Chloride Application Weaken Cadmium Immobilization By Lime In Soil-rice System And The Mechanisms

Lime is widely used as an amendment for Cd-contaminated acidic soil,and its positive remediating effect is generally obvious,but not in all causes.The lime application promotes the absorption of soil Cd by rice plants and increases the Cd accumulation of rice grain in the Cd-contaminated acid soil planted rice plants.There are many factors that might weaken the remediating effect of lime for Cd-contaminated acidic soil,among which the higher concentration of chloride ions（Cl^-）in the soil is an important factor,because it has been confirmed that the addition of Cl-rich materials increased the Cd accumulation in brown rice.Among acidic soils,there are some soils with high Cl^-contents,such as coastal saline soil,and soils with extensive application of Cl-rich materials.In order to better control the pollution risk of Cd on acidic soil and carry out reasonable and safe utilization,it is necessary to further study the influences and mechanisms of Cl^-on the remediation of Cd by lime in Cd-contaminated soils.In this study,the possible mechanisms that the effect of Cl^-on the remediation of Cd by lime in Cd-contaminated acidic paddy soil were investigated using the methods of soil chemistry,plant physiology,molecular biology,and microbiology through pot experiment,hydroponic experiment,and field experiment.The main conclusions of this study are as follows:（1）In pot experiment,lime significantly reduced the soil Cd availability by increasing the p H value of soil solution by 2.8%～4.3%,decreasing soil Eh by 9.0%～23.7%,significantly reducing the acid-extractable Cd and reducible Cd of soil by 19.54%and 15.94%,respectively,and increasing residual Cd by 22.51%,which made the proportion of Cd²⁺in the soil solution significantly decreased by 31.3%.Lime significantly promoted the formation of iron plaques on root surfaces by 15.09%,significantly decreased the relative expression of the main Cd transporters in rice roots by 17.8%（Os Nramp5）,5.8%（Os Nramp1）,31.4%（Os IRT1）,and 22.7%（Os IRT2）,respectively,thereby reducing the uptake of Cd by rice roots.Lime also significantly reduced the Cd accumulation in different parts of rice plant and the Cd transport form root to stem,soil to rice grain,root to rice grain,and leaf to rice grain.In conclusion,lime reduced the Cd concentration in rice grain by 36.6%by reducing the bioavailability of soil Cd,reducing the absorption of soil Cd by rice roots,and decreasing the migration and transport of Cd between various parts of rice plants.（2）In hydroponic experiment,Cl^-addition promoted the Cd absorption by rice roots by inducing up-regulation of the relative expression of Os Nramp5,Os Nramp1 and Os IRT1 transporters in roots,increasing the biomass of rice roots,the total root length,root surface area,root volume and the total root tips,significantly reducing the root oxygen loss by 9.1%～25.8%.Cl^-addition reduced the formation of glutathione（GSH）,phytochelatinpeptides（PCs）,and non-protein thiols（NPT）in rice roots,increased the energy spectrum of Cd in rice root cells and the Cd distribution in the cross section of rice roots,which increasing the free Cd concentration and improving the Cd mobility and bioavailability in root cells.In addition,Cl^-addition induced up-regulation of the relative expression of Os HMA2 transporter gene in rice roots,which promoted the Cd transport from root to shoots,and ultimately facilitated the accumulation of Cd in shoots.（3）In pot experiment,compared with the lime treatment,the mixed addition of lime and Cl^-increased the soil Cd solubility by reducing the soil p H and soil Eh,which inhibited the precipitation of Cd S and promoted the dissolution of soil Cd,significantly increasing the water-soluble Cl-Cd complex（Cd Cl⁺and Cd Cl₂）,and increasing the ratio of the aid-extractable and reducible Cd resulting in improving the solubility of soil Cd.At the same time,the mixed addition of lime and Cl^-significantly promoted the reductive dissolution of Fe/Mn oxides in soil,so that the concentration of Fe²⁺and Mn²⁺in the soil were increased by 5.7 times and 8.0 times,respectively,compared with the lime treatment.Higher Fe²⁺and Mn²⁺contents in soil promoted the release of Cd²⁺by competing with Cd²⁺for soil absorption sites,further increasing the concentration of water-soluble Cd（2.5～3.75 times higher）and improving the soil Cd bioavailability.The mixed addition of lime and Cl^-not only reduced the formation of iron plaques on root surfaces by 29.49%～53.08%,compared with the lime treatment,but also weakened the barrier effect of iron plaques on the absorption of Cd by rice roots,thereby promoting the absorption of Cd by rice roots.The mixed addition of lime and Cl^-significantly increased the relative expression of the major Cd transporters in rice roots with the increase of 23.3%～44.4%（Os Nramp1）,25.2%～35.1%（Os Nramp5）,50.2%～91.0%（Os IRT1）and 71.2%～91.0%（Os IRT2）,compared with lime treatment,which further improved the soil Cd uptake by rice roots.In addition,the mixed addition of lime and Cl^-promoted the migration and transport of Cd in the soil-rice system,and increased the Cd contents of rice grains by 0.94～1.1 times.（4）In Cd-contaminated paddy rice soil amended with lime,the addition of Cl^-significantly promoted the absorption and accumulation of Cd in soil-rice system by increasing the soil Cd availability,promoting the uptake of soil Cd by rice roots,and increasing the transport of Cd between various parts of rice plant.These results of the field experiment further verified the results of pot experiment and hydroponic experiment.（5）In conclusion,this study revealed the main mechanisms from the perspectives of soil chemistry,botany and molecular biology of the effect of Cl^-on weakening the remediating effect of lime on Cd-contaminated acidic soil.The weakening effect is more pronounced under the higher Cl^-condition.For some acidic soils with high Cl^-content,such as coastal saline soils and soils with a large of Cl-containing materials,this factor must be considered when selecting soil amendment materials and formulating the measurement of soil remediation.On Cd-contaminated soils,the addition of amendments with high Cl^-contents should be avoided.