A Comparative Review of Pollutant Fate and Climate Implications in Constructed and Natural Wetlands

Wetlands remove nitrogen, phosphorus and organic pollutants, yet the same biogeochemical processes also release carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O); improving water quality and affecting the climate are therefore two sides of one system. The comparison between constructed and natural wetlands in this review covers pollutant fate and greenhouse-gas effects together, and it also asks whether their evidence bases are comparable at all. A structured search of Web of Science and Scopus gave 60 candidate papers (2007–2026); 29 of them were kept as the core evidence base (14 focused mainly on constructed wetlands, 11 mainly on natural wetlands, and 4 on both), and they include laboratory and pilot experiments, field and long-term monitoring, modelling studies, reviews and bibliometric analyses. The paper gives a narrative comparison and does not claim to be a systematic review or a meta-analysis. Both wetland types remove pollutants through the same basic processes, such as sedimentation, adsorption, plant uptake and microbial transformation; the difference between them is one of emphasis and not of kind. Constructed wetlands are managed systems and remove about 60% of total nitrogen and 64% of total phosphorus, but they may emit N2O; natural wetlands store more carbon, but they are a source of CH4. Critically, the two types are usually reported with different indicators, units and system boundaries, so they cannot be ranked by a single removal efficiency; only carbon and the three gas fluxes are directly comparable, and only when the boundary and time horizon are stated. The evidence reviewed here is sufficient to compare the processes and functions of the two wetland types, but not to rank them by overall efficiency. Fair comparison will require paired study designs, long-term mass balances and complete greenhouse-gas accounting.