The effects of CO₂ concentration and nitrogen supply on the contents of soil organic carbon and total nitrogen in Calamagrostis angustifolia wetlands of Sanjiang Plain was investigated.An experiment was carried out with treatments differed in nitrogen supplies(0,5 and 15 g/m2) and CO₂ levels(350 and 700 μmol/mol) with OTC(open-top chamber) equipments.The results show that with additional nitrogen,the soil organic carbon varied but not significantly with increases of 0.5%~1.8%.The elevated CO₂ decreased the contents of soil total nitrogen.The average reduction of NH⁺₄—N due to the elevated CO₂ elevation was 8.2%,8.9% and 9.7%,for the three nitrogen levels,respectively.The contents of NO^-₃—N decreased similarly under higher CO₂ levels.Both NH⁺₄—N and NO^-₃—N decreased to a greater degree with the high N supply(15 g/m2).Soil available N was the key factor in affecting the response of the plants to elevated CO₂.