A review on the mechanism and affecting factors of nitrous oxide emission in constructed wetlands

Constructed wetlands (CWs) are considered important sources of nitrous oxide (N2O). Various reports in the literature indicate that CWs have high N2O emission rates. The release of N2O from CWs treating wastewater emissions range from −16.7 to 188 mg N2O m−2day−1. N2O in CWs is produced mainly by nitrification, denitrification, nitrifier denitrification, and nitrate-ammonification. Denitrification is considered the major source of N2O under most conditions. In recent years, two main methods of sampling N2O gas in CWs have been employed, including the headspace equilibration technique and the closed static chambers technique. N2O emission may be affected by various operating parameters and environmental conditions. One of the main environmental factors affecting the removal of nitrogen in CWs is dissolved oxygen, which affects nitrification and denitrification processes, thus greatly influencing N2O emission. CW gas dynamics is affected mainly by season and weather conditions, especially temperature and moisture. Aquatic plants, flow regime, oxidation–reduction potential, nitrate concentration, C/N ratio and other factors can affect N2O emission in CWs.