[Heavy Metal Pollution Evaluation and Key Restricting Factors Analysis of the Soil-rice System in the Typical Rice...

A total of 180 paired soil-rice samples were collected from two districts in the western grain-producing area of Chongqing， using a systematic stratified sampling method. The study evaluated the contamination status of five heavy metals-cadmium （Cd）， lead （Pb）， chromium （Cr）， mercury （Hg）， and arsenic （As）-in both the regional soil and rice. The study also analyzed key soil physical and chemical properties， such as soil pH， organic matter （OM）， and cation exchange capacity （CEC）， to explore the critical soil factors that constrain the accumulation of heavy metals in rice. The results showed that among the five heavy metals， only Cd and Pb exceeded the standard limits， with exceedance rates of 9.4% and 0.5%， respectively. The spatial distribution map of soil heavy metals generated through the spatial interpolation method indicated that Cd exhibited significant spatial variation within the study area， showing a "dual-core" pattern of high-concentration areas in the northeast and southwest. Cr concentrations were higher in the northern and central areas， but overall， the concentrations were close to background values， while the distribution of other elements was more uniform. In rice grains， the exceedance rate of Cd was 10%， indicating a higher bioaccumulation potential and greater potential harm. The exceedance rate of Cr was 2.23%. The comprehensive quality index of the soil-rice system showed that the overall pollution level of the region was relatively low， with no severe pollution observed. The majority of areas were classified as clean or lightly polluted， accounting for 49% and 31%， respectively. In the correlation analysis of Cd accumulation in rice， it was found that soil organic matter （OM） was the main factor for reducing the bioavailability of Cd （r=-0.27， P<0.001）， and total phosphorus （TP） also exhibited a significant inhibitory effect on Cd accumulation （r=-0.15， P<0.05）. Therefore， managing soil organic matter and applying phosphorus fertilizer appropriately could reduce the transfer of Cd to rice grains. The comprehensive quality evaluation of the soil-rice system showed that the regional pollution level was generally controllable， but some areas require dynamic monitoring and remediation measures to ensure food safety and ecological health.