| As agricultural waste,unreasonable disposal of walnut green husk will cause serious pollution to the water and soil environment.Meanwhile walnut green husk contains a large number of active functional groups,which may affect the environmental behavior of heavy metals in the soil after being applied to the soil.Deeply digging the value of walnut green husk in the field of heavy metal pollution control is significant in theoretical and practical for improving the rural environment and promoting the development of heavy metal repair technology.This research focuses on the changes in the soil-plant-microbial system closely related to phytoremediation after the application of the heavy metal activator WGH(Walnut Green Husk)developed from walnut green husk,as well as the distribution and morphological changes of heavy metals during the whole process involving a series of experiments.This research is divided into three parts as a whole: First,the activation ability of WGH on heavy metals in soils is verified through single factor experiment and response surface methodology(RSM).And preliminary exploration of its mechanism of action is made through analysis of the form of heavy metals and fourier-transform infrared spectroscopy(FTIR)analysis of WGH.At the same time,using Cd and Zn hyperaccumulating plant with Sedum plumbizincicola as the test material,comprehensive analysis of soil rhizosphere/bulk heavy metal forms and contents,changes in soil basic properties,changes in plant physiological indicators,changes in plant heavy metal content of root and stem and leaves after applying WGH to explore the effect of WGH on the enhanced absorption of Cd and Zn by sedum from different perspectives.Finally,16 S r RNA gene amplicon sequencing and other techniques were used to compare the microbiological differences of rhizosphere/non-rhizosphere soil microorganisms and plant endophytes under different treatments,and explore the potential effects of changes in the microbial community after the application of WGH on the extraction of soil heavy metals by plants.The main results of the experiment are summarized as follows:1.Through single factor experiment and RSM,the experimental conditions(p H,concentration and extraction time)for WGH extraction of soil heavy metals were screened and optimized,in order to achieve the best heavy metal extraction effect.Results showed that the heavy metal leaching agent of WGH has a certain ability to extract Cd,Pb and Zn in the soil in this study.RSM optimizing experimental conditions made the highest extraction efficiency reach 39.71%,15.80% and 15.46% for Cd,Pb,and Zn,respectively.Tessier sequential extraction results showed that the main reduction phase in soil for the three heavy metals(Cd,Pb,and Zn)after leaching is the iron and manganese oxidation phase,which reduced by 90.89% and 49.33% and 40.39% respectively under the treatment of 0.1g/m L WGH.FTIR analysis showed that the functional groups in the water-soluble extraction of WGH included hydroxyl(-OH),carbonyl(-C=O)and carbon-nitrogen double bonds(-C=N)etc.,which may have a chelating effect to remove heavy metals in the soil.2.The pot experiment was divided into two groups: direct application of walnut green husk powder(WGH powder)and application of walnut green husk water extract(WGH water extract),each with 3 concentration gradients.Results showed that high uptake efficiencies of Cd and Zn by Sedum plumbizincicola appeared at the 0.03 g/m L and 0.05 g/m L WGH water extract treatments,respectively,achieving 171.79% and 110.68% compared with those of the CK treatment.WGH powder/WGH water extract significantly increased the contents of Tessier exchangeable heavy metals in soil and improved the soil fertility.Meanwhile,the biomass of Sedum plumbizincicola root,stem and leaf at the 0.03 g/m L WGH water extract treatment were 182.59%,117.97%,and 141.08% to which of the CK treatment,and SOD activity was 251.39% to which of the CK treatment,respectively,but no significant change in chlorophyll content.3.By analyzing the rhizosphere/bulk soil microorganisms and plant endophytes in the pot experiment found that the addition of WGH significantly enriched the microorganisms of Proteobacteria and Actinobacteria.The results of beta diversity analysis showed that many environmental factors,such as p H,SOM,CEC,etc.,are all related to the community structure of microorganisms.Through LEfse analysis,it was found that Flavobacterium of phylum the Pseudomonas and Pseudomonas of the phylum Bacteroides were specifically enriched in the treatment of applying WGH water extract.Through LEfse analysis,it was found that Pseudomonas of Proteobacteria and Flavobacterium of Bacteroides were specifically enriched in the treatment of WGH water extract.It is worth noting that both of these two bacteria can produce IAA(indole-3-acetic acid)and siderophore,which further demonstrates that the addition of WGH water extract is associated with the biomass of Sedum plumbizincicola significantly.The analysis results of plant endophytes showed that WGH can significantly affect its community structure and promote the enrichment of some bacterial communities such as Streptomyces and Bradyrhizobium in the treatment of WGH water extract.After analyzing the metabolic processes of these microorganisms,they are all conducive to plant growth or the absorption of heavy metals.In general,the addition of walnut green husk affects the environmental behavior of heavy metals in the soil-plant-microbe system,thereby promoting the accumulation of heavy metals by Sedum plumbizincicola,achieving the goal of enhancing the efficiency of phytoextraction.Meanwhile a comprehensive analysis of the relationship among WGH-soil-plants-heavy metals-microbes provides new ideas and data support for recycling walnut green husk and strengthening measures for phytoextraction.It also provides new ideas for resource utilization of walnut green husks,prevention of wastewater pollution of walnut husks,as well as enhancement of phytoextraction. |
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