Elevated CO2 and nitrogen availability have interactive effects on canopy carbon gain in rice.

•  Here we analysed the effects of CO2 (Ca ) elevation and nitrogen availability on canopy structure, leaf area index (LAI) and canopy photosynthesis of rice (Oryza sativa). •  Rice was grown at ambient and elevated Ca (c. 200 µmol mol-1 above ambient, using the free-air CO2 enrichment, FACE) and at two N availabilities. We measured leaf area, area-based leaf N contents and leaf photosynthesis, and calculated net daily canopy photosynthesis. •  FACE plants had higher light-saturated rates of photosynthesis (Pmax ) and apparent quantum yields than ambient plants, when measured at their own growth CO2 . Ca elevation reduced the total leaf N in the canopy (Nleaf ) but had no effect on LAI, and the average leaf N content (Nleaf /LAI) was therefore reduced by 8%. This reduction corresponded well with our model predictions. Leaf area index increased strongly with N availability, which was also consistent with our model. •  Calculated canopy photosynthesis increased more strongly with Nleaf under elevated than under ambient Ca . This indicates that there is an N × Ca interactive effect on canopy carbon gain. This interaction was caused by the increase in LAI with N availability, which enhanced the positive effect of the higher quantum yield under Ca elevation.