Elevated CO2 increases carbon allocation to the roots of Lolium perenne under free-air CO2 enrichment but not in a controlled environment

Grass swards often show a higher root-shoot ratio of dry matter (R : S-DM) at elevated [CO2] than at ambient [CO2]. However, it is not known whether this is a result of a sustained increase in C allocation to the roots. The effects of free-air CO2 enrichment (FACE) on carbon allocation to roots in established Lolium perenne (perennial ryegrass) swards during regrowth were investigated by means of C-14 labelling and compared with data from a controlled environment experiment. Elevated [CO2] in the field increased root dry matter (109%), R : S-DM (44%) and, in accordance, root-shoot C-14 ratios (R : S-14C, 39%), as well as the intercept of the allometric line. By contrast, elevated [CO2,] did not affect the allometry of the plants under controlled conditions. However, at low N concentration in the nutrient solution, the R : S-DM and the intercept of the allometric line increased. It is suggested that the remarkably different responses to elevated [CO2] under field and controlled conditions result from differences in both N availability for growth, and C sink strength of the shoots.