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

Here we analysed the effects of CO₂ (C_a) 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_a (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 (P_max) and apparent quantum yields than ambient plants, when measured at their own growth CO₂. C_a elevation reduced the total leaf N in the canopy (N _leaf) but had no effect on LAI, and the average leaf N content (N_leaf/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_leaf under elevated than under ambient C_a. This indicates that there is an N x C _a 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_a elevation.