Soil Threshold Of Cadmium And Lead In Major Chinese Wheat-producing Areas

Chinese environmental quality standards for soils(GB 15618-1995) has been implemented for 20 years, which is making by ecological effect method depending on less samples, for example, the soil-plant system is concerned, based mainly on 1980 s & 1990 s of short-term pot experiment and field surveys, and gradually showing some deficiencies with the society development, in general, site-specific soil environment evaluation was investigated a lot, but researchers are rarely dealt with nationalwide areas of land. This paper aims to obtain the effect of heavy metal(Cd, Pb) toxicity tests on wheat, the bioaccumulation factor of eight wheat varieties in different concentrations of heavy metals, the transfer of different parts of wheat on heavy metal in different soil types, the main soil factors that controls the enrichment of heavy metals of grain and quantify the relationship with the bioaccumulation factor, then critical threshold of heavy metals can be derived according to the food hygiene standards. In addition, species sensitivity distribution method, combined with heavy metal bioaccumulation factor and prediction models for 8 wheat grain, established of soil threshold models for cadmium and lead based on soil physical and chemical properties, that could benefit the development of soil environmental quality criteria and food safety of wheat.1. The tested wheat varieties screening: two kinds of typical soil(acid, alkali), national eight wheat varieties, pot experiment.(1) First, we got access to the bioaccumulation factor and the sensitivity level of Cd, Pb through the study of heavy metals on Chinese 8 wheat varieties, and the results showed that, regardless of the wheat grown in red soil(acidic) or lou soil(alkaline), different varieties of heavy metal enrichment capacity greater differed on the same soil, bioaccumulation factor of 8 varieties of grain for cadmium and lead were approximately 2, 2.4 times, respectively. Comprehensive study of 8 wheat varieties in two soils, Xiaoyan 22 cultivar were moderately sensitive to pairs of Cd, Pb.2. Ecological effect method to deduce the soil environmental threshold of wheat varieties: heavy metal toxicity prediction model of wheat grain(Xiaoyan 22), cross-species extrapolation(1) We collected a number of farmland soils in wheat-producing areas to plant cultivar Xiaoyan 22, by adding exogenous Cd, Pb through the way of pot experiment, then the multiple regression functions between heavy metal bioaccumulation factor of wheat grain and soil properties was established based on different calculation of bioaccumulation(total or added), and the results revealed that soil properties affected the accumulation of Cd, Pb from soil to plants, and pH and organic carbon(OC) content were main factors, among which pH can be explained cadmium prediction models of variation of 61.7%-79.6%, while the pH and OC can explain the variability of lead models of 66%-80%.(2) Using a cross-species extrapolation approach, the Cd and Pb uptake prediction model for cultivar Xiaoyan 22 was satisfactorily applied to 7 additional non-modeled wheat varieties to develop a prediction model for each variety, which demonstrated the feasibility of extrapolation of wheat toxicity prediction model, what is more, the extrapolated prediction model for other varieties in two soils(red soil, lou soil) predicted well on bioaccumulation factor of metals in grain, generally within a 2-fold deviation range from the measured bioaccumulation factor, and added method was overall slightly better than the total method. According to the food hygienic limits of wheat grain, soil threshold of heavy metal were back-calculated by the prediction models based on soil properties.3. Ecological risk assessment method of deriving the soil environmental safety threshold of wheat: bioaccumulation factor of normalization, selecting the appropriate cumulative probability function, soil threshold model of heavy metal(1) According to the prediction equation of each species for heavy metal, the bioaccumulation factor of eight varieties planted in red soil and Lou soil was normalized to various standard soil scenario(pH and OC), the results showed that the variation of bioaccumulation factor of 8 varieties decreased significantly after normalization, which were reduced 5.13-7.9, 2.39-2.96 times(integrated total and added method) compared to non-normalization, indicating that the normalization process can reduce the influence of soil properties to plant uptake of metals, and resolve the problem that the toxicology data from different experimental conditions before applying species sensitivity distribution method for calculating soil threshold.(2) Compare the fitting accuracy of soil Cd, Pb critical value of eight varieties using the different cumulative probability distribution function(Burr â…¢, Log-normal, Log-logistic, Gamma), the results indicated that whether in acidic, neutral or alkaline soil, Burr â…¢ was best prediction for soil critical value(Integrated total and added method). When cumulative probability was low, the effect of 4 functions fitting was overall close to each other for the Cd treatment, especially for calculation of Log-normal and Log-logistic, and the soil Cd critical value for Burr â…¢ function was moderate; while the fitting results of 4 kinds of function differed slightly, in which soil Pb threshold of Burr â…¢ was less conservative for the Pb treatment.(3) According to the Cd, Pb toxicity prediction model of wheat grain, we constructed species sensitivity curves of eight wheat varieties based on Burr â…¢ under typically acidic, neutral, alkaline soil conditions, finally, the soil Cd, Pb threshold model were established based upon the cumulative probability distribution, the results indicated that the soil threshold models calculated by different concentrations of heavy metals approached to the national soil environmental quality standard, while added method was slightly better than the total method. Our continuous soil threshold equations for Cd, Pb based on soil properties were more scientific than the present soil standard which set a fixed value for acidic, neutral and alkaline soil, respectively, and the research can provide reference information for the development of food safety of wheat and soil criteria of heavy metal.