Studies On Influencing Factors Of Greenhouse Gases Emissions And Carbon Budget Estimation In Horizontal Subsurface Flow Constructed Wetlands

Constructed wetlands（CWs）,as a representative technology of Nature-based Solution（Nb S）,are widely used worldwide to treat domestic,agricultural,industrial,and leachate wastewater.CWs have the advantages of high pollutant removal efficiency,stable effluent quality,low operational investment and simple maintenance.Organic pollutants,nitrogen and phosphorus nutrients are widely distributed among the various wastewaters treated in CWs.However,due to the regular influx of wastewater,CWs may release large amounts of greenhouse gases（GHGs）during the removal of pollutants,this will reduce the environmental and ecological benefits of CWs.In addition,CWs as unique ecosystems that mimic natural wetlands,and the carbon sequestration capacity of CWs should not be overlooked.Therefore,it is necessary to study the influencing factors of greenhouse gas emissions from CWs,quantify the GHGs emission and estimate the carbon balance of CWs.In this study,we investigated the effects of influent and environmental conditions on the pollutant removal capacity and greenhouse gas emissions of horizontal subsurface flow（HSSF）CWs.The mesocosm experiment uses simulated wastewater based on the secondary effluent standard configuration of a wastewater treatment plant.At the same time,the carbon input and output of the HSSF CWs ecosystem are estimated to assess whether the CWs act as a carbon source or a carbon sink.The response of CWs to each influencing factor was obtained through experiments,and the CWs were assessed as either a carbon source or a carbon sink.The main findings of this study are as follows:（1）Through Meta-analysis,it can be concluded that the greenhouse gas emissions from CWs were influenced by many factors,including the structure of CWs（i.e.,CWs type,substrate,and plants）,the influent water characteristics（such as C/N ratio and salinity,etc.）and temperature.When comprehensively considering water purification capacity and greenhouse gas emissions,compared with the surface flow and vertical subsurface flow CWs,HSSF CWs are relatively better choice;biochar-amended CWs are conducive to improving pollutant removal efficiency and reducing nitrous oxide（N₂O）emissions;When considering the reduction of greenhouse gas emissions from CWs,using the appropriate plant is more important than increasing species richness;most studies have shown that the N₂O emission from CWs is the lowest when the influent C/N ratio is 5;high water salinity would reduce the purification ability of pollutants,and at the same time reduced the emission of carbon dioxide（CO₂）and methane（CH₄）;high ambient temperature might increase the emission of CH₄ and N₂O.（2）The HSSF CWs planted with three common wetland plants（Cyperus alternifolius,Typha orientalis,and Acorus calamus）and a mixture of the three plants have been established in this study.The different CWs exhibited different water purification capacities and emitted different levels of greenhouse gases.The monoculture CWs with C.alternifolius and polyculture CWs had a higher capacity to remove organic matter and nutrients（nitrogen and phosphorus）rather than CWs planted with T.orientalis and A.calamus.For GHGs emissions,there were no significant differences in CH₄ emission fluxes between different CWs.N₂O emission fluxes were significantly lower（p<0.05）in CWs with C.alternifolius（0.106 mg/m²/h）and polyculture CWs（0.0026 mg/m²/h）than T.orientalis（0.425 mg/m²/h）and A.calamus（0.464 mg/m²/h）CWs.The lowest global warming potential（GWP）was detected in the polyculture CWs.Based on microbial community analysis,the relative abundance of microorganisms associated with pollutant degradation and transformation was found to be higher in the polyculture CWs.（3）Influent water concentration affects the removal of CWs pollutants and greenhouse gas emissions.High influent conditions decrease the pollutant removal efficiency of CWs.At high influent conditions,the pollutant removal efficiencies of chemical oxygen demand（COD）,nitrate（NO₃^--N）,ammonium（NH₄⁺-N）,and total phosphorus（TP）for each CWs were 87.60-91.92%,58.05-69.93%,53.40-79.55%and37.57-99.61%,respectively.In this study,the higher influent COD hardly impacts CH₄and carbon dioxide（CO₂）emissions from each CWs.In contrast,high influent total nitrogen（TN）significantly increased N₂O emissions.The percentages of N₂O-N/TN removal from CWs with C.alternifolius,T.orientalis,and A.calamus and polyculture CWs at high and low TN loads were 3.8%,1.5%,1.4%,2.58%and 2.23%,0.87%,-0.27%and 1.83%respectively.Taking into account the pollutant cleaning capacity and greenhouse gas emissions,the polyculture CWs at high influent loads resulted in better pollutant removal and reduced N₂O emission.（4）The ambient temperature impacts the transport and transformation of pollutants in CWs.Additionally,temperature affects pollutant removal efficiencies and GHGs emissions by influencing the microbial activity and community structure of CWs.The removal capacity of NH₄⁺-N,NO₃^--N and TN decreased with lower ambient temperature;the temperature had no significant effect on the removal of COD from CWs;and temperature did not affect the removal of TP from CWs with C.alternifolius and polyculture CWs.The CH₄ emission flux of CWs planted with C.alternifolius and T.orientalis was negatively correlated with temperature,while the CH₄ emission flux of CWs planted with A.calamus and polyculture CWs was not significantly correlated with temperature.For CO₂ emission,there was a linear positive correlation between CO₂ emission flux and temperature in CWs planted with C.alternifolius and A.calamus,while there was a linear negative correlation between CO₂ emission flux and temperature in polyculture CWs.For N₂O emissions,the N₂O emissions from CWs of C.alternifolius and A.calamus decreased significantly with decreasing temperature.Overall,CWs planted with different plants at different temperatures showed variability in pollutant removal efficiencies and GHGs emissions.（5）Under the operating conditions of this study,the carbon budget of the different CWs was estimated to show that all CWs were carbon sinks.The average annual carbon sequestration of CWs was 4.90 kg C/yr/m²、3.31 kg C/yr/m²、1.78 kg C/yr/m² and 2.12kg C/yr/m².Compared to the amount of carbon fixed by plant and soil fixation,the CH₄-C emissions over the whole experimental period were very small and even the CWs became CH₄ sinks（negative CH₄ emission fluxes）.However,whether CWs are carbon sources or sinks also depends on their age,operation and environmental boundary conditions,such as location and climate.Therefore,further studies are needed to assess the position of CWs as carbon sources or sinks in the future.