The fate of photosynthetically‐fixed carbon in <i> Lolium perenne</i> grassland as modified by elevated CO₂ and sward management

• 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 CO2 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.