Abstract: This study examined how flooding affects decomposition and carbon and nitrogen release from single-species and mixed-species litter of three dominant wetland plants (Phragmites australis (Cav.) Trin. ex Steud, Deyeuxia angustifolia (Kom.) Y. L. Chang, and Glyceria spiculosa (F. Schmidt) Roshev.) in Khanka Lake, Northeast China. Based on a laboratory simulation experiment, results showed that after 240 d of decomposition, the mass loss rates of P. australis, D. angustifolia, and G. spiculosa were 15.13%, 20.3%, and 21.1%, respectively, with P. australis exhibiting the slowest decomposition rate. Mixed litter generally decomposed more rapidly than single-species litter. The mixture of G. spiculosa and D. angustifolia showed the highest mass loss rate (23.83%), exceeding that of G. spiculosa mixed with P. australis (22.20%) and P. australis mixed with D. angustifolia (19.10%). Mixing effect analysis indicated that mixtures containing G. spiculosa primarily produced synergistic effects during decomposition, whereas P. australis–D. angustifolia mixed litter showed stage-specific antagonistic effects. Litter decomposition was significantly and negatively correlated with initial litter C : N ratio (r=−0.178) and C : P ratio (r=−0.181), indicating that initial stoichiometric composition was a key regulator of litter breakdown under flooded conditions. All litter types showed net carbon release during decomposition, as indicated by carbon relative return index values above zero (CRRI>0). In contrast, net nitrogen release occurred only in D. angustifolia litter (nitrogen relative return index, NRRI>0), whereas P. australis litter and its mixture with G. spiculosa exhibited net nitrogen accumulation (NRRI<0). Carbon loss occurred primarily through CO₂ emissions (81.7%–88.1%), followed by dissolved organic carbon (7.1%–11.3%) and CH₄ emissions (1.5%–8.3%). These findings show that litter decomposition and nutrient release in flooded wetlands are strongly controlled by initial litter chemistry, but that mixed-species litter can generate non-additive effects that can not be inferred from single-species decomposition patterns alone.