Determination Of Soil Cadmium Safety Threshold Based On Quadratic Discriminant Analysis Model

Soil heavy metal threshold refers to the content of heavy metals in soil without affecting the yield and biological quality of agricultural products and the maximum amount of surface water and groundwater pollution.The main purpose of this paper is to establish a risk assessment model for assessing the risk of Cd in rice and then confirm soil cadmium safety threshold.We started from three aspects to achieve this goal that the selection of soil-rice system cadmium transfer indexes by k-means algorithm,the establishment of rice cadmium safety risk assessment model by quadratic discriminant analysis model(QDA),and the calculation of soil cadmium safety thresholds by decision surface formula and kriging interpolation using the data of 2012-2014 in Xiang Tan.The calculation of soil cadmium safety threshold provides a basis for the evaluation of the adaptability of agricultural products and the division of the forbidden region.The final result as follows:(1)The cadmium transfer accumulation index of the soil-rice system constructed in the study was divided into three levels.The first-level indicator included three indicators: soil cadmium content,soil cadmium activity,and rice cadmium accumulation.The second-level indicator included pollution sources,soil physical and chemical properties,synergetic cations,accompanying anions,crops,hydrology,meteorology and soil cadmium content indicator containing soil background content,precipitation,irrigation water,atmosphere,inputs.The third-level indicators were the detailed indicators which can be monitored or surveyed,including 132 specific indicators.(2)Three indicators including soil pH,soil cadmium and organic matter were selected by K-means clustering,and the three selected indicators were proved effective and can be used to represent the information of all indicators by k-means verification.(3)A quadratic discriminant analysis model was established to assess the risk of rice cadmium with pH,soil organic matter,soil cation exchange capacity,and soil cadmium content,and the decision tree model and logistic regression model were used as comparison models.The accuracy of the three models were calculated and the final result was: the accuracy of the quadratic discriminant analysis model was 74%,while the accuracy of the decision tree and the logistic regression model were 66 % and 68 %,respectively.There was no significant difference in the stability of the three models.(4)The correlation between soil pH and other three factors(soil organic matter,soil cation exchange capacity,soil cadmium)was higher in the quadratic discriminant analysis model lower accuracy prediction group than in the higher accuracy group;The correlation coefficients between soil organic matter and soil cation exchange capacity,soil cadmium were higher in the quadratic discriminant analysis model lower accuracy prediction group than in the higher accuracy group.The correlation between soil cation exchange capacity and soil cadmium was higher in the quadratic discriminant analysis model lower accuracy prediction group than in the higher accuracy group,but the correlation coefficient was negative.(5)The soil cadmium safety threshold was backcalculated by the quadratic discriminant analysis model.The results showed that more than 80% soil cadmium safety thresholds were less than 0.3 mg/kg,which was less than the secondary standard specified in the Soil Environmental Quality Standard.380 groups calculated soils cadmium threshold data were compared with the corresponding soil cadmium content data,and the rationality of the soil cadmium threshold predicted by the second discriminant analysis model backdating algorithm was tested.The results showed that 66 % of soil cadmium thresholds reversed by the second discriminant analysis model was reasonable.The soil cadmium safety thresholds of the whole area ranged from 0.15 to 0.40 mg/kg,mainly concentrated in 0.2-0.3 mg/kg and was few above 0.3mg/kg.50 points in 2014 were used to verify the rationality of soil cadmium safety threshold interpolation results.The results showed that the reasonable number of points was 35 and the reasonality was 75 %.