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Horizontal subsurface flow constructed wetland (SSF CW) provides good conditions to denitrification but the ability of this system to nitrify NH4+-N is limited.The performance response of SSF CW to simulated wastewater with different COD/N (chemical oxygen demand: TN in influent) ratios was studied during an 8-month period in greenhouse conditions to improve the removal of NH4+-N.In this study,several wetland litters were hydrolyzed to get relevant carbon extracting solution,which was applied to the wetland systems.During the nitrification process,the variation of dissolved oxygen (DO) with COD/N ratios had a significant effect on the removal of NH4+-N.It was found that the total nitrogen (TN) removal efficiency increased with COD/N ratios.Appropriate control of the carbon or nitrogen source concentration and the COD/N ratio in the influent could achieve the optimal nitrogen removal.