxide budget.