Elevated CO(2) and nitrogen availability have interactive effects on canopy carbon gain in rice.

• Here we analysed the effects of CO (C ) 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 C (c. 200 µmol mol above ambient, using the free-air CO 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 (P ) and apparent quantum yields than ambient plants, when measured at their own growth CO . C elevation reduced the total leaf N in the canopy (N ) but had no effect on LAI, and the average leaf N content (N /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 N under elevated than under ambient C . This indicates that there is an N × C 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 C elevation.