Contrasting effects of elevated CO₂ on old and new soil carbon pools

Soil organic carbon (SOC) is the largest reservoir of organic carbon in the terrestrial biosphere. Though the influence of increasing atmospheric CO₂ on net primary productivity, on the flow of newly fixed carbon below ground, and on the quality of new plant litter in ecosystems has been examined, indirect effects of increased CO₂ on breakdown of large SOC pools already in ecosystems are not well understood. We found that exposure of California grassland communities to elevated CO₂ retarded decomposition of older SOC when mineral nutrients were abundant, thus increasing the turnover time of SOC already in the system. Under elevated CO₂, soil microorganisms appeared to shift from consuming older SOC to utilizing easily degraded rhizodeposits derived from increased root biomass. In contrast to this increased retention of stabilized older SOC under elevated CO₂, movement of newly fixed carbon from roots to stabilized SOC pools was retarded; though root biomass increased under elevated CO₂, new carbon in mineral-bound pools decreased. These contrasting effects of elevated CO₂ on dynamics of old and new soil carbon pools contribute to a new soil carbon equilibrium that could profoundly affect long-term net carbon movement between terrestrial ecosystems and the atmosphere.