Effects Of Microplastics On Nitrification And Denitrification In Sediments Of Surface Flow Wetland

Microplastics are a new type of pollutant that can be discharged into freshwater systems through domestic sewage or sewage plant tailwaters and settles into sediments.It has been reported that microplastics can affect the nitrogen cycling process in environment by increasing porosity,releasing additives and changing the structure of microbial communities.Surface flow wetland is a common sewage treatment process,which can remove nitrogen from water through the nitrification and denitrification reactions of sediment.The effects of microplastics on nitrogen cycling in surface-flowing wetland sediments have not been studied.By adding microplastics to surface flow wetland sediments,the effects of polyvinyl chloride（PVC）and polylactic acid（PLA）on the N concentration of overlying water,sediment biofilm biomass,microbial oxygen consumption rate,enzyme activity and microbial community structure in surface flow wetland sediments were investigated,so as to explore the effects of microplastics on nitrification denitrification reactions and their mechanisms in surface flow wetland sediments.The detection results of overlying water quality indexes showed that higher concentration of microplastics in sediments could reduce the concentration of TN in water,but lead to the accumulation of organic nitrogen.The addition of PVC microplastics could also lead to the accumulation of NH₄⁺-N.Compared with control,at exposure level of 5.0wt%,the concentration of TN in overlying water of PVC and PLA groups decreased by 16.12%and18.24%,respectively.The concentration of organic nitrogen in overlying water increased by404.19%and 365.22%,respectively.The concentration of NH₄⁺-N increased by 51.77%in PVC treatment and decreased by 25.77%in PLA treatment.The results showed that microplastics were the carrier of microbial colonization and biofilm growth in sediments.With the increase of microplastics concentration,biofilm biomass increased significantly.The mature disintegration of the biofilm into the water resulted in the accumulation of organic nitrogen in the overlying water.At the exposure level of 5.0wt%,the amount of biofilm in the PVC and PLA treatments increased by 5.66%and 5.13%,respectively,compared with control.detection results of dissolved oxygen profile of the interface between sediment and water and oxygen consumption rate of the sediment showed that microplastics in the sediments become a kind of organic carbon substrates,resulting in higher sediment microbial activity,reducing the penetration depth of oxygen in the sediments;detection results of key enzyme activities of nitrification and denitrification in sediment showed that:microplastics in sediments lead to an increase in the activity of key enzymes in the nitrogen cycling,which is positively correlated with the concentration of microplastics.At 5.0wt%exposure,compared with control,The AMO activity in PVC and PLA treatments was increased by 47.32%and 57.13%,HAO activity was increased by 53.46%and 62.30%,NR activity was nearly three times,NIR activity was about 1.5 times,HZS activity was increased by 31.07%and 66.70%,respectively.Metagenomic analysis of sediment microbial communities showed that microplastics had significant effects on microbial community structure and metabolic function.The results showed that theαdiversity index of PVC and PLA treatments was higher than that of the control group,and the higher the concentration was,the higher theαdiversity index was.Compared with control,the relative abundance of genes associated with negative NO reduction reaction was lower in microplastics treatments.The relative abundance of genes associated with nitrate assimilation and dissimilation-reduction was higher in PVC treatment.The relative abundance of genes associated with nitrate assimilation-reduction reaction was higher in PLA treatment.The research results clarified the influence process and mechanism of PVC and PLA microplastics on nitrification and denitrification in surface flow wetland sediments,providing a reference for further exploration of the influence of microplastics on nitrogen cycling in environmental media.