Application Of Base Cations To Regulate Cd Absorption And Accumulation In Rice

Cadmium(Cd)is a heavy metal element with accumulation toxicity.Excessive Cd enters the soil due to human related production activities,resulting in soil Cd contamination.The root system of crops can absorb Cd in the soil and accumulate in the edible parts of plants,thus entering the human body through the food chain and endangering human health.Rice(Oryza sativa L.)is a kind of crop that is easy to enrich Cd,even when the content of Cd in soil is very low,the root system of rice tends to enrich a large amount of Cd in rice.Base cations are the basic physical and chemical properties of soil and are important reference for soil fertilization and remediation.At present,the technology of remediation of Cd contaminated soil has affected the contents of soil base cations to a certain extent.This paper hypothesized that changing the soil cation exchange capacity could regulate Cd translation in rice soil system and Cd absorption and accumulation in rice by application of base cations.Based on the molar ratio of four base cations(K:Ca:Na:Mg = 2.8:10.9:1.0:1.3)in the tested soil,the mixed solution of base cations was prepared by using analytical purity of KCl,CaCl2,NaCl and MgCl2,and the mixed solution was applied to light(about 0.5 mg·kg-1)and moderate(about 1.5 mg·kg-1)Cd contaminated soil with the level of 0,0.2,0.8 cmol·kg-1(marked as CK,T1,T2,respectively).Pot experiments were carried out to study the influences of different concentrations of base cations on the bioavailability of Cd in soil,and accumulation of Cd in various rice organs of two different cultivar rice(Xiang-Wanxian 12 and Wei-You 46,abbreviated as XWX12 and WY 46),and the grain yield.Moreover,the mechanisms of controlling Cd translocation in soil rice system by regulating base cations were described,and thus providing a new theoretical basis and technical ideas for remedation of Cd contamination paddy soil.The man research fingdings have been expatiated below.(1)At 5 different growth stages of two rice varieties,with the increase of base cations application,the soil pH showed a downward trend,with a decrease range of0.24-0.96 units.Moreover,the application of base cations had a greater impact on the soil pH of rice which was planted in moderate Cd contaminated soil,and had a significant change to the soil pH of XWX12.(2)Base cations application had an effect on soil CEC,the CEC increased first and then fluctuating decreased during the development of rice growth.In light Cd-contaminated soils,the highest CEC value of rice XWX12 appeared at booting stage(32.4 cmol·kg-1),the lowest at dough stage(22.0 cmol·kg-1),while the highest and lowest CEC values of WY46 soils appeared at maturing stage(34.5 cmol·kg-1)and dough stage(18.1 cmol·kg-1),respectively.In moderate Cd-contaminated soils,the highest and lowest CEC values of XWX12 soils still appeared at booting stage(38.8 cmol·kg-1)and dough stage(20.7 cmol·kg-1),while those of rice WY46 soils showed the highest CEC values at tillering stage(42.5 cmol kg-1)and the lowest CEC values at dough stage(18.1 cmol·kg-1).In two Cd-contaminated soils,the application of base cations changed soil CEC of WY46 more significantly.(3)With the increase of base cations application,the content of CaCl2-Cd of two rice rhizosphere soil decreased with the extension of rice growth in two Cd-contaminated soils.In light Cd-contaminated soils,base cations treatment reduced the content of CaCl2-Cd of XWX12 at the tillering stage,filling stage and dough stage by 42.9-61.4%,10.0-30.8%and 16.7-26.4%,respectively.In moderate Cd-contaminated soils,the CaCl2-Cd contents in tillering stage,booting stage,filling stage and dough stage decreased by 7.2-19.3%,38.0-46.5%,40.2-42.1%and 21.4-28.0%,respectively,moreover,the CaCl2-Cd content increases sharply in maturing stage with high concentration of base cations treatment(T2 treatment).In case of WY46,CaCl2-Cd content of the soil at tillering stage,dough stage and maturing stage decreased by 23.2-29.6%,16.2-35.9%,15.6-38.6%respectively in light Cd-contaminated soils,while in moderate Cd-contaminated soils,the CaCl2-Cd content at dough stage and maturing stage only reduced by 24.8-31.8%and 53.3-71.1%.(4)In light Cd-contaminated soils,base cations treatment reduced TCLP-Cd content by 65.6-67.9%during maturation stage,after transplantation,the contents of TCLP-Cd in the soils of XWX12 and WY46 decreased by 27.7-31.2%,33.2-34.6%and 13.3-21.9%,35.5-39.9%at booting and filling stages respectively.In moderate Cd-contaminated soils,base cations treatment had no significant effect on reducing the TCLP-Cd content of matured soils,but increased TCLP-Cd content in soils at different growth stages.The TCLP-Cd content of XWX12 and WY46 rhizosphere soils at maturing stage decreased by 3.74-6.54%and 10.4-21.9%.The effect of base cations on TCLP-Cd content in light Cd-contaminated soils was stronger than that in moderate Cd-contaminated soils.(5)The effects of base cations application on contents of Cd in various rice organs of two different cultivar rice were different by rice growth stages and soil contamination level.In the two Cd-contaminated soils,with the increase of the base caions application,the Cd content in the roots of two rice decreased in each growth stage,in ranging of 29.0 to 91.4%.The Cd content of stem was significantly reduced,and the largest drop was 83.3%,and the reduction effect was more significant in light Cd-contaminated soils.In the moderate Cd-contaminated soil,the application of base cations significantly increased Cd content of leaves at tillering stage,the increase is 5-10 times,and the content of Cd in WY46 leaves decreased by 213.5-512.8%at maturing stage.The content of Cd in brown rice of XWX12 at the filling period has an increasing trend,while has a decreasing effect to WY46.To the maturing stage,planted in light and moderate Cd-contaminated,the content of brown rice of XWX12 and WY46 were reduced by 53.8-62.3%,41.5-49.2%,10.2-25.0%and 23.2-36.0%respectively.In light Cd-contaminated soils,the Cd content of brown rice of two rice plant were between 0.015-0.04 mg·kg-1,and in moderate Cd-contaminated soils,the Cd content of brown rice of two rice plant were between 0.022-0.038 mg·kg-1,both were lower than the national limit for pollutants in food(0.2 mg·kg-1).(6)There were significant differences in the accumulation of K,Ca,Na and Mg in various rice organs by the application of base cations during the rice growth.The contents of each element in stem and leaves increased with the increase of the base cations application,and increased with the extension of rice growth,however,the contents of each element in brown rice gradually decreased following the extension of rice growth.There was a significant negative correlation between the content of Cd and Mg in the leaves of XWX12 in light Cd-contaminated soils,and there was also a significant negative linear correlation between the content of Cd and Na in the rice roots of WY46 in the moderate Cd-contaminated soils.(7)The application of base cations has an increasing effect on the biomass of XWX12 and WY46 exposed to light Cd-contaminated soils,and the biomass in maturing stage was increased by 8.51-30.4%and 40.5-40.8%,respectively.The application of high concentration of base cations could also increase the dry weight of XWX12 brown rice by 21.8%.In case of moderate Cd-contaminated soils,it has a reducing effect,the total biomass of rice XWX12 and WY46 in the maturing stage could be reduced by 11.2-27.6%and 3.20-9.70%,respectively;and the dry weight of brown rice could be reduced by 3.95-40.1%and 22.3-22.8%,respectively.Correlation analysis showed that,in this experiment,there was no correlation between rice biomass and soil CEC,and soil CEC had little effect on rice biomass,while the content of CaCl2-Cd was significantly negatively correlated with rice biomass.The higher the bioavailability of Cd in soil,the stronger the toxicity to rice,and then the greater downtrend for rice growth.