Abstract: Elevated CO₂ and O₃ concentrations could affect ecosystem carbon (C) cycling. Isoprene (ISO) and monoterpenes (MTs) are the intermediate products in plant C metabolism, the synthesis and release of which are important pathways of ecosystem C cycling. We conducted a 4-month fumigation experiment with two-year-old Phoebe bournei seedlings. Four treatments, namely the control (CK), elevated CO₂ concentration (EC), elevated O₃ concentration (EO) and simultaneously elevated CO₂ and O₃ (EC+EO), were set up to examine the effects of elevated CO₂ and O₃ concentrations on ISO and MTs release and photosynthetic parameters in P. bournei. The results showed that after one month fumigation, a significant increase in MTs fluxes (P<0.05) and a 5.6% decrease in ISO flux were observed with the EC treatment compared with CK. After 4 months’ fumigation, the ISO and MTs fluxes decreased under EC, EO, and EC+EO treatments. The reduction was the strongest under EC+EO treatment, with the ISO and MTs fluxes being reduced by 76.4% and 89.6%, respectively. The decrease of fluxes might be attributed to the changes of net photosynthetic rate by changing the content of photosynthetic pigments and adjusting stomatal conductance under different treatments, which directly affects C source of ISO and MTs metabolism. The main components of MTs were α-pinene and limonene. Under elevated CO₂ and O₃ concentrations, net photosynthetic rate gradually decreased with fumigation duration. The effects of independently and simultaneously elevated CO₂ and O₃ concentrations on ISO and MTs fluxes differed. When we investigated the effects of elevated CO₂ and O₃ concentrations on ecosystem C cycling, compound factor interaction experiments should be taken into consideration, to provide more reliable data support for understanding the responses of ecosystem C cycling to global climate change.

Key words: elevated CO₂ concentration, elevated O₃ concentration, isoprene, monoterpene, photosynthetic parameter.