Nitrogen Fertilization Modified the Responses of Schima superba Seedlings to Elevated CO₂ in Subtropical China

There are very few studies about the effects of relatively higher CO₂ concentration (e.g., 1000 μmol·mol^-1) or plus N fertilization on woody plants. In this study, Schima superba seedings were exposed to ambient or eCO₂ (550, 750, and 1000 μmol·mol^-1) and N fertilization (0 and 10 g·m^-2·yr^-1, hereafter: low N, high N, respectively) for one growth season to explore the potential responses in a subtropical site with low soil N availability. N fertilization strongly increased leaf mass-based N by 118.38%, 116.68%, 106.78%, and 138.95%, respectively, in different CO₂ treatments and decreased starch, with a half reduction in leaf C:N ratio. Leaf N was significantly decreased by eCO₂ in both low N and high N treatments, and N fertilization stimulated the decrease of leaf N and mitigated the increase of leaf C:N by eCO₂. In low N treatments, photosynthetic rate (Pn) was maximized at 733 μmol·mol^-1 CO₂ in August and September, while, in high N treatments, Pn was continuously increased with elevation of CO₂. N fertilization significantly increased plant biomass especially at highly elevated CO₂, although no response of biomass to eCO₂ alone. These findings indicated that N fertilization would modify the response of S. superba to eCO₂.