Atrazine Degradation Characteristics,Degradation Genes Distribution And Bacterial Community Succession In Different Soils

Atrazine is commonly used in maize after sowing and before emergence,which is one of the most widely used herbicides in China.Due to the relatively high persistence,environmental toxicity,and endocrine disturbance of atrazine,its extensive use not only leads to the long-term residual pollution in agricultural soil,but also poses a potential threat to the ecological environment and human health.Therefore,it is of great scientific and practical significance to study the degradation characteristics and mechanism of atrazine in different soil environments for bioremediation of contaminated soil.In this thesis,the typical s-triazine herbicide atrazine was used as the target compound,and 10 representative soils from different regions were selected to construct indoor simulated soil ecosystem.We studied the degradation characteristics of atrazine in different types of soils,the diversity and abundance of atrazine degradation genes by qPCR technology,and the succession of microbial community in the atrazine-contaminated soils by 16 S rDNA amplicon high-throughput sequencing.Meanwhile,combined with the isolation and identification of the atrazine-degrading bacterial strains,the degradation mechanism of atrazine in different types of soil was revealed.The results will provide a scientific basis for bioremediation of the atrazinecontaminated soils.The main results are summarized as follows:(1)The degradation of 10 mg/kg of atrazine in different types of soils showed a degradation trend of initially fast and then slow.The degradation dynamics of atrazine in all treatments followed the 1st-order kinetics equation,and the degradation half-life followed an order: DXAL-C soil > BT-L/AKS-S soil > BJ-L soil > JH-L soil > SQ-L soil > ZY-L soil > DD-L soil > BD-L soil > BZ-L soil.Compared with the unsterilized soil,the degradation half-life of atrazine in the sterilized soil increased by 3.68-71.40-fold,indicating that soil microorganisms played a major role for atrazine degradation in different soils.In addition,the higher the soil organic matter(OM)content,total nitrogen(TN)content and cation exchange capacity(CEC),the longer the degradation half-life of atrazine.(2)Atrazine treatment significantly increased the absolute abundance of atz degradation genes in the soil.The increase in the absolute abundance of atzC gene was the largest in 10 types of soils,while that of atzA gene was the smallest;the trzD gene was only detected in the BZ-L soil,indicating that the subsequent degradation process of atrazine from cyanuric acid to biuret occurred after the initial degradation of atrazine.The co-occurrence network analysis between soil bacterial genera and atrazine degradation genes showed that the number of soil bacterial genera carrying atzC gene was the most,and that carrying the trzD gene was the least.Meanwhile,the number of bacterial genera carrying one atrazine degradation gene in the network was significantly higher than that carrying four atrazine degradation genes.In addition,soil organic matter(OM)content,available phosphorus(AP)content,total nitrogen(TN)content and hydrolyzable nitrogen(HN)content were significantly negatively correlated with atrazine degradation genes.(3)Atrazine treatment could alter the composition of soil microbial community structure.With the extension of atrazine treatment time,soil bacterial diversity(Observed OTUs and Shannon index)displayed a suppression-stimulation trend,indicating that atrazine treatment could reduce the soil microbial diversity at the initial treatment stage,and then gradually recovered to the original level.Principal component analysis showed that the microbial community structure in the atrazine-treated soils was significantly different from that in control soils.Atrazine treatment increased the relative abundance of Ochrobactrum,Nocardiopsis,Lactobacillus and Brevibacterium,especially the relative abundance of Lactobacillus significantly increased by 3.38%-12.11% compared with the control soil,while the relative abundance of Conexibacter,Solirubrobacter and Micromonospora decreased significantly.In addition,the number of nodes in the ATZ-30 soil accounted for only 4.6%,but the positive correlation was100% in the co-occurrence network of soil bacterial genera.Meanwhile,the topological characteristics of soil bacterial co-occurrence network showed that the modularity index in ATZ-30 soil was higher than that in CK-30 soil,indicating that soil microorganisms may have a synergistic effect on the degradation of atrazine.(4)Four atrazine-degrading bacterial strains AT1,AT2,AT3 and AT4 were isolated from the atrazine-treated soils,and identified as bacterial genera Rhizobium,Stenotrophomonas,Brevibacterium and Bacillus,respectively by 16 S rDNA sequences.The degradation rate of 10 mg/L atrazine by these 4 isolates was 17.56%-30.55% within14 d after inoculation.Meanwhile,the high-throughput sequencing analysis of 16 S rDNA amplicon showed that the relative abundance of these four isolates in the atrazine-treated soil was significantly higher than that in the control soil after treatment for 30 d,indicating that these four potential degradation bacterial genera were involved in the degradation of atrazine in the soil.