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 mu mol 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 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 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 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.