Pathways and biological mechanisms of NO emission reduction by adding biochar in the constructed wetland based on N stable isotope tracing.

Constructed wetlands (CWs) added with biochar were built to study pollutant removal efficiencies, nitrous oxide (NO) emission characteristics, and biological mechanisms in nitrogen transformation. The results showed that biochar addition enhanced the average removal rates of ammonium (NH-N), total nitrogen, and chemical oxygen demand by 4.03-18.5%, 2.90-4.99%, and 2.87-5.20% respectively while reducing NO emissions by 25.85-83.41%. Based on N stable isotope tracing, it was found that nitrification, denitrification, and simultaneous nitrification and denitrification were the main processes contributing to NO emission. The addition of biochar resulted in maximum reduction rates of 71.50%, 80.66%, and 73.09% for these three processes, respectively. The relative abundance of nitrogen-transforming microbes, such as Nitrospira, Dechloromonas, and Denitratisoma, increased after the addition of biochar, promoting nitrogen removal and reducing NO emissions. Adding biochar could increase the functional gene copy number and enzyme activity responsible for nitrogen conversion, which helped achieve efficient NH-N oxidation and eliminate nitrite accumulation, thereby reducing NO emissions.