Effects Of Different Types Of Conditioners On The Bioactivity And Passivation Mechanism Of Cd In Paddy Fields

The problem of food security caused by Cd pollution in paddy fields has a bearing on the national economy and people’s livelihood and cannot be ignored.In order to explore the effects of different types of conditioners on the bioactivity and passivation mechanism of Cd in paddy fields,and establish a technical model for safe utilization of Cd in paddy fields with mild Cd while producing and repairing,this study selected a weak acid and mild Cd polluted paddy field in Daye City,Hubei Province,and conducted in situ passivation repair tests of conditioning agents and comparison tests of rice varieties with low Cd accumulation during 2021-2022.The optimal and stable soil conditioner and rice varieties with low Cd accumulation were selected.On the basis of field experiment in the first year,soil conditioner and rice pot experiment were designed to explore the passivation mechanism of different types of conditioners on Cd.The main results of the experiment are as follows:（1）The investigation of soil pollution in the test area showed that the total Cd content in the selected test area was 0.52～0.62 mg·kg^-1,the soil p H value was between 5.76～6.42,and the soil was continuously acidified,which belonged to the weakly acidic and mildly Cd polluted rice field.The annual sedimentation fluxes of Cd,Pb and As were79.93,138.75 and 715.38 mg·（667 m²·a）^-1,respectively,and the content of Cd in irrigation water was 0.021～0.066 ug?L^-1.There was no Cd pollution in the atmosphere or in irrigation water.It can represent lightly single Cd polluted farmland in Daye City.（2）The effects of different types of conditioning agents on Cd contaminated paddy fields were significantly different.In the two-year experiment,different types of soil conditioners can reduce the available Cd content in soil and the Cd content in brown rice by 4.83%to 25.21%and 8.05%to 79.57%,respectively.Among the 20 kinds of conditioner treatments,the inorganic conditioner that can make the Cd content of rice grain reach the standard(<0.2 mg·kg^-1)are:P11(in 2021and 2022:0.103 mg·kg^-1and0.121 mg·kg^-1,respectively),P12(in 2021and 2022:0.108 mg·kg^-1and 0.126 mg·kg^-1,respectively),P13(in 2021 and 2022:0.175 mg·kg^-1and 0.177 mg·kg^-1,respectively),P15(2022:0.135 mg·kg^-1),P16(2022:0.157 mg·kg^-1),Only P3(0.185 mg·kg^-1)of organic conditioner reached the standard.In general,the effect of inorganic conditioner on Cd reduction was better than that of organic conditioner.The application of all kinds of conditioners did not cause obvious yield reduction,but some of them could significantly increase rice yield.Considering the overall yield,P12 conditioner is recommended as the remediation conditioner for lightly Cd polluted paddy fields in Hubei Daye.（3）There were differences in Cd uptake and accumulation among different rice varieties,which could be divided into low,medium and high Cd accumulation rice varieties.The Cd uptake and accumulation of rice is influenced by both the rice variety and the environment.The experiment of rice varieties showed that Cd accumulation and yield of brown rice varied significantly among different rice varieties.In 2021 and 2022,the Cd content and yield of 36 rice varieties were 0.021～0.396 mg·kg^-1、187.76～654.10 kg·（667m²）^-1 and 0.016～0.736 mg·kg^-1、220.40～726.28 kg·（667m²）^-1,respectively.Among them,the grain Cd content of the qualified varieties in 2021 from low to high is:V2(0.021 mg·kg^-1)<V3(0.030 mg·kg^-1)<V8(0.121 mg·kg^-1)<V14(0.138 mg·kg^-1)<V7(0.145 mg·kg^-1)<V11(0.155 mg·kg^-1)<V4(0.161 mg·kg^-1)<V5(0.169 mg·kg^-1)<V6(0.183 mg·kg^-1)<V12(0.185 mg·kg^-1)<V13(0.197 mg·kg^-1).The rice varieties in 2022 from low to high are:V26(0.016 mg·kg^-1)<V2(0.022 mg·kg^-1)<V3(0.024 mg·kg^-1)<V27(0.024 mg·kg^-1)<V28(0.025 mg·kg^-1)<V4(0.156 mg·kg^-1)<V5(0.169 mg·kg^-1).Cd accumulation of most conventional rice was lower than that of hybrid rice,and yield of most hybrid rice was higher than that of conventional rice.The main variety（GLYX 66）in the test area has exceeded the national limit seriously for two years,so it is not suitable for local planting.Considering Cd content and yield,it is recommended to plant hybrid rice V3（AY 1）.（4）The passivation mechanism of different types of conditioners on soil Cd in paddy field can be summarized as soil science mechanism and rice physiology transport mechanism.In the field passivation-repair experiment,inorganic cooking agents P11 and P12（A and B）and organic conditioning agents P5 and P4（C and D）were selected.In the rice variety experiment,GLYX 66 and LXY 130 with high and medium Cd accumulation were selected for indoor rice pot experiment.The study showed that the application of various conditioning agents could significantly increase soil p H value.Just applied inorganic conditioner（A,B）and organic conditioner（C,D）increased soil p H value by1.19～1.47 and 0.47～0.71 units,respectively.After rice maturation,inorganic conditioner（A,B）still significantly increased soil p H value by 1.09～1.30 units,while organic conditioner A and B had little effect on p H increase.The increase of soil p H promoted the adsorption and precipitation of Cd²⁺by soil colloid,which may be the main soil mechanism of Cd passivation by inorganic conditioners.The main soil mechanism of organic conditioners may be the chelation and chelation of Cd by soil colloid.The results of pot experiment showed that the organic matter content of soil could be significantly increased by 20.33%～29.00%.After rice ripening,due to crop-soil interaction,each treatment increased soil organic matter content in different degrees,but the increase of organic conditioners was greater than that of inorganic conditioners.（5）The cumulative Cd transport of different rice varieties in pot experiment revealed the mechanism of Cd transport in rice physiology,which is influenced by the interaction of soil transport antagonistic ions and gene expression.In general,the Cd content in different parts of rice was in the order of root>straw>grain.The key step of Cd transport in rice was Cd transport from soil to rice root,and the BCF root/soil was CK>C>D>B>A.the reduction of Cd transport capacity from soil to rice root under inorganic conditioning agents was stronger than that under organic conditioning agents.This process is co-regulated by Os HMA2 and Os HMA3 genes in different rice varieties.Overexpression of Os HMA2 and Os HMA3 can effectively reduce Cd accumulation in grain.（6）Field and pot experiments showed that the application of various soil conditioners could effectively increase the soil p H value and reduce the content of available Cd in soil,and thus effectively reduce the Cd uptake of rice in soil.Correlation analysis in field experiments showed that soil p H was negatively correlated with available Cd content.The application of various soil conditioners can reduce the content distribution ratio of exchangeable Cd in soil,and transform it to the less active and more stable organic binding state and residual state of Cd,which reduces the activity and migration of Cd in soil.