Physiological Response And Pollutant Removal Characteristics Of Vertical Subsurface Flow Constructed Wetlands Under Nitrogen Stress

Nitrogen concentration directly affects plant metabolism,organ development,and rhizosphere effects,which in turn make constructed wetlands exhibit different rhizosphere microbial community structures,activities,and pollutant removal characteristics.However,the response mechanisms of microbial habitats and microbial life activities in this process have not been Completely explained.In addition,compared with conventional deep constructed wetlands,excessive root growth in shallow constructed wetlands can enhance biological performance,reduce external energy input,and enhance pollutant removal.The strengthening mechanism of removal remains to be studied.For this purpose,two depths（shallow-bed constructed wetland:0.1m and conventional constructed wetland:0.6m）subsurface constructed wetland and three nitrogen concentrations（nitrogen deficiency:0mg/L,conventional nitrogen concentration:60mg/L,high nitrogen concentration）were carried out.:600mg/L),the physiological response of three wetland plants（Iris,Spathiphyllum and C.indica）,as well as the response mechanism of rhizosphere microenvironment redox conditions,rhizosphere microbial community structure and activity.the result shows:（1）Under high nitrogen and nitrogen deficiency stress,the malondialdehyde（MDA）content of Iris and Spathiphyllum was maintained at a low level,but the final MDA content of C.indica was up to 3.5 to 4.5 times the initial value.The active enhancement of total antioxidant capacity（T-AOC）and peroxidase（POD）activities to nitrogen stress,while the adaptability of T-AOC and POD activities of C.indica was poor.Restricting the root growth space resulted in a maximum increase of 57.55%in MDA content in iris and C.indica,but at the same time the maximum increase in T-AOC content reached 52.90%,indicating that restricting root growth space will aggravate membrane lipid peroxidation,but plants can increase stress resistance by enhancing to defend against coercion.（2）High nitrogen stress promoted the photosynthesis of iris and white crane taro,and the actual quantum yield of PSII(Φ_PSⅡ)increased by 135%,while the photosynthesis of C.indica was inhibited,andΦ_PSⅡdecreased by 62.86%.Nitrogen deficiency stress resulted in the inhibition of photosynthesis in three plants.Under nitrogen stress,the three plants enhanced their stress resistance by increasing heat dissipation,and the maximum increase of the non-photochemical quenching coefficient was 56.67%.Restricting the root growth space increased theΦ_PSⅡof the three plants,and also enhanced the heat dissipation and stress resistance of the plants.The maximum increase ofΦ_PSⅡwas 19.05%.（3）High nitrogen stress promoted the development of aerenchyma in the roots of Iris and Spathiphyllum,and the root porosity was at most 1.40 times that of the control group,while the root porosity of C.indica root was only 55%of the control group under this condition.Nitrogen deficiency stress inhibited plant root biomass but increased root porosity by up to 31.70%compared with the control group.Restricting root growth space will promote the development of aerial tissue in the root system,which is significantly larger than that of conventional wetlands in terms of root number and root dry weight.（4）High nitrogen stress promoted the radial oxygen loss（ROL）rate of Iris and Spathiphyllum,which was up to 12.37%higher than that of the control group,while the minimum ROL rate of C.indica was only 11.44%of that of the control group.Under nitrogen deficiency stress,the photosynthesis of plants was inhibited,and the oxygen production capacity decreased,resulting in the ROL rate of the three plants being significantly lower than that of the control group.The ROL rate of plants in shallow-bed constructed wetlands is higher than that of conventional constructed wetlands because restricting root growth space can enhance plant stress resistance,photosynthesis,and the development of root aerial tissue.The variation characteristics of ROL rate under nitrogen stress were significantly positively correlated with the various characteristics of photosynthesis and aerenchyma development,indicating that photosynthesis and aerenchyma development of plants under nitrogen stress had different response mechanisms,which in turn affected the actual oxygen production capacity and oxygen Differences in transport capacity occur,ultimately altering the ROL rate.（5）Nitrogen stress alters plant ROL rates by altering plant physiological responses,which in turn modulates rhizosphere microbial community structure and activity by changing DO concentrations.Thauera and Paracoccus are the dominant genera in rhizosphere microorganisms,which belong to Proteobacteria.Under high nitrogen stress,the abundance of Thauera and Paracoccus in the rhizosphere microbes of Iris and Spathiphyllum was higher,while the abundance of Thauera in the rhizosphere microbes of C.indica was only 20.12%of the control group.Under high nitrogen stress,the increase of ROL rate and DO concentration of Iris and Spathiphyllum resulted in the largest increase of ammonia monooxygenase（AMO）and periplasmic nitrate reductase activity（Nap）by 23.64%and 18.52%,and nitrate reductase（Nar）The greatest decrease in activity was 33.93%;while the decrease of ROL rate and DO concentration in C.indica resulted in the greatest decrease of AMO and Nap activities by 75.05%and 40.82%,and the greatest increase of Nar by 21.89%;the oxygen secretion rate of plants under nitrogen deficiency stress was lower and the lack of substrates As a result,the activities of dominant bacteria and key enzymes were reduced.（6）Under high nitrogen stress,the higher ROL rate of Iris and Spathiphyllum increased the DO concentration and microbial diversity in the wetland and promoted the removal of COD,N,and P.The highest removal rate of TN was 68.25%;under this condition,the ROL rate of C.indica was increased.lower,reducing the DO concentration and microbial diversity,inhibiting the removal of pollutants,and the lowest TN removal rate was only 18.49%.Under the condition of limited root growth space,plant roots were overgrown and the ROL rate increased,which enhanced the removal of pollutants in constructed wetlands.