Effect Of Influent Salinity On The Selection And Application Of Macrophyte Species In Floating Constructed Wetlands

As one of the important processes of sewage treatment,constructed wetland technology has more and more extensive applications in urban wastewater treatment,industrial tail water treatment,sewage treatment in agricultural aquaculture,etc.As one of the important components of constructed wetlands,macrophytes play important ecological roles.Aquatic plants in constructed wetlands often suffer from stresses such as excessive pollution and high salinity,causing their poor growth and affecting their functions.Therefore,the selection of aquatic plants is an important task for constructed wetlands.A large number of studies have been carried out in related fields,and the comparison results of different studies on aquatic plants are quite different,even opposite,leading to a narrow range of application of these results For this problem,we propose a hypothesis:the difference in influent water conditions is the main reason for the different comparison results of different researches.In order to verify this hypothesis,five macrophyte species floating constructed wetland（FCW）tests were carried out under different influent salinity conditions,and we analyzed the dynamic changes of growth status,purification function,and characteristics of rhizosphere microbial community.To achieve the following goals:（1）quantifying the effect of salinity on pollutant removal efficiency（RE）of macrophyte species in FCWs,（2）Evaluating the growth status of macrophytes by multiple morphological and physiological parameters under varying salinities,（3）Comparing the differences in the rhizosphere bacterial community and diversity between macrophyte species under varying salinities,and（4）Identifying the relationship between macrophyte characteristics and pollutant REs as well as the dominant indicative characteristics of macrophytes in determining purification function.In this study,five macrophyte species were selected:Oenanthe javanica（Oe）,Iris seudacorus（Ir）,Acorus calamus（Ac）,Thalia Dealbata（Th）and Typha angustifolia（Ty）to construct floating constructed wetland systems.The results of the test under the different influent salinity conditions show that:（1）With the increase of salinity,the removal rate of pollutants in high salt wastewater generally decreases,and some macrophyte species,such as Iris seudacorus,can effectively resist this decline.At low salinity,the removal rate of chemical oxygen demand（COD）of five kinds of plants ranges from 34.2%to 45.6%,and the removal rate of COD drops to-44.0-24.9%at high salinity.The average removal rate of other plants except Iris seudacorus（Ir）is-27.5%;the nitrate nitrogen（NO₃^--N）removal rate is consistent with the change of the chemical oxygen demand（COD）removal rate,the removal rate range is 68.8-98.1%at low salinity,and the removal rate is reduced to 16.7-73.6%at high salinity,the average removal rate of nitrate nitrogen of macrophyte species other than Iris seudacorus（Ir）is only 23.0%.The average coefficient of variation of interspecific chemical oxygen demand（COD）,ammonia nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）and total phosphorus（TP）increased from 28.6%at low salinity to high salinity 91.3%of the total,indicating the importance of selection of macrophyte species under high salinity conditions.（2）The selection of macrophyte species based on morphological and physiological parameters varies greatly due to salinity and parameters,indicating that the research results under a certain condition may have a relatively narrow scope of application.At low salinity,there are relatively few parameters related to pollution removal(such as root catalase（CAT）,the number of bacteria observed(S_obs)),and with the increase of influent salinity,more macrophytes parameters（biomass（TB,UB）,root activity（RA）,lignin content（Lig）,etc.）have a significant correlation with pollutant removal rate,indicating that the importance of macrophytes parameter selection in representing pollutant removal rate is reduced.Therefore,influent salinity is a key factor that needs to be considered in the selection and application of macrophytes,especially under soilless conditions.（3）Rhizosphere microorganisms play an important role in the purification of water quality by floating constructed wetland plants and the response to environmental changes.The increase in influent salinity will significantly reduce the abundance and diversity of microorganisms.At low salinity,the rhizosphere Shannon diversity index（SHDI）is 5.0-6.0 at the genus level.As the salinity increases,it drops to a minimum of 3.0-3.8,and then rises slightly to 4.3-4.9.The number of bacterial species observed(S_obs)have similar results.The increase of influent salinity will cause the rhizosphere bacterial community to show convergent or convergent evolution.The evolution of different aquatic plant species is slightly different,and the ability of the rhizosphere bacterial community to maintain stability varies significantly with the species of macrophytes.The Ir species showed the highest degree of stability(SI_salinity),with an average of 82.5%.Oe and Ty species had the lowest stability in the rhizosphere bacterial community under different salinities,with an average of 68.8%and 69.0%,respectively.（4）Rising salinity will change the structure of the microbial community.At low salinity,the rhizosphere microbial community has a complex composition,The number of OTUs of the five plants is 5238,and the proportion of each colony is relatively uniform.As the salinity increases,the number of OTUs dropped to 3832,and the microbial community structure was simplified.At high salinity,salt-tolerant colonies（such as Pseudomonas）occupy a dominant position,and the relative abundance range of the five plant species rose from 4.2-15.0%at the low salinity to high salinity 12.4-34.8%.There are certain similarities or differences in the composition of rhizosphere microorganisms between species.This characteristic will be affected by species and salinity,and species is the main determinant.