Greenhouse gas emissions and control measures for constructed wetland: A systematic review

Constructed wetlands (CWs) have gained global acceptance as sustainable ecological systems for wastewater remediation, offering economic viability and operational energy efficiency. However, their paradoxical role as significant sources of greenhouse gases (GHG) such as CH₄, CO₂, and N₂O presents an environmental challenge, potentially offsetting their ecological benefits through climate impacts. Here we systematically examine the evolution of CWs technology, analyze global trends in GHG emissions, and current emission flux data. Measured fluxes of CO₂, CH₄, and N₂O from CWs range from 4.32 to 961.04 mg m^-2·h^-1, 0.05-245.23 mg m^-2·h^-1, and 0.0013-49.75 mg m^-2·h^-1, respectively. Regionally, elevated CH₄ and CO₂ emissions are observed in urban areas of Southeast Asia, southern China, eastern America, and tropical regions, while higher N₂O fluxes are detected in urban areas of Europe, China, Africa, and South America. These trends underscore the urgent need for effective mitigation strategies. This review further explores key factors influencing GHG emissions from CWs, including temperature, wetland configuration, influent quality, emerging contaminants (e.g., antibiotics and microplastics), and hydrological conditions. By uncovering the mechanisms behind CH₄, CO₂, and N₂O emissions, we propose targeted design and operational strategies to minimize GHG outputs. Ultimately, the empirical findings yield actionable guidance for optimizing engineering protocols in environmental governances, promoting the transition of CWs toward more sustainable and low-carbon wastewater treatment systems, thereby contributing to global climate change mitigation and water pollution control.