Mostly based on assumptions derived from controlled-environment studies, predicted future atmospheric CO ₂ concentrations [CO ₂] are expected to have considerable impacts on carbon (C) turnover in agro-ecosystems. In order to allow the in situ examination of C-transformations in the plant-soil system of arable crop rotations under future [CO ₂], a free air carbon dioxide enrichment (FACE) experiment (550 μmol mol ^-1 CO ₂) was started at Braunschweig, Germany in 1999. The crop rotation under investigation comprised winter barley, a cover crop (ryegrass), sugar beets and winter wheat. Assessments of CO ₂ effects included the determination of above- and belowground biomass production, measurements of canopy CO ₂- and H ₂O- fluxes, soil microbial biomass and in situ soil respiration. The results obtained during the 1st crop rotation cycle (3 years) showed that for the selected crops elevated [CO ₂] entailed significant positive effects (P < 0.05) on aboveground (6%-14% stimulation) and belowground biomass production (up to 90% stimulation), while canopy evapotranspiration was reduced. This resulted in increased soil water content. Also, depending on crop type and season, high CO ₂ stimulated in situ soil respiration (up to 30%), while soil microbial biomass did not show significant responses to elevated [CO ₂] during the first rotation cycle.