Accumulation Characteristics Of Cadmium In Rice And Main Control Factors In Major Grain Producing Areas Of Sichuan And Chongqing

According to National Survey Report on Soil Pollution in 2014,the over-standard rate of points on cultivated soil is 19.4%in China,of which light,mild,moderate,and severe pollution account for 13.7%,2.8%,1.8%,and 1.1%,mainly polluted by Cd,As,Hg,Pb and Cr.Especially in the southwest and central south areas,the pollution is the most serious.After the farmland soil is polluted by heavy metals,it poses a serious threat to the soil ecological environment and the safety of agricultural products.In recent years,excessive levels of Cd in paddy soils and rice have been found in many areas of China,attracting widespread attention from the government,the public and academia.In order to further know the characteristics of Cd pollution,the accumulation of Cd in rice and its main influencing factors in regional farmland soils,and to provide a scientific basis for the classification management and remediation of Cd-contaminated farmland,we simultaneously collected paddy soil and rice samples from the major grain-producing areas in Sichuan and Chongqing,and studied the pollution characteristics and bioavalability of Cd in paddy soils to evaluate the pollution risk.We analyzed the accumulation characteristics and pollution risk of Cd in rice,and obtained the main control impact factors of Cd accumulation.Taking the Jiangjin district of Chongqing as an example,according to the Cd pollution level of soil and rice,we explored the classification of soil pollution in rice fields,and proposed control measures for safe planting of rice.The following main research results were obtained:（1）In the soil samples in Chongqing,the total Cd contents ranged from 0.14 to 1.11mg·kg^-1（n=152）with an average value of 0.38 mg·kg^-1.The total Cd contents in soils were higher than the Chongqing background value(0.133 mg·kg^-1).According to"Soil Environmental Quality:Control Standards for Soil Pollution Risk of Agricultural Land（Trial）"（GB 15618-2018）,the exceeding standard rate of Cd in soil was 25.0%in Chongqing.The single-factor pollution index evaluation found that paddy soils in Chongqing were mainly non-polluted（75.1%）,light pollution accounted for 23.7%,and light and moderate pollution accounted for 1.2%,and no serious pollution was found.The evaluation of the potential ecological hazard index found that the ecological risks of Cd pollution were mainly minor risk（72.8%）,moderate and higher risks accounted for 27.2%,and no high and extremely high risk points were found.It should be noted that the p H values of paddy soils ranged from 5.18 to 7.68 in Chongqing,the bioavaible Cd contents ranged from 0.02 to 0.80 mg·kg^-1,the average value was 0.25 mg·kg^-1,and the bioavailability coefficients ranged from 46.5%to 83.2%,indicating high migration possibility of Cd from soil to rice.The total Cd contents of paddy soils in Sichuan Province ranged from 0.09 to 1.59mg·kg^-1（n=713）with an average value of 0.50 mg·kg^-1.According to the soil environmental quality standards for agricultural land,the Cd excess rate of paddy soil was 28.9%in Sichuan.The single-factor pollution index evaluation found that paddy soils in Sichuan were mainly non-polluted（73.5%）,minor pollution accounted for 24.5%,and mild and moderate pollution accounted for 2.0%.No serious pollution was found.The evaluation of the potential ecological hazard index found that the ecological risks of Cd pollution were mainly minor risk（73.4%）,medium and higher risks accounted for 26.6%,and no high and extremely high risk points were found.Soil p H ranged from 4.14 to 9.38,available Cd contents ranged from 0.01 to 1.25 mg·kg^-1 with an average value of 0.24mg·kg^-1,and bioavailability coefficients ranged from 41.5%to 59.7%.The bioavailability coefficients were lower than those in Chongqing,but migration possibility of Cd from soil to rice should be noted.（2）The Cd contents of rice in Chongqing ranged from 0.003 to 0.663 mg·kg^-1（n=142）with an average value of 0.144 mg·kg^-1.According to food hygiene standard(Cd<0.2 mg·kg^-1),the Cd over-standard rate of rice was 24.8%in Chongqing.The average enrichment coefficient of Cd in rice was 0.38.The hazard quotients of rice Cd pollution is 0.001～0.076,and they were all less than 1,indicating that Cd in rice would not cause obvious impact on human health,but there had potential harm.The Cd contents in rice in Sichuan ranged from 0.005 to 0.730 mg·kg^-1（n=713）with an average value of 0.138 mg·kg^-1,and the Cd over-standard ratewas 25.1%.The average enrichment coefficient of Cd in rice was 0.29.The hazard quotients of rice Cd pollution ranged from 0.001 to 0.084,and they wre all less than 1,indicating that Cd in rice would not cause obvious impact on human health,but there had potential harm.（3）The correlations were found between Cd content in rice and soil available Cd content,p H,cation exchange capacity（CEC）and soil organic matter（SOM）in Chongqing with correlation coefficents of 0.649（p<0.01）,0.288（p<0.05）,0.223（p<0.05）and 0.16（p>0.05）,respectively.The order of impact factors was soil available Cd content>soil p H>CEC>SOM.Based on the above correlation analysis,a multiple regression prediction model was proposed between rice Cd content and influencing factors:log Cd_rice=0.753log Cd_available+1.212logp H+0.016log CEC+0.337log SOM–1.934The correlation coefficient of this equation was 0.551（p<0.01）.Combined with Pearson correlation analysis,principal component analysis and multiple regression prediction model,the main control factors that affect the accumulation of rice Cd were soil available Cd content,soil p H and CEC in Chongqing.The correlations were found between Cd content in rice and soil available Cd content,soil p H,CEC and SOM in Sichuan with correlation coefficents of 0.599（p<0.01）,-0.063（p<0.05）,-0.05（p>0.05）and 0.181（p<0.01）,respectively.The order of impact factors were soil available Cd content>SOM>p H>CEC.According to the results of principal component analysis,the main control factors affecting the accumulation of rice Cd were soil p H and CEC in Sichuan.Based on the above correlation analysis,a multiple regression prediction model was proposed between rice Cd content influencing factors:log Cd_rice=0.81log Cd_available–0.66logp H+0.425log SOM-0.031log CEC–1.642The correlation coefficient of this equation is r=0.607（p<0.01）.Combining Pearson correlation analysis,principal component analysis and multiple regression prediction model,we can know that the main controlling factors that affect the accumulation of rice Cd are soil available Cd content,soil p H and SOM in Sichuan.（4）The total Cd contents of paddy soil ranged from 0.09 to 1.61 mg·kg^-1（n=301）in Jiangjin District of Chongqing with the average value of 0.36 mg·kg^-1.The Cd contents of rice ranged from 0.002 to 0.753 mg·kg^-1（n=301）with the average value of 0.108mg·kg^-1.The soil p H ranged from 4.29 to 8.09,and the available Cd contents ranged from0.01 to 0.70 mg·kg^-1 with the average value of 0.23 mg·kg^-1.According to soil environmental quality standard of agricultural land and the limitation standard of Cd content for food hygiene,the"Cd pollution zone and safe use map of farmland soil in Jiangjin District"was drawn by using Arc-GIS software.The paddy soils in the northern and southern regions of Jiangjin District were in safe zone,which did not pose a hazard to rice production and should be given priority protection.The paddy soils in the eastern and western regions of Jiangjin District were in basic safe zone,and should be used safely and prevent external Cd input.