As atmospheric CO increases there will almost certainly be alterations in soil carbon fluxes. It is likely that such alterations will be accompanied by changes in the partitioning of carbon between organic structures and to soil processes. These changes have the potential for further altering the structure and function of terrestrial ecosystems. While there has been increasing recognition of the importance of soil-mediated responses to global climate change, the nature and magnitude of these responses are not well understood. In an effort to expand our assessment of the significance of belowground responses to rising atmospheric CO , a workshop has been organized that resulted in the peer-reviewed contributions that are contained in this volume.