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Sponge iron (s-Fe0) is a porous metal with the potential to be an electron donor for denitrification.This study aims to evaluate the feasibility of using s-Fe0 as the substrate of wetland mesocosms.Here,wetland mesocosms with the addition of s-Fe0 particles (CW-Fe) and a blank control group (CW-CK) were established.The NO3--N reduction property and water quality parameters (pH,DO,and ORP) were examined at three COD/N ratios (0,5,and 10).Results showed that the NO3--N removal efficiencies were significantly increased by 6.6 to 58.9％ in the presence of s-Fe0.NH4+-N was mainly produced by chemical denitrification,and approximately 50％ of the NO3--N was reduced to NH4+-N,at the COD/ratio of 0.An increase of the influent COD/N ratio resulted in lower chemical denitrification and higher bio-denitrification.Although chemical denitrification mediated by s-Fe0 led to an accumulation of NH4+-N at COD/N ratios of 0 and 5,the TIN removal efficiencies increased by 4.5％-12.4％.Moreover,the effluent pH,DO,and ORP values showed a significant negative correlation with total Fe and Fe (Ⅱ) (P ＜ 0.01).High-throughput sequencing analysis indicated that Trichococcus (77.2％) was the most abundant microorganism in the CW-Fe mesocosm,while Thauera,Zoogloea,and Herbaspirillum were the primary denitrifying bacteria.The denitrifiers,Simplicispira,Dechloromonas,and Denitratisoma,were the dominant bacteria for CW-CK.This study provides a valuable method and an improved understanding of NO3--N reduction characteristics of s-Fe0 in a wetland mesocosm.