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

Increased atmospheric carbon dioxide ( CO 2 ) 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 CO 2 (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‐ N 2 fixers and one N 2 fixer, in subtropical China from 2006 to 2009. Elevated CO 2 decreased or had no effects on N : P ratios in plant tissues of tree species. N addition, especially under elevated CO 2 , lowered N : P ratios in the N 2 fixer, and this effect was significant in the stems and the roots. However, only one species of the non‐ N 2 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 CO 2 and N addition were mainly associated with the increases in P concentrations. Our results imply that elevated CO 2 and N addition could facilitate tree species to mitigate P limitation by more strongly influencing P dynamics than N in the subtropical forests.