| The activity and environmental risk of Cadmium(Cd)are determined by their speciation distribution.So far,the researches on Cd bioavailability and its regulatory mechanism have been mainly focused on the pollution of exogenous sources such as mines and industries.However,the results do not represent the environmental behavior of Cd root in the natural background.The study of Cd derived from natural background had been only a preliminary investigation,but there was rarely reports on the environmental behavior of Cd.Thus,it was of great realistic and scientific significance to study the environmental behavior of Cd in the high background soils for improving the soil environmental quality standard.In this study,the soil samples were collected from typical parent material-derived paddy soil in the high Cd background of Guangxi province.Using diffusion gradient in thin films(DGT)technique to accurately characterize the activty of Cd in the different parent materials.Combined with pot experiment and anaerobic soil culture experiment to study Cd environmental chemical behavior and its influencing factors in paddy soil-rice system under the reaction of organic-sulfur-iron cycle.It provides scientific basis for preventing and controlling the pollution of rice Cd and guaranteeing the quality and safety of agricultural products.The major results are as follows:1.Cd lability in the typical parent materials-derived soils in Guangxi provinceDiscovered through investigation,the soils samples derived from limestone,sandshale and Quaternary red earth had a total Cd content exceeding the maximum permissible concentration(0.3 mg·kg-1),with the mean of 3.45,2.40 and 0.69 mg·kg-1,respectively.The order of total Cd content was limestone > Quaternary red earth > sandshale.The exchangeable fraction(F1)of sandshale,limestone,and Quaternary red earth were taking up 50%,56%,and 39%,respectively of the total Cd.For the rice grains harvested from the soils derived from limestone,sandshale,and Quaternary red earth were 0.59,1.06 and 0.16 mg·kg-1,respectively,and 95%,67%,and 28% had a Cd content exceeding the maximum permissible concentration(0.2 mg·kg-1)for grain.The results indicated that the survey areas had serious ecological and environmental health risks in soil-agricultural products.The results of regression analysis show that p H and total Cd were the key factors on Cd DGT,Cdsol and Cd Ca Cl2 in the three parent material-derived soils.CEC,total Fe,Clay,SOM and total Cd were the key factors on Cd HNO3 and Cd DTPA in limestone and sandshale soils.In Quaternary red earth soil,p H was the key factors on Cd HNO3 and Cd DTPA.Random forest showed that DOM and soil p H played an important role to Cd accumulate in rice grains.In limestone soil,soil p H was among the top important factors influencing Cd accumulation in rice grains.By contrast,Mn DCB?p H and FeDCB were more important than other physicochemical factors of Quaternary red earth soil influencing Cd accumulation in rice grains.Cd measured by diffusive gradients in thin films(DGT)technique was best reflects Cd bioavailability,being significantly and positively related to Cd content in rice plant,indicating that DGT was more accurately assesse Cd bioavailability in rice.2.Effect mechanisms of water managements on Cd lability in three parent materials-derived soilsCd concentration in rice plant and rhizoshere was with the order of non-flooding> intermittent flooding>continuous flooding in three paddy soil types under different water managments.Grain Cd concentration was 0.88 and 0.25 mg·kg-1 in limestone and Quaternary red earth soils under NF conditions,and 77% and 20% exceed the maximum permissible concentration(0.2 mg·kg-1),respectively.These results indicated that intermittent flooding and continuous flooding condition can effectively reduce the absorption and accumulation of Cd in rice plants.The effect of water management on Cd availability and plant Cd content was flollwing with the order of limestone>Quaternary red earth>sandshale.The changes of the rhizosphere environment were influenced by water management.During continuous flooding condition,decreased oxidation-reduction-potential(ORP)resulted in increased soluble ferrous iron [Fe(II)] and dissolved organic carbon(DOC)concentrations compared to non-flooding and intermittent flooding condition in the three types soils.Continuous flooding resulted in an enhanced formation of Fe plaque compared to that under non-flooding conditions,which may related to the oxidation of large quantities of Fe(II).Cd concentration absorbed onto root surface were no significant differents under water treatment in sandshale soil.For limestone and Quaternary red earth soils,Cd concentration absorbed onto root surface was with the order of non-flooding > intermittent flooding > continuous flooding.Decreased of p H in sandshale soil,and increased in limestone and Quaternary red earth under continuous flooding condition.By contrast,p H was the opposite trend under NF condition.CdDGT was significantly and negtively related to Fe DCB and Mn DCB,indicating that root plaque act as a scavenger of Cd to reduce Cd availability in the rhizosphere in limestone and Quaternary red earth-derived soils.Under non-flooding condition,Cd DCB was significantly and positively related to Cd DGT and Fe DCB,indicating that Cd absorbed by Fe planque was depended on Cd avialibale concentration.Furthermore,plant Cd concentration was significantly and positively related to Cd DCB and Fe DCB,indicating that Fe plaque may be served as a “source” to promote Cd uptake by rice under non-flooding condition.The migration and release kinetics of Cd in soil was effected by water management.The resupply capacity of soil Cd from solid phase to liquid phase was increased,and relatively releasable Cd capacity from solid phase was also increased under non-flooding condition.Under continuous flooding condition,the resupply capacity of soil Cd from solid phase to liquid phase was decreased,and relatively releasable Cd capacity from solid phase was also decreased.The results indicating that Cd would like to transfer to the soil solid phase in the form of weak combination state under NF condition,when Cd concentration was uptake by plant in soil solution,Cd can be more rapid release from soil solid phase.3.The coupled effect of humic acid-ferrihydrite-sulfate on Cd labilityAnaerobic culture experiment showed that Cd DGT and Cd Sol concentration,as a whole,were decreased with time at all treatments.On the 64 th day,Cd DGT concentration was following the order of treatment S+Fe > CK?HA+S?S?CK > HA+S+Fe > HA?Fe?HA+Fe,indicating that the addition of humic acid and ferrihydrite will significantly reduce Cd activity.Adding humic acid and ferrihydrite at the same time,due to the complexation of both will occupy part of adsorption point each other,so there was no significant difference when compared to a single add,indicating that compound added will occupy part of adsorption point.Anaerobic culture significantly affected the composition of soil bacteria.In the phylum lever,anaerobic culture reduced the relative abundance of Proteobacteria?Gemmatimonadetes and Actinobacteria and enhanced the relative abundance of Acidobacteria?Chloroflexi and Verrucomicrobia.In the genus lever,anaerobic culture reduced the relative abundance of microbials,such as Sphingomonas,Bacillus,Candidatus Koribacter,Ramlibacter,Ellin6067,Micromonospora,Lactococcus,Gemmatimonas,however,at the treatment of HA+S+Fe,the relative abundance of Gemmatimonas was increased.Anaeromyxobacter was the main Iron-reducing bacterium in the study,and anaerobic culture significantly increased the relative abundance of Anaeromyxobacter. |
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