AMF-phragmites Australis Symbiosis For In-situ Remediation Of Heavy Metal Compound Pollution In River Sediment

In recent years,with the improvement of urbanization level in China,the problem of urban river pollution has become increasingly prominent.Although river water treatment technology has become increasingly mature,sediment pollution is difficult to cure,resulting in frequent water pollution accidents.The treatment and restoration of sediment has become a key issue for river treatment.The traditional methods of dredging and ex-situ treatment not only have a huge amount of engineering,but also easily cause secondary pollution.We need to develop an economical and environmentally friendly in-situ ecological restoration technology to address the practical needs of urban river sediment pollution which is difficult to control,in line with the road map of ecological civilization construction and green development proposed in the report of the 19th National Congress of the CPC.This study is aimed at the serious pollution of heavy metals in river sediments in cities.From the perspective of environmental protection and economy,the method of improving phytoremediation by constructing a phytomycorrhizal fungi-phragmites symbiosis system(AMF-PS)was used to compensate for the low adaptability and small adsorption capacity of traditional phytoremediation.Exploring the response of different symbiotic systems to composite polluted sediments and the optimal AMF-PS tolerance to heavy metals at different concentrations was focused on.The best symbiotic system was selected with the symbiosis infection rate and growth and development parameters of phragmites as morphological indicators and the chlorophyll fluorescence and content of heavy metals in the phragmites as physicochemical indicators.The best tolerance concentration of CK and Ri-PS was investigated by morphological,physicochemical indicators and the changes of heavy metals effective state in the tolerated process;Combined with the nutritional indicators of the sediment and phragmites,the response of the root system to different concentrations of heavy metal tolerance,the redistribution of heavy metals in the plant and the microbial community in the plant rhizosphere sediment,a comprehensive analysis of the mechanism of Ri-PS to increase the tolerance of phragmites to heavy metal was studied.The main conclusions are as follows:(1)Three types of AMF-PS(Fmo-PS,Ri-PS and As-PS)were successfully respectively constructed with Fmoses(Fmo),Rhizogenes(Ri)and Asphaeroides(As).The infection rates of the three groups of AMF-PS were similar.And their plant height and root length were slightly heigher than the CK group.(2)The 31-day tolerance screening experiment of Fmo-PS,Ri-PS and As-PS in actual heavy metal compound contaminated sediment showed that Ri-PS had the best performance in promoting phragmites growth and reducing heavy metal content in the sediment.It also showed that the anaerobic environment of sediment did not reduce the infection rate both of three AMF-PS groups during the experiment.The growth of phragmites was promoted in AMF-PS groups.The promotion in Ri-PS group was the most obvious,and its the stem length was always at the leading level during the 31-day experiment.The stem,leaf,root weight and biomass of the Ri-PS and As-PS groups were significantly higher than those of the CK group,while those of the CK group were higher than those of the Fmo-PS.The Ri-PS group could alleviate the photoinhibition effect of phragmites.the F_m(maximum fluorescence value)of the Ri-PS was 0.256,and meanwhile that of the CK group was only 0.199.The effective remove of six heavy metals mainly by increasing the biomass achieves the purpose of removal was observed in the Ri-PS group.Through comparison,the Ri-PS group performed best during the tolerance screening experiment.And it was decided to choose the Ri-PS group as the experimental group for subsequent experiments.(3)A 78-day tolerance experiment of the Ri-PS group in different concentrations of heavy metal sediments was conducted and obtained the removal effects of the six different concentrations of heavy metals.The study found that the infection rate of each experimental group of Ri-PS remained above 9%and in the second stage of the tolerance cycle,the growth rate of phragmites stem length was the fastest.F_m and F_v/F_mboth of Ri-PS and CK groups decreased significantly with the increase of heavy metal concentration.Meanwhile the F_m of Ri-PS group was higher than that of CK group at the same concentration.There was a significant decrease of F_m at?-20 concentration in the Ri-PS group.However,the similar decrease of F_m occurred at a concentration of?-10 in the CK group.The total absorbed amount of the six heavy metals in the Ri-PS group was higher than that in the CK group at most concentrations.The phragmites could actively control the absorption of Cd and control the absorption of Cr and Pb to a certain extent.The effective concentration of Cd,Cu,Pb,and Zn decreased during tolerance experiment,but there was no significant difference between the Ri-PS group and the CK group.After the tolerance experiment,the concentrations of heavy metals in the sediments of the two experimental groups decreased,and most of the Ri-PS groups decreased more than the CK group.The Ri-PS group showed higher removal rates of?-5 and?-20 concentrations,which were 44.43%and 35.02%,respectively.Comprehensive consideration is suggested to use phragmites alone as a remediation plant,when concentration of the compound contaminated sediment is suitable for III-5to IV-10.If the sediment concentration is higher than IV-10,Ri-PS remediation is recommended and the best concentration is between?-10 and?-15.(4)The mechanism of increases the tolerance of plants to heavy metal contaminated sediments in Ri-PS group was explored.Compared with the CK group,the Ri-PS group could significantly promote the phragmites to absorb N and P,meanwhile reduced the N and P content in the sediment.In the Ri-PS group,the production of antioxidant enzymes such as SOD,POD and CAT reduced.And the generation of reactive oxygen species reduced in the phragmites body,and the tolerance concentration of phragmites to heavy metal stress increased.At the same time,Ri-PS could significantly increase the phragmites rhizosphere S-UE activity,enhance s DHA activity,slightly increase S-ACP activity,and help accelerate the decomposition rate of urea and phosphate in the sediment environment.At low concentrations,Ri-PS could promote the transfer of Cd,Cr,Ni and Pb to the leaves,but at high concentrations,it reduced the transfer of heavy metals to the leaves.But the ability to regulate heavy metals in the stem was not strong.There were significant differences in microbial populations at different concentrations,but there were no significant differences in microbial communities in different experimental groups of the same concentration,indicating that the concentration of heavy metals was the main reason for the change in microbial community.The change trend of Simpson and Shannon index indicated that in the range of phragmites tolerant heavy metals,the bacterial agent reduced the microbial community diversity,but when the concentration exceeds the heavy metal tolerance range,the bacterial agent increased microbial diversity.