Perfluorooctanoic acid and concomitant microplastics pollution impact nitrogen elimination processes and increase N2O emission in wetlands through regulation of the functional microbiome

Per- and polyfluoroalkyl substances (PFASs), typical groups of emerging contaminants (ECs), can accumulate in wetland systems and adsorb onto the surface of microplastics (MPs), resulting in composite pollution. However, the effects of PFASs and their composite pollution with MPs on the ecological processes and functions of wetlands remain largely unknown. We studied the effects of perfluorooctanoic acid (PFOA) and its combined pollution with two types of MPs (polylactic acid (PLA) and polyethylene (PE)) at low and high concentration levels on nitrogen elimination processes and NO emissions in wetlands as well as the associated microbial mechanisms over three months. The results showed that PFOA inhibited nitrification in wetland sediment (P < 0.05), most pronouncedly with the composite pollution of PFOA and MPs. NO₃⁻ isotope tracing analysis showed that anammox and denitrification rates were both significantly inhibited by PFOA contamination, especially at high concentrations, while co-presence of MPs, especially PLA, weakened the inhibitory effect of PFOA on anammox and denitrification rates. The contribution of anammox to nitrogen elimination declined under PFOA and its composite pollution with high concentrations of MPs. Overall, PFOA and its composite pollution with MPs weakened the nitrogen removal capability of the wetlands. PFOA presence increased N₂O emissions (by 43.4-343 %) from the wetlands, and its composite pollution with MPs, particularly with PLA, further exacerbated N₂O emissions (by 35.6-197 %), evidencing a concentration- dependent effect. The increases were primarily attributed to that PFOA and MPs contamination regulated the community structure of the functional microbiome and reduced the abundance of ammonia-oxidizing and N₂O-reducing bacteria. DO, nitrogen (NH-N or NO-N) and dissolved organic carbon (DOC) concentrations were the key environmental factors influencing nitrogen loss rates in the wetlands. PFOA and its composite pollution with MPs regulated the nitrogen loss processes and NO emission in the wetlands following distinct pathways. This study provides new insights into the impacts of PFASs and their composite pollution with MPs on nitrogen transformation and N₂O emissions in wetlands and the indispensable management of wetlands under continuous inputs of ECs.