Down-Regulation of Photosynthesis to Elevated CO₂ and N Fertilization in Understory Fraxinus rhynchophylla Seedlings

(1) Background: Down-regulation of photosynthesis has been commonly reported in elevated CO2 (eCO(2)) experiments and is accompanied by a reduction of leaf nitrogen (N) concentration. Decreased N concentrations in plant tissues under eCO(2) can be attributed to an increase in nonstructural carbohydrate (NSC) and are possibly related to N availability. (2) Methods: To examine whether the reduction of leaf N concentration under eCO(2) is related to N availability, we investigated understory Fraxinus rhynchophylla seedlings grown under three different CO2 conditions (ambient, 400 ppm [aCO(2)]; ambient x 1.4, 560 ppm [eCO(2)1.4]; and ambient x 1.8, 720 ppm [eCO(2)1.8]) and three different N concentrations for 2 years. (3) Results: Leaf and stem biomass did not change under eCO(2) conditions, whereas leaf production and stem and branch biomass were increased by N fertilization. Unlike biomass, the light-saturated photosynthetic rate and photosynthetic N-use efficiency (PNUE) increased under eCO(2) conditions. However, leaf N, Rubisco, and chlorophyll decreased under eCO(2) conditions in both N-fertilized and unfertilized treatments. Contrary to the previous studies, leaf NSC decreased under eCO(2) conditions. Unlike leaf N concentration, N concentration of the stem under eCO(2) conditions was higher than that under ambient CO2 (4). Conclusions: Leaf N concentration was not reduced by NSC under eCO(2) conditions in the understory, and unlike other organs, leaf N concentration might be reduced due to increased PNUE.