An experiment with treatments differing in nitrogen supply(0

5and 15g/m2)and CO2 level(360and 700μmol/mol)with OTC(open top chamber)equipment was carried out to investigate the effects of elevated CO2concentration on the carbon and nitrogen accumulation of Calamagrostis angustifoliain freshwater marsh of the Sanjiang Plain.Results showed that elevated CO2concentration increased carbon fixation ofplant by 19.3%

24.4% and 24.6% under the NN

MN and HN treatments

respectively.Elevated CO2concentration also had a great impact on the carbon allocation in different parts of Calamagrostis angustifolia

as the carbon distribution in root increased to different degrees under all the nitrogen treatments.Elevated CO2 concentration reduced the nitrogen content of Calamagrostisangustifoliaapparatus. The largest decrease in leaf and stem both appeared in heading period(14.4% and 19.5%)

while the decrease in root appeared in dough period(17.4%).The decrease of nitrogen content in apparatus was due to diluting effect byplant speeding growth under elevated CO2 concentration.