Asurvey Of Cadmium Contamination In Soil And Rice And Investigation On The Soil Cadmium Limit

Soil heavy metal pollution is a severe problem in China;however,the existing soil environmental quality standard(GB15618-1995)does not fit the demands of the soil environmental lanagement today.Some suggestions are put forth for amendment of the existing soil environmental quality standards in China,including amendment of the criteria for highly toxic heavy metals,like Cd.Cd-polluted soils pose a hazard to safety quality of agricultural products and bring about harmful effects on eco-environment and human health.It is,therefore,of important significance to develop reasonable standards for Cd in soil environment in China to the enhancement of management of Cd contamination in soil environment.Cd is one of the major pollutants in rice grains,for rice plants can absorb Cd easily.Thus,it becomes the potential important pollutant for the food supply and food safety in China.In order to recognize Cd pollution in rice grains of Cd-polluted soils in China,a nationwide survey was carried out in 2015.Pot experiments were also conducted to examine the effects of the Cd content in 8 soils selected from the low Cd-polluted farmland on the Cd concentration in rice with adding different concentrations of Cd of 0,0.25,0.5.1.0.2.0 and 4.0 mg/kg in 2015.The main results were found as following:(1)The survey results showed that the 82%paddy soil samples exceeded the limit for Cd of the Chinese standard for Soil Environmental Quality(0.3 mg/kg,pH<7.5;0.6 mg/kg,pH>7.5).59%rice grain samples had more Cd than the Chinese Food Health standard(0.2 mg/kg).There were significant correlations between the concentrations of Zn and Pb(r=0.85,p<0.01),suggesting co-contamination of these elements in soil samples.89%and 71%,74%and 67%,89%and 53%of soils and grains in Hunan province,Guangdong province and Guangxi province exceeded the limit for Cd of the Chinese standard for the soil environmental quality(GB 15618-1995)and the food health standards(GB2762-2012),respectively.The results of multiple regression lodels showed that the background level of Cd in soils,the pH values of soils and Cd concentrations in grains had some correlation(R2=0.13,p<0.01).Results show that Grade ? criteria for Cd of the Chinese soil environmental quality(GB 15618-1995)standard are more stringent than the international soil environment standards for agricultural lands.(2)The pot experiment results show that when the added Cd concentrations in soil are increased from 0 to 4 mg/kg,the Cd concentrations in aboveground rice are increased significantly.The Cd concentrations in straw are 4-6 times higher than that in grains.Regression analysis indicated a linear relationship(R2>0.95,p<0.01 or p<0.05)between Cd concentrations in soils and Cd concentrations in aboveground rice with Cubic,Sigmoid and Logarithm mathematical models,respectively.Regression analysis method was applied to work out a significant linear relationship(R2=0.89,p<0.05),which indicate that soil threshold values of Cd are closely related to soil pH and soil clay.For rice soil,different Cd threshold values were derived from the related food health standards(GB2762-2012).The soil threshold values of Cd vary widely,Ningbo paddy soil 4.37,Xiangxiang paddy soil 4.11,Hechi red soil 2.24,Dujiangyan paddy soil 1.29,Yichang paddy soil 1.06,Hefei paddy soil 0.91,Quanzhou paddy soil 0.72,and Xuyi yellow soil 0.70 mg/kg dry weight.This study will provide technical support for developing and revising the soil environment standards for Cd in soils for different land uses in China.