| Soil lead pollution has become a long-term and severe environmental problem.The damage of soil microecology due to heavy metal stress is one of the main causes of soil degradation.Plant growth promoting rhizobacteria(PGPR)in coordination with plant remediation of heavy metal contaminated soil has attracted more and more attention as a green bioremediation method.However,different growth-promoting bacteria have different growth-promoting effects on different plants and soil improvement.In this study,two growth-promoting bacteria of Pseudomonas sp.B1(Pseudomonas sp.)and D8(Pseudomonas montalli sp.)and two-year-old Melia azedarach Linn.seedlings were selected as the research objects.Pot experiment was carried out with three levels of lead contamination(0mg·kg-1,100mg·kg-1 and 500mg·kg-1)and three inoculation treatments(uninoculated,B1 inoculated and D8 inoculated).0,0-B,0-D,100 Pb,100 Pb-B,100 Pb-D,500 Pb,500 Pb-B,500 Pb-D.To study the effects of inoculated growth-promoting bacteria on the growth and enrichment characteristics of Melia azedarach seedlings,rhizosphere soil properties of Melia azedarach,and the response of microbial community structure and diversity to different concentrations of lead pollution.The main results are as follows:1.In the untreated group,the plant height and ground diameter RGR of Melia azedarach seedlings decreased with the increase of lead concentration in the soil,and the growth of Melia azedarach seedlings was significantly inhibited at the level of 500 mg·kg-1.At 100 mg·kg-1 level,compared with the uninoculated group,after inoculation with B1 or D8,the plant height and ground diameter increased slightly but not significantly.At 500 mg·kg-1 level,the plant height and ground diameter RGR of B1 and D8 were increased significantly.It is speculated that inoculation of rhizosphere growth-promoting bacteria can alleviate lead toxicity and promote plant growth under high concentration of lead pollution.2.The enrichment coefficient and transport coefficient of Melia azedarach did not change significantly with the increase of lead concentration in soil.The enrichment and transport coefficients of inoculated B1 and D8 were significantly increased at 100 mg·kg-1 and 500 mg·kg-1 levels,and the transport coefficients of inoculated B1 were significantly higher than those of inoculated D8 at 100 mg·kg-1 and 500 mg·kg-1 levels.3.In the untreated treatment,with the increase of soil lead concentration,pH,TOC,TN,TP and AP had no significant change,while urease activity significantly decreased,invertase activity significantly increased.At 100 mg·kg-1 level,the soil chemical properties of B1 and D8 were not significantly changed,but the activities of sucrase and acid phosphatase were significantly increased.At 500 mg·kg-1 level,soil pH of B1 and D8 inoculation decreased significantly,while AP increased significantly.The activities of sucrase,catalase and acid phosphatase increased significantly.Principal component analysis showed that lead pollution was the main factor causing soil property differentiation,while inoculation of growth-promoting bacteria was the secondary factor.4.The α-diversity of soil bacterial community was not significantly changed by lead contamination and inoculation.The results of non-metric multidimensional scale analysis showed that the effect of lead pollution on bacterial community was greater than that of inoculation,and the effects of inoculation B1 and D8 on bacterial community were also significantly different.Bacteroidetes,Firmicutes,Proteobacteria and Actinobacteria were the dominant bacteria at phylum level in the lead contaminated and inoculated group;At class level,Bacteroidia,Clostridia,Gammaproteobacteria,Actinobacteria,Alphaproteobacteria and Bacilli are dominant.Compared with the uninoculated group,The abundance of Bacteroidia,Clostridia and Bacilli increased in B1 inoculated significantly group at 100 mg·kg-1 level;The abundance of Bacteroidia,Actinobacteria and Bacilli increased in D8 treatment group.The abundance of Actinobacteria and Bacilli in B1 and D8 groups increased significantly at 500 mg·kg-1 level.5.Symbiotic network analysis shows that,inoculation with B1 mainly increased the number of key bacteria in Proteobacteria,inoculation with D8 increased the number of key bacteria in Firmicutes,Proteobacteria and Actinobacteria.Through the interaction between the key bacteria and the dominant species in the bacterial community,a different microbial network structure was formed in the soil from the uninoculated group.The networks of inoculations were simpler than those of the uninoculated group.Moreover,the network structure of inoculation B1 and D8 also showed significant differences.Compared with the D8 group,the negative connection ratio of the network formed by inoculation B1 was larger,so the bacterial network formed by inoculation B1 was more stable than that formed by inoculation D8.6.Redundancy analysis showed that in group B1 urease,TN and sucrase were the key environmental factors affecting bacterial community structure.In D8 group,TN,TP,AP and sucrase were the key environmental factors affecting bacterial community structure.VPA showed that soil enzymes had the greatest influence on bacterial community structure in B1 group,and soil physical and chemical properties had the greatest influence on bacterial community structure in D8 group.In conclusion,inoculation of rhizosphere growth-promoting bacteria changed the network structure and composition structure of the rhizosphere soil microbial community by influencing the number and distribution of key bacteria in the soil,improving the physical and chemical properties of soil and promoting soil enzyme activity.This change resulted in the increase of beneficial bacteria,which further improved the soil microecological environment with lead pollution,and thus promoted the growth of Melia azedarach with lead pollution and its ability to enrich and transport lead.The results of this study revealed the mechanism of growth promoting bacteria in the synergistic remediation of heavy metal contaminated soil by Melia azedarach,and provided a theoretical basis for the microbial-plant synergistic remediation of heavy metal contaminated soil. |
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