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

Abstract

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. 700μmolmol(-1) ) alone and together with N addition (100kgNha(-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.