Biochar and hematite amendments suppress emission of CH and NO in constructed wetlands.

Constructed wetlands (CWs) are considered a widely used cost-effective technology for pollutant removal. However, greenhouse gas emissions are a non-negligible problem in CWs. In this study, four laboratory-scale CWs were established to evaluate the effects of gravel (CW), hematite (CW), biochar (CW), and hematite + biochar (CW) as substrates on pollutants removal, greenhouse gas emissions, and associated microbial characteristics. The results showed that the biochar-amended CWs (CW and CW) enhanced the removal efficiency of pollutants, with 92.53 % and 93.66 % of COD and 65.73 % and 64.41 % of TN removal, respectively. Both single and combined inputs of biochar and hematite significantly reduced CH and NO fluxes, with the lowest average of CH flux obtained in CW (5.99 ± 0.78 mg CH m h) and the least NO flux in CW (287.57 ± 44.84 μg NO m h). The substantial reduction of global warming potentials (GWP) was obtained in the applications of CW (80.25 %) and CW (79.5 %) in biochar-amended CWs. The presence of biochar and hematite mitigated CH and NO emissions by modifying microbial communities with higher ratios of pmoA/mcrA and nosZ genes abundances, as well as increasing the abundance of denitrifying bacteria (Dechloromona, Thauera and Azospira). This study demonstrated that biochar and the combined use of biochar and hematite could be the potential candidates as functional substrates for the efficient removal of pollutants and simultaneously reducing GWP emissions in the constructed wetlands.