Pyrolusite-mediated denitrifying anaerobic methane oxidation for simultaneous enhancement of nitrogen removal and greenhouse gas mitigation in wetlands.

Constructed wetlands are widely used for secondary effluent treatment, but the denitrification process is limited by the low C/N ratio, accompanied by greenhouse gas emissions. A coupled wetland (MDCW) was developed by integrating pyrolusite and denitrifying anaerobic methane oxidation (DAMO) microorganisms. It achieved 95.75 % and 87.40 % removal of TN and COD, while reducing CH4 and N2O fluxes by 75.81 % and 69.13 %, respectively. Pathway decoupling analysis revealed that heterotrophic denitrification (38.12 %), DAMO (32.49 %), Mn-mediated nitrate reduction (26.98 %), and endogenous denitrification (2.41 %) jointly drove NO3- removal, while DAMO and Mn-induced methane oxidation contributed 55.24 % and 44.76 % to CH4 consumption. Microbial community and gene analysis showed that Candidatus Methanoperedens and Candidatus Methylomirabilis were enriched and the relative abundances of functional genes such as mcr, mdh, narK and nor were upregulated significantly. Therefore, Mn(IV)/Mn(II) cycling acted as a redox interface, optimizing electron allocation from CH4/COD oxidation to NO3- reduction for low-carbon nitrogen removal.