Effects Of Rhizosphere Microenvironment Of Different Plant Assemblages On The Performance And Greenhouse Gas Emission Reduction Of Constructed Wetlands

In recent years,under the social demand of"building a beautiful China based on ecological priority and green development",constructed wetland（CW）has been attached to great importance as an ecological project of water treatment.However,the high greenhouse gas（GHG）emissions from constructed wetlands greatly reduce their comprehensive ecological and environmental benefits.Therefore,it is urgent to explore the GHG emission mechanism of artificial wetlands and develop technical measures that can simultaneously achieve efficient water quality purification and GHG emission reduction.Wetland plants,as the main body of artificial wetlands,play an important role in regulating GHG emissions.The current relevant research mainly focuses on the impact of plants on GHG emissions from constructed wetlands at the macro scale,but from the micro-scale analysis,the key process and microbiological driving mechanism of plant rhizosphere environment on GHG emissions in constructed wetlands are still unclear.This study conducted hydroponic experiments and constructed wetland simulation（CWs）experiments on four commonly used constructed wetland plants and their six plant combinations.Through exploring the impact of the rhizosphere microenvironment of different plant combinations on the performance of constructed wetlands and the ability to reduce GHG emissions,we screened out plant combinations that can simultaneously improve the sewage treatment effect and reduce GHG emissions.The main experimental conclusions are as follows:（1）In the hydroponics experiment,the physiological and ecological characteristics of different wetland plants and assemblages were studied.It was found that the assemblages of plants could accelerate the growth and development of plants.Dissolved organic carbon（DOC）and radial oxygen loss（ROL）were also increased,and the growth rate of plants was positively correlated with the DOC secretion rate and ROL rate of roots.In all experimental systems,the system configured with plant assemblage“Lythrum salicaria+Phragmites australis”has the highest growth rate,root DOC secretion rate and ROL rate.Therefore,from the perspective of promoting plant growth,“Lythrum salicaria+Phragmites australis”has the most advantages.（2）The results of CWs experiments showed that plant assemblages could effectively improve the performance of CWs.Compared with single-species plant-planted CWs,the CWs with mixed-species increased the removal efficiency of COD（6.9–12.0%）,TP（9.0–13.7%）and TN（5.2–11.0%）.Among them,the CW equipped with“Lythrum salicaria+Phragmites australis”had the highest removal efficiency of COD（82.8%）,TP（59.4%）and TN（68.9%）,which greatly developed the CW pollutant removal ability.In addition,the dissolved organic matter（DOM）in the effluent was determined.According to the results of parallel factor analysis,the CW equipped with“Lythrum salicaria+Phragmites australis”had a higher F_maxvalue and more protein-like substances,which provided conditions for optimizing the performance of CW.（3）The GHG of the systems with different wetland plants and assemblages were measured in the hydroponics and CWs experiments respectively.The results showed that although the average emission fluxes of CO₂,CH₄,and NO₂in mixed systems are higher than those in single systems,by calculating the global warming potential（GWP）,it was found that the system configured with“Lythrum salicaria+Phragmites australis”had the lowest GWP,311.72±0.13 mg m^-2h^-1CO_2-eq（hydroponics experiment）and 561.80±0.22 mg m^-2h^-1CO_2-eq（CWs experiment）.Therefore,in this study,the plant assemblage“Lythrum salicaria+Phragmites australis”had the lowest net greenhouse effect.（4）The microbial community in the rhizosphere microenvironment of CW was analyzed,and it was found that the plant assemblage“Lythrum salicaria+Phragmites australis”significantly increased the richness and diversity of microorganisms in the CW,increased relative abundance of Proteobacteria and Actinobacteriota related to organic degradation,and promoted the growth and reproduction of Acidobacteriota that can improve the ROL rate.The high-throughput sequencing results of functional genes showed that the CW with“Lythrum salicaria+Phragmites australis”had a higher nir S abundance,which realized a higher TN removal rate.In addition,the higher abundance of nos Z,pmo A abundance and lower abundance of mcr A in this system also further explained its lower GWP.To sum up,“Lythrum salicaria+Phragmites australis”is the best combination of CW plants that can simultaneously achieve higher pollutant removal efficiency and lower GHG.