Interactive effects of rising CO2 and elevated nitrogen and phosphorus on nitrogen allocation in invasive weeds Mikania micrantha and Chromolaena odorata

Global environmental changes and biological invasions are important environmental problems. A better understanding of their interactions will help us improve management for plant invasion control under global change context. Mikania micrantha and Chromolaena odorata are two of the most notorious invasive weeds in southern China. They have caused serious damage to semi-natural forests, pastures, crops and plantations. In this study, they were grown under elevated and ambient CO2 (700 +/- A 50 and 380 +/- A 50 mu mol mol(-1), respectively) in open-top chambers in south China botanical garden and treated with fertilizers in the form of soluble nitrogen (N), phosphorus (P), N + P (NP), and with tap water as control. Elevated CO2 had significant positive effects on most variables for M. micrantha, and N addition had positive effects on most variables for C. odorata. Elevated CO2 and N or NP addition increased Rubisco content, photosynthetic rates and photosynthetic N use efficiency of both weeds, which improved their resource capture ability and resource utilization efficiency, and improved their growth capacity. SDS-insoluble protein decreased in response to elevated CO2 in both weeds indicating a decrease in their mechanical defense when CO2 elevated. P addition decreased the quantity of N invested in the photosynthetic apparatus under ambient CO2 in both weeds, but this effect was reduced under elevated CO2, suggesting that high soil P will not benefit for the growth of the two weeds under current CO2 levels but less so if CO2 levels continue to rise. From the results we predict that the invasion potential of both weeds will decrease in the high P and low N area at present CO2 concentration, but increase in the high N deposition area, especially with elevated CO2.