| Wheat and corn are the main staple food crops in Hebei Province.Human activities,such as historical sewage irrigation,metal smelting,and the overfertilization using fertilizers with heavy metals,caused the concentrations of heavy metals in farmland soils and crops exceeding their safety limitations around the province.In addition,cadmium(Cd)and arsenic(As)contaminated-farmland soil has attracted much more attention.Heavy metals contamination in soil should inevitably affect the activities of plants,microorganisms and related soil enzymes in the soil.In order to accurately carry out the safe utilization of contaminated farmland or soil remediation,it is necessary to explore the biological response of plants and rhizosphere microorganisms to different levels of cadmium and arsenic in soil.Therefore,the study was conducted to explore the responses of wheat and corn to different Cd/As contaminated soils.To investigate the variation of microbial flora,enzymes activities and microbial communities in the rhizosphere of wheat and corn,to analyze the relationship between the soil properties and structure of microbial communities,which would provide strong support for the utilization and remediation of farmland soil contaminated by Cd and As.The main results are shown as follows:(1)Clarified the responses of wheat and corn to Cd/As contaminated soil.It has been shown that the concentrations of Cd and As in corn grains did not exceed the National Food Safety Standards Limits of Contaminants in Food sunder moderately and heavily Cd/As contaminated soil.However,the levels of Cd in 70%of the wheat grains samples exceeded the safety limitation,and As in wheat-grain content did not exceed the standard.Therefore,Cd and As in corn grains had no carcinogenic risks to adults and children,whereas Cd and As in wheat grains had carcinogenic risks to human health.Moreover,the bioaccumulation of Cd and As in wheat grains were 11.57-fold and 9.09-fold higher than those of corn grains,respectively.Cd accumulation in the plants of wheat and corn was significantly stronger than As accumulation.The ratio of Cd from straw to grain in wheat was 10.32-fold larger than that of As.(2)It was explored that variation patterns of the fractions of Cd/As in rhizospere and non-rhizosphere of wheat and corn.The fraction of specific-sorbed was a predominant species of As in non-rhizosphere of wheat and the availability of As in wheat rhizosphere was lower than that in non-rhizosphere.There was not significant difference in Cd availabilities between rhizosphere and non-rhizosphere of wheat and corn,however,the availability of Cd in this Cd/As contaminated and calcareous soil was quite high,accounting for more than 60%of total content.Arsenic in rhizosphere of corn was mainly associated with the fractions of non-specific and specific sorbed,amorphous and poor-crystalline hydrous oxides of Fe/Al or Fe/Mn.As availability in rhizosphere was lower than that in non-rhizosphere,it was opposite for Cd in the rhizosphere and non-rhizosphere in corn.(3)Clarified the characteristics of microbial flora and community in rhizosphere and non-rhizosphere of wheat and corn in Cd/As contaminated soils.Compared with moderate pollution levels,the numbers of actinomycetes in the rhizosphere of wheat,rhizosphere and non-rhizosphere of severely contaminated soil were significantly reduced by 76.25%,15.12%and 34.35%compared with moderately Cd/As contaminated soil.Moreover,the activities of alkaline phosphatase and β-glucosidase decreased by 17.41%and 30.64%in rhizosphere of wheat and invertase activity reduced by 34.24%in the rhizosphere of corn.The contamination Cd and As in soil did not impact the biodiversity of bacterial and fungal communities in the rhizosphere,but influenced significantly the composition and structure of microbial communities.In the heavily contaminated soil,the relative abundances of bacterial phyla Chlorocurcus,Methylomirabilota and fungal phylum Oil Chytridia in the rhizosphere of wheat significantly increased by 37.36%,93.74%and 96.91%compared with the moderately polluted soil,while the relative abundances of bacterial phyla Patella and Bacteroides and fungal phylum Mycophyta and Oil Chytridia in the rhizosphere of corn reduced by 39.73%and 49.04%,14.08%and 85.33%,respectively.Soil organic matter,available P,K,Cd and As,catalase and β-glucosidase shifted remarkedly the structure of bacterial communities in wheat rhizosphere,and β-glucosidase had shifted the structure of bacterial communities in corn rhizosphere.In addition,soil pH,CEC,SOM,Total Cd and As,available As,invertase and β-glucosidase had changed the structure of fungal communities in rhizosphere of wheat.soil pH,available K,Total Cd and As,available As and invertase had changed the structure of fungal communities in rhizosphere of wheat.In summary,the bioaccumulation of Cd and As in wheat grain and their translocation from straw to grain were stronger than those of corn,and the levels of Cd and As in wheat grain had carcinogenic risks to human health.Moreover,the Cd/As contaminated soil had no effect on the biodiversity of bacterial and fungal communities in the rhizosphere,but influenced significantly the composition and structure of microbial communities. |
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