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 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.