Microbiota and genetic potential for reducing nitrous oxide emissions by biochar in constructed wetlands.

The denitrification process in constructed wetlands (CWs) is responsible for most of the nitrous oxide (NO) emissions, which is an undesired impact on the ecology of sewage treatment systems. This study compared three types of CWs filled with gravel (CW-B), gravel mixed with natural pyrite (CW-BF), or biochar (CW-BC) to investigate their impact on microbiota and genetic potential for NO generation during denitrification under varying chemical oxygen demand (COD) to nitrate (NO-N) ratios. The results showed that natural pyrite and biochar were superior in enhancing COD (90.6-91.2 %) and NO-N removal (90.0-93.5 %) in CWs with a COD/NO-N ratio of 9. The accumulation of NO-N during the denitrification process was the primary cause of NO emission, with the fluxes ranging from 95.6-472.0 μg/(m·h) in CW-B, 92.9-400 μg/(m·h) in CW-BF, and 54.0-293.3 μg/(m·h) in CW-BC. The addition of biochar significantly reduced NO emissions during denitrification, while natural pyrite had a lesser inhibitory effect on NO emissions. The three types of substrates also influenced the structure of microbiota in the biofilm, with natural pyrite enriched nitrogen transformation microorganisms, especially for denitrifiers. Notably, biochar significantly enhanced the abundance of nosZ and the ratio of nosZ/(norB + norC), which are critical factors in reducing NO emissions from CWs. Overall, the results suggest that the biochar-induced changes in microbiota and genetic potential during denitrification play a significant role in preventing NO production in CWs, especially when treating sewage with a relatively high COD/NO-N ratio.