Aging Process And Microbial Adaptability Of Different Heavy Metal Contaminated Soils

Currently,heavy metal pollution is one of the most serious environmental problems.It not only affects the normal ecological function of soil,but also enriches into human body through the food chain and endangers human health.Therefore,it is urgent to investigate the pollution characteristics of heavy metals to develop suitable heavy metal remediation technologies.Soil enzyme activity,as a sensitive indicator for evaluating soil quality,can be used to monitor the changes in soil ecosystem structure after heavy metal contamination.Microorganisms,due to their prevalence and sensitivity to environmental changes,can be a useful indicator for monitoring environmental health.Thus,in this study,Shanxi cinnamon soil and Liaoning brown earth were selected as the research objects,and three different levels of cadmium（Cd）,mercury（Hg）,arsenic（As）,lead（Pb）,and chromium（Cr）were set according to the standard,and a 12-month simulation experiment was conducted.We studied the available content of heavy metals and determined the soil enzyme activity periodically,meanwhile,we explored the characteristics of microbial community structure with the type,concentration of heavy metal and incubation time.The negative cohesion index（|Neg|）and topological parameters of co-occurrence network were calculated to obtain the trend of microbial community stability with time,at the same time,the common and specific microorganisms were found in the stable communities.It may provide theoretical guidance for heavy metal contaminated soil remediation by microbes.The major results are as follows:1.The available contents of different heavy metals in Shanxi cinnamon soil and Liaoning brown earth showed a decreasing trend with incubation time,decreasing rapidly in the early incubation period（Month 0-6）and stabilizing in the latter incubation period（Month 6-12）.The kinetic process of heavy metal available content over time was well fitted by parabolic diffusion equation,which showed the aging rate of As was the fastest compared to other heavy metals under severe pollution conditions,with the aging rate constants（|K_p|）of 21.03%^-1d^-0.5 in Shanxi cinnamon soil and 24.98%^-1d^-0.5in Liaoning brown earth,respectively.By calculating the time for heavy metals to reach balance,the average time in the Liaoning brown earth was shorter than Shanxi cinnamon soil（Liaoning brown earth:5.16 months;Shanxi cinnamon soil:6.57months）.The multiple stepwise regression analysis indicated that soil p H was the main factor affecting the time required to reach the balance of heavy metals.2.Under heavy metal contamination,the enzyme activities in both soils varied considerably from Month 0 to 9 and stabilized from Month 9 to 12.Hg showed higher inhibition of urease,sucrase in Liaoning brown earth.At the end of incubation（Month12）,the inhibition rates of these two enzymes were 80.0%and 90.0%,83.4%and 86.0%under mild and severe contamination,respectively.Cr showed higher inhibition of urease,acid and alkaline phosphatase in Liaoning brown earth.At the end of incubation（Month 12）,the relative activities of urease were 2.2%and 3.0%of the control;the relative activities of acid phosphatase were 21.9%and 16.0%of the control;and the activities of alkaline phosphatase were 33.2%and 19.1%of the control under two contaminated levels,respectively.The enzyme stability index（FS）of the two soils under heavy metal in the Month 12 were calculated,and it was found that the mean values of the enzyme stability index in Shanxi cinnamon soil were greater than Liaoning brown earth（under mild contamination,Shanxi cinnamon soil:0.793,Liaoning brown earth:0.394;under severe contamination,Shanxi cinnamon soil:0.694,Liaoning brown earth:0.253）.Soil p H,organic matter（SOM）,total nitrogen（TN）,available phosphorus（AP）and available potassium（AK）were the main factors that affected enzyme functional stability.3.The dominant phyla in both soils under different levels of heavy metal contamination were Proteobacteria,Acidobacteriota,Actinobacteriota,and Firmicutes.Proteobacteria is the most abundant phylum in heavy metal contaminated soil,Verrucomicrobiota and Planctomycetota are the specific dominant phylum in Shanxi cinnamon soil;Cyanobacteria is the specific dominant phylum in Liaoning brown earth.The topological parameters of co-occurrence network（clustering index,mean path length,network diameter）and negative cohesion index were used to predict the stability of the microbial community.The clustering index decreased,the mean path length and network diameter increased,and the absolute value of the negative cohesion index increased with incubation time,indicating that the microbial community was gradually stabilized.Structural equation model（SEM）was used to establish the relationship between soil physicochemical properties,heavy metal pollution concentration and microbial community structure and enzyme function stability,and it was obtained that heavy metal pollution had a greater influence on enzyme function stability（p<0.001）and soil properties had a greater influence on community stability（p<0.001）.4.Microorganisms in both soils have different adaptations under different heavy metal contamination.Common microbial genus in Shanxi cinnamon soil is Sphingomonas.There were some specific microorganisms under certain heavy metal contamination,for example,Aeromicrobium under mild Cd contamination,Bacillus,Nocardioides,Actinomadura,Kribbella under mild Cr contamination,Pseudomonas under severe Hg contamination,Escherichia-Shigella,Glycomyces,Actinomadura,Pseudonocardia under severe Cr contamination.The common microorganism existed in contaminated Liaoning brown earth was Chujaibacter.and there are also some specific microorganisms for certain heavy metal contamination,such as Limnochordaceae for Cd mild contamination,Nitrolanceah and Candidatus＿Yanofskybacteria for Hg mild contamination,Bacteroides and Faecalibacterium for As mild contamination,Sphingomonas,Acinetobacter and Pseudarthrobacter for Pb mild contamination.Acidibacter for Cd severe contamination,Rhodanobacter、Roseisolibacter、Nitrolancea for As severe contamination,Limnochordaceae for Pb severe contamination.The results of the random forest model indicated that total nitrogen（TN）,available potassium（AK）,p H,common microorganisms（Common）and soil organic matter（SOM）were essential factors in maintaining the stability of the microbial community.