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 (N ₂ O) 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 N ₂ O generation during denitrification under varying chemical oxygen demand (COD) to nitrate (NO ₃ ^- -N) ratios. The results showed that natural pyrite and biochar were superior in enhancing COD (90.6-91.2 %) and NO ₃ ^- -N removal (90.0-93.5 %) in CWs with a COD/NO ₃ ^- -N ratio of 9. The accumulation of NO ₂ ^- -N during the denitrification process was the primary cause of N ₂ O emission, with the fluxes ranging from 95.6-472.0 μg/(m ² ·h) in CW-B, 92.9-400 μg/(m ² ·h) in CW-BF, and 54.0-293.3 μg/(m ² ·h) in CW-BC. The addition of biochar significantly reduced N ₂ O emissions during denitrification, while natural pyrite had a lesser inhibitory effect on N ₂ O 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 N ₂ O 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 N ₂ O production in CWs, especially when treating sewage with a relatively high COD/NO ₃ ^- -N ratio.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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