Greenhouse gas emissions from field-scale surface-flow constructed wetlands fed with different polluted water types: Environmental and microbial drivers

Constructed wetlands (CWs) are widely employed for the purification of various polluted water types (PWTs) and are hotspots of greenhouse gas (GHG) emissions. However, the effect of influent PWTs on GHG emissions from CWs is poorly understood. In this study, GHG (CO2, CH4 and N2O) emissions from three groups of field-scale riparian surface-flow CWs (SFCWs), fed with agricultural runoff (CW-AR), urban runoff (CW-UR), and tailwater from wastewater treatment plants (CW-T), were compared. Moreover, the driving mechanisms were comparatively explored relative to nutrient and dissolved organic matter (DOM) properties, as well as microbial communities and activities. The results showed distinct variations in nutrients, DOM, and microbial characteristics among the SFCWs. The CO2 emission flux was highest in CW-UR (139.6 mg m- 2 h- 1), the CH4 flux peaked in CW-AR (4.3 mg m- 2 h- 1), and N2O presented the highest flux in CW-T (1.0 mg m- 2 h- 1). N2O was the primary contributor to the global warming potential in CW-UR (38.0 %) and CW-T (52.3 %), while CH4 (46.4 %) contributed the most in CW-AR. Partial least squares-structural equation modelling showed that the CO2 fluxes in CW-UR were directly related to the proportion of aquatic Polynucleobacter and sedimentary Thiobacillus, and indirectly associated with the NO3--N-dominated N parameters. The CH4 emission in CW-AR was directly related to the proportions of sedimentary Nitrososphaeraceae and Bathyarchaeia, as well as aquatic Nitrososphaeraceae and Methanosaeta, and indirectly related to carbon-to-nitrogen ratio parameters. The N2O emission in CW-T was directly related to the proportions of aquatic Limnohabitans and carbon-to-nitrogen ratio parameters, and indirectly to water temperature. Our findings emphasize that PWTs with distinct water quality properties regulated the GHG emissions of the SFCWs, primarily through their influence on DOM and microbial characteristics. This study provides new insights into the mechanisms involved in distinct GHG emission fluxes from CWs fed with different PWTs, which can be useful for improving the management of CWs and optimizing their benefits.