The Research On Cd Speciation Changes,Mobilization Characteristics And Cd Migration Impedance In Soil-Rice System

The toxicity and behavior of Cd in the soil-rice system depend on Cd forms,soil environmental conditions,and rice growing.Rice absorbs the soluble Cd in the soil,while the rice root exudation acidifies the rhizospheric environment at the same time,promoting the mobility or solubility of Cd in the soil,affecting the re-distribution of Cd forms in the soil.Comprehending the characteristics of Cd uptake and accumulation in rice and the activation effect of rice on soil Cd is conducive to take measures to regulate the migration and transformation of Cd from soil to rice.Therefore,it is of great significance to study the activation of soil insoluble Cd by rice and the environmental behavior of activated Cd.A sand culture experiment was conducted to investigate the interaction between rice and insoluble Cd,especially the mobilization of insoluble Cd.Four rice cultivars(WX:low-Cd conventional rice,XX:high-Cd conventional rice,YY:low-Cd hybrid rice and JY:high-Cd hybrid rice)and four insoluble Cd(CdS,[Cd3(PO3)2],CdCO3 and CdSe)were selected for this sand culture experiment operated in a plant growth chamber.The experiment quantitatively investigated the mobilization of insoluble Cd and the cellular and chemical forms distribution of mobilized-Cd in rice root and shoot.Pot experiment ? was carried out to illuminate the soil Cd forms under rice-planted and non-planted treatments.Two typical parental soils,yellow clayed soil(YCS)derived from shale parent material and granitic sandy soil(GSS)derived from granitic parent material,.were chosen in this pot experiment.Exogenous Cd was applied to simulate Cd stress on soils.The distributes of Cd fractions in the soil of rice-planted treatments and no rice-planted treatments and the uptake and accumulation of Cd in rice tissues(roots,stems,leaves,husks,and brown rice)at different rice growing stages(tillering,heading and maturity stages)were measured and the response of rice to soil insoluble Cd was found.Correlation analysis determined the relation between mobilization of the residual form of Cd in the soil and Cd accumulation in brown rice.Pot experiment ? was executed to ascertain the effect of remediation measures on Cd migration in different parental soils.The effects of two soil amendments(lime[SH]and Si-Ca-Mg fertilizer[GF])on distributes of Cd fractions in the soil of rice-planted treatments and non-planted treatments and the uptake and accumulation of Cd in rice were investigated.The predominant results showed that as followed:(1)The results in the sand culture experimeInt showed that both of the four rice cultivars can mobilize the insoluble Cd.The total amounts of mobilized-Cd range from 29.08 ?g plant-1 to 84.63?g plant-1,and high-Cd and hybrid rice cultivars have a greater ability to mobilize the insoluble Cd.Hybrid and high Cd-tolerance rice can mobilize?137(?)insoluble Cd in the sand culture experiment.In addition.,the mobilized-Cd was prime distributed in the cell wall fraction of rice root and shoot.In addition,the protein and pectates integrated Cd(extracted with NaCl)and the undissolved Cd phosphate(extracted with HAc)were the major chemical form in root and shoot of rice.Regression analysis presented that the mobilization of insoluble Cd by rice plants depends on the interaction of root secretion phytosiderophore and acidity change of rhizosphere solution.(2)The results in the pot experiment I showed that the concentrations of Cd in rice tissues increased along with the addition of exogenous Cd.The concentrations of Cd in rice tissues followed the order:roots>stems>leaves>brown rice>husks,and difference of the distribute of Cd contents in rice tissues between soil types was discovered under high Cd stress.The difference of distributing of Cd fractions on YCS and GSS was also found between rice-planted and no rice-planted treatments.Cd fraction in soil was mainly in residual-Cd and organically bound to-Cd under no rice-planted treatment,while for rice-planted treatment,Cd fraction in soil was mainly in acid extractable-Cd in the soil,which indicates the effect of rice on the transformation of other three Cd fractions to acid extractable fraction in the soil.Rice increased the remobilization of residual-Cd in the soil,and the remobilization rate increased with the increment of exogeneous Cd,up to 28%.Correlation analysis showed that the remobilization rate of residual-Cd in soil was positive significant and extremely positive significant correlated with total Cd accumulation and brown rice Cd and the number of phytosiderophores(PS)secreted by rice root.This study implies that rice secrets PS to remobilize insoluble Cd in the soil,and then uptakes the remobilized Cd.The prediction model of Cd in rice and the characteristic equation of rice accumulation of Cd were applied to calculate the critical values of Cd which were 0.98 mg·kg-1 in YCS,and 0.86 mg·kg-1 in GSS,respectively.In consideration of the effect of remobilization of soil insoluble Cd on Cd accumulation in rice grain,we propose acid extractable Cd threshold for GSS(0.25 mg·kg-1)and YCS(0.28 mg kg-1)could be utilized.Therefore,the process of remobilization of insoluble Cd in soil-rice-system and the establishment of soil environmental quality standard by bioavailable Cd should be highly emphasized in further studies.(3)The results of the pot experiment ? showed that application of SH and GF significantly reduced Cd accumulation in rice in YSS and GSS,and the maximum reduction of Cd in GSS reached 319 ?g·pot-1 with SH treatment.SH and GF significantly decreased and increased the proportion of Acid-Cd and Res-Cd in the soil.Rice-planted treatments activated Res-Cd,compared with no rice-planted treatments,and SH and GF restrain the remobilization process.Significant positive correlations were found between remobilization rates of Res-Cd in soil and Cd content in brown rice.Multiple regression revealed that applying GF at a dosage of 1.296 g kg-1 and of 1.246 g·kg-1 in YCS and GSS is an ideal method to control soil acidity.From the above,the soil security threshold values and the soil environmental capacities of the two different parent materials varied greatly,and the effectiveness and mechanism of different soil amendments also varied,and there is the mobilization of insoluble Cd by rice.Therefore,it is suggested that the formulation of environmental quality standards and the implementation of Cd-polluted amendment control measures should be targeted at the difference in soil type and bioavailability of Cd.This study focused on the activation effect of rice on insoluble Cd and its response to activated Cd,which may not only provide new research ideas for the bioavailability of soil Cd but also provided certain theoretical guidance for the evaluation of soil environmental quality.