Inducing Redundant Root Development Enhances Heavy Metal Removal Properties Of Wetland Plants

With the rapid development of industry and agriculture,a large amount of heavy metal elements including Pb and Zn have been discharged,causing serious damage to the water environment.As a restoration technology that simulates natural wetlands,constructed wetlands are often used to treat heavy metal-polluted wastewater.Compared with conventional deep constructed wetlands,inducing redundant root growth greatly increases root biomass,changes the wetland rhizosphere environment and microbial community structure,and theoretically has the potential to significantly improve the removal efficiency of constructed wetlands for heavy metals.The characteristics of heavy metal pollution removal have not yet been clarified.The paper takes three kinds of wetland plants（canna,calamus,and rush）as the research object,and explores the low concentration（L）and high concentration（H）Pb,Zn heavy metal pollution,0.1 m（shallow bed）and 0.6 m（conventional depth）The removal characteristics of heavy metals in constructed wetlands and the stress characteristics of plants and rhizosphere microorganisms,clarified the mechanism of induced redundant growth of roots on the removal of heavy metals in constructed wetlands,the results showed that:（1）Under conditions of induced redundant root development,the plant root system will be stressed and form a root mat.The increase of DO concentration,Eh and microbial diversity will strengthen the removal of Pb and Zn in the constructed wetland,and the removal rate is 98.67%and 88.49%.At the same time,the effluent concentrations of COD,NH₄⁺-N,TN,TP were also lower than those in conventional deep constructed wetlands.（2）With the increase of Pb and Zn pollution concentrations,the malondialdehyde（MDA）content in canna and calamus leaves increased significantly,but the MDA content in rush leaves decreased under high concentrations of Pb and Zn pollution,this is because the activity of total antioxidant capacity（T-AOC）of rushes was stimulated under the pollution of Pb and Zn,and the activity of T-AOC of canna and calamus had poor adaptability.Inducing redundant root development resulted in a maximum increase of58.13%in MDA content and 56.12%in T-AOC content in canna and calamus.The results showed that inducing redundant root development significantly increased the level of membrane lipid peroxidation,but plants were also able to respond to stress and damage from the external environment by enhancing their stress resistance.（3）Inducing redundant root development will increase root biomass,which is significantly better than conventional wetland in terms of root number and root dry weight.Low concentrations of Pb and Zn promoted the development of root aerenchyma of canna,calamus and rush grass,and the root activity was 1.40 times that of the control group at most.High concentrations of Pb and Zn inhibited plant root biomass.（4）Inducing the redundant development of roots can enhance the stress resistance of plants and the development of root aerenchyma.The ROL rate of plants in shallow constructed wetlands was higher than that in conventional constructed wetlands.Low concentrations of Pb and Zn promoted the ROL rate of canna,calamus and rush,which increased by up to 12.37%compared with the control group.The photosynthesis of plants under high concentrations of Pb and Zn pollution was inhibited,resulting in the ROL rate of the three plants being significantly lower than that of the control group.（5）The induced redundant development of roots significantly increased the iron plaque（IP）content of canna,calamus,and rushes.The IP content of the three plants in the shallow bed constructed wetland was higher than that in the conventional deep constructed wetland,which was attributed to the induced root system Redundant development enhanced three plant ROL rates.The induced redundant development of roots significantly enhanced the ability of plants to absorb and fix heavy metals,and the enrichment ability of Pb and Zn in shallow constructed wetlands was better than that in conventional deep constructed wetlands.Pb and Zn were mainly distributed in the cell walls and soluble fractions of aboveground and underground tissues of canna,calamus and rush grass.Low concentrations of Pb and Zn pollution are conducive to the transfer of metals to the interior of the cell,and the plant absorption is more obvious.The accumulation ratio of Pb and Zn in the cell wall increases under high concentration pollution.（6）Inducing redundant root development significantly increased the abundance and diversity of rhizosphere microorganisms by improving the wetland microenvironment.The rhizospheres of the three plants were enriched with Nakamurella,Saccharimonadales,Chloroplast,Mycobacterium,Microbacterium,Azohydromonas and other genera,among which Nakamurella was the dominant fungal genus,and the abundance of Nakamurella in the shallow bed constructed wetland was higher than that in the conventional deep artificial wetland.wetlands.The activities of alkaline phosphatase（AKP）,urease（Urease）,and dehydrogenase（DHO）peaked in the middle of wetland system operation,with the largest increases of 38.21%,50.45%,and 26.18%,and the induced redundant development of roots significantly increased the activities of AKP,Urease and DHO in the rhizosphere of three plants.When the concentrations of Pb and Zn increased,the ROL rate of plants decreased significantly,and the lack of substrates also led to a significant weakening of dominant bacterial species and enzyme activities.