Prediction of the impact of climate change requires the response of carbon (C) flow in plant-soil systems to increased CO2 to be understood. A mechanism by which grassland C sequestration might be altered was investigated by pulse-labelling Lolium perenne swards, which had been subject to CO 2 enrichment and two levels of nitrogen (N) fertilization for 10 yr, with 14CO2. Over a 6-d period 40-80% of the 14C pulse was exported from mature leaves, 1-2% remained in roots, 2-7% was lost as below-ground respiration, 0.1% was recovered in soil solution, and 0.2-1.5% in soil. Swards under elevated CO2 with the lower N supply fixed more 14C than swards grown in ambient CO2, exported more fixed 14C below ground and respired less than their high-N counterparts. Sward cutting reduced root 14C, but plants in elevated CO2 still retained 80% more 14C below ground than those in ambient CO2. The potential for below-ground C sequestration in grasslands is enhanced under elevated CO2, but any increase is likely to be small and dependent upon grassland management.