Abstract: Pot-grown alfalfa (Medicago sativa L. ) were experimented in the open top chambers in which natural air (350 × 10-6, 1 × CO2) and doubled CO2 air (700 × 10-6, 2 × CO2) were continuous blown through bottom respectively. Results showed that the biomass in both shoot and root was increased by the 2 × CO2 treatment. The root/shoot ratio was nearly unchange in the short term treatment and slightly decreased in the longer term treatment. The results differed from those reported in literatures that was attributed to the potgrown condition in this experiment. The nitrogen fixation activities (acetylene reduction) per plant were obviously promoted under the 2 × CO2 condition but the difference of the specific nitrogen-fixing activities between 1 × CO2 and 2 × CO2 treatment was small. It implied that supplement of ATP, NADP or carbon skeleton under the 2 × CO2 condition was not more than that under the 1 × CO2 condition. C/N ratio in the shoot increased in the doubled CO2 treatment group, similar to the reports from other authors; but the ratio increased in the root in the 2 × CO2 treatment group was ascribed to the higher nitrogen absorption from the soil and (or) N-fixing activity of the nodules as compared with that in the 1 × CO2 treatment. The results demonstrated that assimilation and allocation of carbon and nitrogen in legume plant were deeply influenced by the elevated CO2.

Key words: Alfalfa, Doubled CO2, Nitrogen fixation, C/N ratio