Elevated CO2 increases root growth and fine (diam. £2 mm) root growth across a range of species and experimental conditions. However, there is no clear evidence that elevated CO2 changes the proportion of C allocated to root biomass, measured as either the root : shoot ratio or the fine root : needle ratio. Elevated CO2 tends to increase mycorrhizal infection, colonization and the amount of extramatrical hyphae, supporting their key role in aiding the plant to more intensively exploit soil resources, providing a route for increased C sequestration. Only two studies have determined the effects of elevated CO2 on conifer fine-root life span, and there is no clear trend. Elevated CO2 increases the absolute fine-root turnover rates; however, the standing crop root biomass is also greater, and the effect of elevated CO2 on relative turnover rates (turnover : biomass) ranges from an increase to a decrease. At the ecosystem level these changes could lead to increased C storage in roots. Increased fine-root production coupled with increased absolute turnover rates could also lead to increases in soil organic C as greater amounts of fine roots die and decompose. Although CO2 can stimulate fine-root growth, it is not known if this stimulation persists over time. Modeling studies suggest that a doubling of the atmospheric CO2 concentration initially increases biomass, but this stimulation declines with the response to elevated CO2 because increases in assimilation are not matched by increases in nutrient supply.© New Phytologist (2000).