Abstract: By using open top chamber, an experiment with two levels of atmospheric CO₂ concentration (350 and 700 μmol·mol^-1) and three levels of nitrogen supply (0, 5, and 15 g N·m^-2) was conducted to investigate the effects of elevated atmospheric CO₂ and nitrogen supply on the growth of Calamagrostis angustifolia in the freshwater marsh of Sanjiang Plain. Under elevated atmospheric CO₂ concentration, the phenophase of C. angustifolia advanced. Jointing stage was advanced by 1-2 d, and maturity stage was advanced by 3 d. Elevated atmospheric CO₂ promoted the tillering of C. angustifolia, with the increment of tiller number under 0, 5, and 15 g·m^-2 of nitrogen supply being 8.2% (P<0.05), 8.4% (P<0.05), and 5.5% (P>0.05), respectively. Elevated atmospheric CO₂ also promoted the aboveground biomass at jointing and heading stages, the increment being 12.4% and 20.9% (P<0.05), respectively, and increased the belowground biomass at later growth stages, with the increment at dough stage and maturity stage being 20.5% and 20.9% (P<0.05), respectively. The responses of C. angustifolia biomass to elevated atmospheric CO₂ concentration depended on nitrogen supply level. Under sufficient nitrogen supply, the promotion effect of elevated atmospheric CO₂ concentration on the biomass of C. angustifolia was higher.

Key words: Sanjiang Plain freshwater marsh, Calamagrostis angustifolia, elevated atmospheric CO₂ concentration, nitrogen addition, phenophase, biomass, low-carbon behavior, driving mechanism, scenic spot, World Heritage Site., redundancy analysis (RDA)