Down-regulation of photosynthesis and its relation to changes in leaf N allocation and N availability under elevated CO2 after long-term exposure

Down-regulation of photosynthesis in elevated CO2 (eCO2), could be attributed to depletions in nitrogen (N) availability after long term exposure to eCO2 (progressive nitrogen limitation, PNL) or N dilution due to excessive carbon accumulation of nonstructural carbohydrates. To investigate this, we examined N availability, photosynthetic characteristics, and leaf N allocation in Pinus densiflora, Fraxinus rhynchophylla, and Sorbus alnifolia, grown under three different CO2 concentrations, ambient CO2 (aCO2), aCO2 × 1.4 ppm (eCO21.4), and aCO2 × 1.8 ppm (eCO21.8), for 9 years. N availability increased under eCO21.8, and its allocation to chlorophyll (NFchl) and photosynthetic N use efficiency also increased under eCO2. The maximum carboxylation rate, leaf N per mass (Nmass), and N allocation to Rubisco (NFRub), however, were all lower under eCO21.8. There were interactions in NFchl between canopy × species × CO2 concentrations. The greatest changes in N allocation under eCO2 were in the lower canopy of S. alnifolia, the most shade-tolerant species and this species have relatively high flexibility in N operations compared to shade-intolerant species. Based on the reduction in Nmass that was diluted by increased nonstructural carbohydrates and increased N availability, down-regulation of photosynthesis was found to be caused by the dilution and change in N allocations, rather than PNL.