| Heavy metal pollution of farmland soil caused by mining and metal smelting is a serious environmental problem that has threatened human survival.Cadmium（Cd）,a highly toxic metal element,can migrate in the soil-plant system and accumulate in food crops and vegetables,and then threaten human health through the food chain.Therefore,preventing and controlling the content of Cd in edible parts of vegetables is of great significance for improving the utilization rate of contaminated soil and ensuring the safety of vegetables.Selenium（Se）and silicon（Si）,the two kinds of beneficial microelements for plant growth,help to alleviate the toxicity of heavy metal to plants and reduce the accumulation of heavy metals in plants.Artemisia selengensis is a common vegetable with high nutritional and medicinal value,which has a long history of cultivation and extensive planting area in southern China.Meanwhile,the safety of Artemisia selengensis is also a social concern.In this study,the farmland around four typical mining areas in Daye City was selected as the research area.the pollution status and sources apportionment of five heavy metals in the farmland soil around the mining area were compared and analyzed through field investigation,and the pollution degree and distribution characteristics of soil Cd in the farmland around the mining area were defined.Accordingly,the enrichment characteristics and physiological response of Artemisia selengensis to Cd,and the regulation mechanism of exogenous Se/Si on Cd reduction of Artemisia selengensis were explored by hydroponic experiment.On this basis,the effects of exogenous Se/Si on the Cd bioavailability and quality of Artemisia selengensis grown in Cd contaminated soil were studied by pot experiments.Furthermore,the characteristics of rhizosphere soil microbial communities and the response of rhizosphere microenvironmental factors under the regulation of exogenous Se/Si were deeply discussed,and the abundance changes of functional bacteria related to the conversion of cadmium in soil were further analyzed.The mechanism of exogenous Se/Si inhibitting the enrichment of Cd into Artemisia selengensis was revealed from the perspective of microbiome.These main conclusions are as follows:（1）The farmland soils around Daye mining area were polluted by multiple heavy metals such as Cu,Cr,Cd,Pb and As.On the whole,the contents of Cd,Cu,As and Pb in these investigated areas were higher than the standard value,and the exceeding standard rates were 80.00%,61.11%,45.56%and 32.22%,respectively.Especially,Cd was the main characteristic pollution element,which had a higher risk of ecological pollution and wider distribution.Cu,Pb,Cd,As derived from human activities,indicating that anthropogenic activities such as concentrated mining and smelting were the main causes of Cd pollution in farmland.（2）Under Cd stress,the growth of Artemisia selengensis showed a phenomenon of"low promotion and high inhibition"with the change of Cd concentration.Artemisia selengensis exhibited a strong tolerance to Cd,the maximum threshold of Cd tolerance on Artemisia selengensis was 120μmol L–1,and the root was the main site enriching Cd.The subcellular distribution of Cd conternt in the root of Artemisia selengensis was as follows:cell wall>soluble component>organelle,indicating that the root cell wall of Artemisia selengensis played an important role in alleviating Cd toxicity.Cd existed in different chemical forms in root cells,among which salt–soluble cadmium（F–Na CI）accounted for the highest proportion of 61.36%.The results indicated that Cd mainly bound to salt–soluble proteins and compounded in Artemisia selengensis.（3）Under Cd stress,the combined addition of exogenous Se/Si could effectively reduce the Cd content in Artemisia selengensis.The content of roots and shoots was decreased by 59.68%and 61.94%,respectively.Exogenous Se/Si could help maintain the root growth activity under Cd stress,and increase the number of root tips,root surface area,average diameter and volume.Moreover,the addition of Se/Si enhanced the ability of antioxidant system,showing that the content of MDA decreased and the content of GSH and PCs increased;at the same time,the activities of SOD,POD,CAT and other antioxidant enzymes significantly increased.（4）In Cd contaminated soil,the content of Cd in edible parts of Artemisia selengensis was significantly reduced by adding Se(1 mg kg–1)and Si(150 mg kg–1),which had a significant reduction of more than 50%compared with the control.The digestion and dissolution of Cd from edible parts of Artemisia selengensis in gastric phase was significantly higher than that in intestinal tract phase.The bioavailability of Cd was decreased significantly after the application of Se/Si,which was significantly reduced by 28.6%in gastric phase and 43.51%in intestinal tract phase,respectively.In addition,the application of Se/Si can reduce the nitrate content,and increase the content of reducing sugar,soluble protein and vitamin C in edible parts of Artemisia selengensis,thereby improving the quality and value of Artemisia selengensis.（5）16S rRNA was used for high–throughput sequencing analysis of soil microbiome characteristics.The results showed that the addition of exogenous Se/Si in Cd contaminated soil could reduce the absorption of Cd by Artemisia selengensis by improving the rhizosphere microbial community structure.More beneficial Cd resistant strains such as Flavitalea,Labraceae,Bradymonadales and Laceyella were recruited to the rhizosphere.Moreover,the addition of exogenous Se/Si significantly increased the activities of soil enzymes such as catalase,urease,acid phosphatase and sucrase.Combined effects of exogenous Se/Si on the physicochemical properties and microbiome of rhizosphere soil,it could be deduced that exogenous Se/Si can maintain the normal growth of Artemisia selengensis and reduce the uptake of Cd by Artemisia selengensis through regulating the structure of rhizosphere microbial community and the abundance of key functional bacteria,and optimizing the rhizosphere microenvironment. |
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