Nitrogen to phosphorus ratios of tree species in response to elevated carbon dioxide and nitrogen addition in subtropical forests

Increased atmospheric carbon dioxide (CO₂) concentrations and nitrogen (N) deposition induced by human activities have greatly influenced the stoichiometry of N and phosphorus (P). We used model forest ecosystems in open‐top chambers to study the effects of elevated CO₂ (ca. 700 μmol mol^?1) alone and together with N addition (100 kg N ha^?1 yr^?1) on N to P (N : P) ratios in leaves, stems and roots of five tree species, including four non‐N₂ fixers and one N₂ fixer, in subtropical China from 2006 to 2009. Elevated CO₂ decreased or had no effects on N : P ratios in plant tissues of tree species. N addition, especially under elevated CO₂, lowered N : P ratios in the N₂ fixer, and this effect was significant in the stems and the roots. However, only one species of the non‐N₂ fixers showed significantly lower N : P ratios under N addition in 2009, and the others were not affected by N addition. The reductions of N : P ratios in response to elevated CO₂ and N addition were mainly associated with the increases in P concentrations. Our results imply that elevated CO₂ and N addition could facilitate tree species to mitigate P limitation by more strongly influencing P dynamics than N in the subtropical forests.