[Characteristics and Comprehensive Quality Assessment of Heavy Metals in Soil-crop System of High Geological Background Area]

Heavy metals （HMs） pollution in agricultural soil-rice systems has attracted worldwide attention as it directly threatens regional ecological security and human health. To understand the heavy metal pollution of agriculture soil and rice in the high geological background areas, a total of 200 paddy soil and rice samples were collected in southeast Chongqing. The concentrations of arsenic （As）, cadmium （Cd）, chromium （Cr）, copper （Cu）, mercury （Hg）, nickel （Ni）, lead （Pb）, and zinc （Zn） in paddy soil and rice were analyzed. The comprehensive impact index （IICQ） was used to evaluate the quality of heavy metals in soil-rice. The results showed that the mean concentrations of As, Cd, Cr, Hg, and Pb in soil were higher than their corresponding background values of Chongqing soil and the average values of Chinese surface soil, indicating the accumulation of HMs enrichment of these metals in the paddy soils. Compared with China's soil environmental quality standard of agricultural land （GB 15618-2018）, only 20.5% of rice samples containing Cd exceeded the national food safety standard （GB 2762-2022）, though the Cd and Hg sample contamination rates of the sampling sites were 82% and 22.5%, respectively. The evaluation results of heavy metal pollution, such as Cd in soil and crops, were quite different. With the increase in soil pH value, the average content of Cd in the soil gradually increased, but the average content of Cd in rice seeds decreased by 0.245 mg·kg^-1, and the excess rate of rice seeds decreased by 42.14%. The Cd content in rice seeds was significantly correlated with the Cd ion exchange state and water-soluble state in the soil. The higher the soil pH value, the lower the proportion of bioactive components in the soil, and the pH of the soil was the main factor affecting the safety of rice in the study area. The comprehensive quality evaluation results showed that moderate and severe pollution were the main pollution levels in the study area, but most of the rice seeds were in a clean state, a few were slightly polluted, and none showed mild or above pollution. The basic safety of rice was in the alkaline area with a pH>7.5. In strongly acidic soil with pH<5.5 and below, the excess rate of rice seeds could reach 45%. Moreover, it is recommended to apply quicklime or calcareous soil conditioner to improve soil acidification, reduce the activity of Cd bioactive components, and improve the safety of rice and other agricultural products.