Systematic review and meta-analysis of Salix vs. Phragmites australis constructed wetlands

Recent research has highlighted interest in using woody vegetation (such as willow, Salix) in constructed wetlands (CWs) to improve pollutant removal compared to conventional CWs (typically using Phragmites australis). Nonetheless, comparative research has been limited to a few specific sites or pollutants. There remains a need for a comprehensive comparison between woody and herbaceous CWs covering multiple pollutants, as well as greenhouse gas emissions. Here we present the first systematic review and meta-analysis to compile and compare data from studies using Salix species against those using Phragmites australis, covering removal of conventional pollutants and emissions of CO₂, CH₄, and N₂O. The review includes horizontal subsurface flow CWs for municipal/domestic wastewater treatment in temperate/continental climates without pronounced dry seasons. It encompasses primary, secondary and tertiary CWs. Findings suggest that i) Salix species improve TN, TKN, NH₄⁺-N and TP removal efficiency by 16-72 % (absolute change), ii) Salix species and Phragmites australis perform similarly in reducing both BOD and COD, iii) Salix species have 12 % worse TSS removal efficiency (absolute change), and iv) Salix species can sequester 14 tCO2 ha^-1 yr^-1 more than Phragmites australis. BOD and COD findings afford high confidence; however, because of data limitations, there is only medium or low confidence regarding the other findings. For higher confidence, further research is recommended to compare Salix species against Phragmites australis in treating all conventional pollutants, ideally in pilot or operational CWs, for longer durations and across all seasons – and including net greenhouse gas emissions.