Response of 12 weedy species to elevated CO2 in low-phosphorus-availability soil

We conducted an experiment on responses of weedy species from an orchard ecosystem to elevated CO2 (700-800 mu mol mol(-1)) under low phosphorus (P) soil in an environment-controlled growth chamber. Twelve local weedy species, Poa annua L., Lolium perenne L., Avena fatua L., Vicia cracca L., Medicago lupulina L., Kummerowia striata (Thunb.) Schindl., Veronica didyma Ten., Plantago virginica L., Gnaphalium affine D.Don., Echinochloa crusgalli var. mitis (L.) Beauv., Eleusine indica (L.) Gaertn. and Setaria glauca (L.) P. Beauv., grouped into four functional groups (C-3 grass, C-3 forb, legume and C-4 grass), were used in the experiment. The total plant biomass, P uptake, and mycorrhizal colonization were measured. The results showed that the total biomass of the 12 weedy species tended to increase under elevated CO2. But changes in the total biomass under elevated CO2 significantly differed among functional groups: legumes showed the greatest increase in the total biomass of all functional groups, following the order C-3 forbs > C-4 grasses > C-3 grasses. Elevated CO2 significantly increased mycorrhizal colonization and P uptake of legumes, C-3 forbs and C-4 grasses but did not change C-3 grasses. Positive correlations between mycorrhizal colonization and shoot P concentration, and between total P uptake and total biomass were found under elevated CO2. The results suggested that the interspecific difference in CO2 response at low P availability was caused by the difference in CO2 response in mycorrhizae and P uptake. These differences among species imply that plant interaction in orchard ecosystems may change under future CO2 enrichment.