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 mumol 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.