The fate of photosynthetically-fixed carbon in Lolium perenne 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.