The general lack of significant changes in mineral soil C stocks during CO2-enrichment experiments has cast doubt on predictions that increased soil C can partially offset rising atmospheric CO2 concentrations. Here, we show, through meta-analysis techniques, that these experiments collectively exhibited a 5.6% increase in soil C over 2–9 years, at a median rate of 19 g C m−2 yr−1. We also measured C accrual in deciduous forest and grassland soils, at rates exceeding 40 g C m−2 yr−1 for 5–8 years, because both systems responded to CO2 enrichment with large increases in root production. Even though native C stocks were relatively large, over half of the accrued C at both sites was incorporated into microaggregates, which protect C and increase its longevity. Our data, in combination with the meta-analysis, demonstrate the potential for mineral soils in diverse temperate ecosystems to store additional C in response to CO2 enrichment.