| At present,soil pollution in our country is becoming more and more serious,especially the problem of heavy metal pollution in farmland soil is becoming more and more prominent,which seriously threatens food safety and people’s health and safety.Among many remediation methods,phytoremediation technology is widely used in soil heavy metal pollution remediation due to its green environmental protection,environmental friendliness,low cost,not easy to produce secondary pollution,and the ability to achieve large-scale in situ remediation and beautify the environment.However,the plant biomass used for remediation is small,the remediation cycle is long,and the limited tolerance to heavy metals restricts the development of this technology.In addition,the use of microorganism-assisted enrichment for phytoremediation is the focus of research in this field.In this study,the typical farmland brick red soil in Hainan was used as the research object,and six species of plants——Pennisetum sinese Roxb,Leersia hexandra Swartz.Brassica juncea,Ricinus communis L.,Manihot esculenta Crantz,Helianthus annuus L.and a heavy metal tolerant strain——Piriformospora indica were used as research materials.The root bag test was performed using inductively coupled plasma optical emission spectroscopy（ICP-OES）,inductively coupled plasma mass spectrometry（ICP-MS）,high-throughput sequencing,and gas chromatography-mass spectrometry（GC-MS）,synchrotron radiation X-ray fluorescence（XRF）,scanning electron microscopy（SEM）,etc.,the remediation effect of enriched plants on low-and medium-concentration Cd-Cr composite heavy metal polluted soil in Hainan was studied P.indica assists dominant plants to improve the remediation efficiency of Cd-Cr,and finds the dominant combination;explores the microdistribution characteristics of heavy metals in plants and the physical and chemical properties of rhizosphere soil under the condition of plant-microbe joint remediation,heavy metal form and effective state,rhizosphere enzyme activity,root exudates and rhizosphere microorganisms and other regulatory remediation mechanisms,and carried out scientometric analysis in this field.The main results obtained in this study were as follows:（1）Scientometric analysis shows that the field of soil heavy metal phytoremediation has developed rapidly since 2009.The number of publications continues to increase,"Environmental Science and Pollution Research"has the largest number of publications,and the National Natural Science Foundation of China is the most funded institution;it involves multiple research fields,and environmental science and ecology are the main subjects of research in this field;There is extensive cooperation among countries,institutions,and authors.China,India,Pakistan,the United States and Spain are the main countries for research in this field,and Ali S has the largest number of published papers;through keyword clustering,it is found that compound polluted soil and heavy metals are effective Sex,lime soil and heavy metal pollution are the research hotspots in this field,and EDTA,plant rhizosphere-promoting bacteria,biochar,plant physiology,and phytoextraction are the research fronts in this field.Keyword frequency analysis showed that Cd,Pb,and Zn were the most concerned heavy metals,and researchers paid more attention to the tolerance,accumulation,and extraction capacity of plants to heavy metals,and the study of plant rhizosphere environment.（2）Phytoremediation experiments showed that plant height and biomass were significantly reduced after Cd-Cr composite pollution,stem diameter and SPAD value showed different changes,and soil properties showed different changes after heavy metal pollution.Comprehensive evaluation of different plants:（1）King grass has a large biomass,good accumulation and transport efficiency of Cd,and good removal efficiency of heavy metals in soil,becoming a screening plant;（2）Leersia hexandra Swartz.has a good accumulation of Cr,a good transport efficiency for Cd and Cr,and a high biomass to become a screening plant;（3）Brassica juncea has good accumulation of Cd,good enrichment efficiency of Cd and Cr,and good transport of Cr efficiency and good removal rate of Cd become the screening plant;（4）Ricinus communis L.has good accumulation and enrichment of Cd,high Cd and Cr transport efficiency and good removal rate to soil,and becomes the screening plant;（5）Manihot esculenta Crantz has a good accumulation of Cr,a good enrichment effect on Cd and Cr,and a good removal rate of Cr,but its transport efficiency to the shoot is not high,and the small biomass becomes plants to be selected;（6）Helianthus annuus L.has a good accumulation and removal rate of Cr,as well as a large biomass,but the accumulation and enrichment of Cd are not high,and the removal rate of Cd in the soil is also low,so it becomes a candidate plant.In general,the king grass,Leersia hexandra Swartz.Brassica juncea,Ricinus communis L.have become the dominant materials for the next research because of their good remediation effect on Cd-Cr composite pollution.（3）The combined plant-microbe restoration experiment showed that P.indica had strong tolerance to single Cd,Cr,and composite Cd-Cr,and could colonize the roots of king grass,Leersia hexandra Swartz.,Brassica juncea,Ricinus communis L.After inoculation with P.indica,it had a greater impact on plant agronomic traits and soil physicochemical properties;inoculation with P.indica significantly increased the accumulation of Cd in king grass and Leersia hexandra Swartz.but had little effect on Brassica juncea and Ricinus communis L.;Inoculation with P.indica the accumulation of Cr in king grass and Leersia hexandra Swartz.increased,followed by Brassica juncea,which had no effect on Ricinus communis L.It has the least effect;there is no obvious regularity on the transport coefficients of Cd and Cr in different plants;after inoculation with P.indica,the removal rate of Cd in the soil of king grass,Leersia hexandra Swartz.and Brassica juncea is significantly improved,and the effect on Ricinus communis L.is small;on king grass the removal rate of Cr in the soil of and Leersia hexandra Swartz has the best effect and has no effect on castor.In general,P.indica inoculation has a good remediation effect on king grass and Leersia hexandra Swartz and has a good promotion effect on plant agronomic traits and rhizosphere environment.Therefore,king grass-P.indica and Leersia hexandra Swartz.-P.indica became the dominant combination for subsequent experimental research.（4）The pot experiment study showed that inoculation of P.indica significantly improved the state of enzyme activity and the form of heavy metals in the soil of king grass and Leersia hexandra Swartz.Two-way ANOVA showed that biomass,p H,EC,Eh,CEC,Zeta potential,urease activity,catalase activity,residual state of Cd,reducible state of Cr,exchangeable state,residual state,Cr6+,plant Cd content in inoculated P.indica（A）and heavy metal concentration in soil（B）had a very significant（P<0.01）interaction,and had a significant interaction with SOM,A-Cd,and Cr in plants（P<0.05）Correlation analysis showed that the contents of Cd and Cr in plants were significantly negatively correlated with plant height and biomass（P<0.01）,and were significantly positively correlated with p H,Eh,invertase and acid phosphatase（P<0.01）.Therefore,soil physicochemical properties and soil enzyme activities have a great influence on the forms of heavy metals in soil.（5）The root bag test study showed that the vascular column is the main channel for Cd accumulation and transport in the root system of rhizoma chinensis.The epidermis,cortex and vascular column can enrich Cr,but the vascular column plays the main role,the inoculation of P.indica can significantly increase the vascular column transport channels and the enrichment range;epidermal cells are the main area of Cd enrichment in the stem of king grass,and all parts of the stem can enrich Cr,and after inoculation with P.indica,significantly the enrichment of Cd to the middle of the stem was increased,and the absorption of Cr in various parts of the stem was improved;epidermal cells were the main area of Cd enrichment in the leaves of the king grass,and Cr was mostly enriched in the lower epidermis of the leaves of the king grass under low-concentration pollution,with the increase of heavy metal concentration,enriched in the epidermis of leaves.After inoculation of P.indica,the enrichment content of Cd and Cr in each part could be significantly increased.The content of Cd in king grass was significantly positively correlated with palmitic acid in root exudates（P<0.01）,and was significantly positively correlated with octadecanoic acid and methyl pentadecanoate（P<0.05）;Cyclopentasiloxane and cadaverine had a very significant negative correlation（P<0.01）,a very significant positive correlation（P<0.01）with dibutyl phthalate and palmitic acid,and a very significant positive correlation with methyl pentadecanoate（P<0.01）.There was a significant positive correlation（P<0.05）,and root exudates had an important effect on soil physical and chemical properties.The contents of Cd and Cr in king grass were significantly positively correlated with the bacterial phylum Bacteroidetes,Proteobacteria,Deinococcus-Thermus and Bacillus（P<0.01）,and were significantly correlated with WPS-2,Acidobacteria and Bacillus phylum.Chloroflexbacteria showed a very significant negative correlation（P<0.01）.There was a significant positive correlation with rhodes phylum of fungi（P<0.05）.After the rhizosphere soil of king grass was polluted by Cd-Cr compound,the function of rhizosphere bacteria was significantly affected.After inoculation with P.indica,the nitrogen fixation ability of bacteria in low-concentration polluted soil was improved,and the chemical heterotrophy of bacteria in medium-concentration soil was increased.function and cellulolytic function.It mainly changed the respiration of sulfur compounds in low-concentration soil,and in the middle-concentration treatment,it mainly changed the nitrogen fixation ability and nitrification of soil bacteria.Based on the above studies,it is believed that the king grass,Leersia hexandra Swartz.,Brassica juncea and Ricinus communis L.are effective materials for remediation of Cd-Cr composite polluted soil.After inoculation with P.indica,the remediating ability of Cd-Cr of P.indica was significantly increased.Therefore,the combination of king grass-P.indica and Leersia hexandra Swartz.-P.indica has good research value and significance in the remediation of Cd-Cr composite polluted soil. |
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