Nitrogen fertilization and developmental stage alter the response of Lolium perenne to elevated CO2

Plant response to elevated atmospheric CO2 may depend on the carbon sink st rength, determined by the availability of resources other than CO2, and the developmental stage. In a 2-yr field experiment with model swards of Lolium perenne, the effect of CO2 enrichment (FACE) on yield and allocation of dry mass (DM) and N wer e examined under three N fertilization treatments during vegetative and rep roductive growth. During vegetative growth, in the highest N treatment, the greatest increase in DM yield occurred at elevated CO2; there was no change in DM allocation . By contrast, at low N, residual biomass, but not yield, increased under C O2 enrichment, and the tillers were shorter. During reproductive growth, un der CO2 enrichment DM yield increased similarly across all N treatments; th ere was no change in DM and N partitioning. The mean weight and height of t he reproductive tillers increased. At high N availability, or during reproductive growth, L. perenne swards ov ercome carbon-sink limitation and show a strong yield response to elevated CO2. Biomass allocation and the height of the plants, in response to elevat ed CO2, clearly depend on N fertilization and developmental stage.