Ecological Risks of Heavy Metals in Soil‐Rice Systems of Carbonate‐Derived High Geochemical Background Areas in Eastern China

ABSTRACT Heavy metal contamination in soils poses substantial environmental and health risks, particularly in areas with high geochemical backgrounds. While extensive research has examined heavy metal pollution in karstic areas of southwestern China, similar studies in karstic regions of eastern China remain limited. This study investigated heavy metal risks in soil‐rice system derived from carbonate rocks in Zhejiang province, eastern China, characterised by mid‐subtropical monsoon climate. Analysis of eight heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) in 43 topsoil samples, 7 paddy soil profiles, and 15 rice grain samples revealed varying degrees of metal enrichment. Heavy metals in soil showed the following enrichment sequence: Cd (4.69) > As (1.72) = Hg (1.72) > Ni (1.49) > Cr (1.37) > Pb (1.16) > Zn (1.13), with Cu showing no enrichment. Among all metals, Cd demonstrated the most severe contamination, with 79.1% of samples exceeding farmland pollution thresholds. The comprehensive potential ecological risk index ranged from 24.4 to 131.4, which indicates slight to moderate contamination levels, with Cd as the primary contributor. Notably, 23.3% of samples had a Cd geo‐accumulation index > 2, and 48.8% showed moderate to high ecological risk levels, particularly in north‐ and south‐facing valleys. Rice grain analysis revealed that 60% of samples exceeded permissible Cd limits (0.2 mg/kg). Sequential extraction revealed that soil acidification from weathering processes increased Cd bioavailability from 9.05%–29.0% in parent material to 39.7%–54.8% in topsoil through systematic redistribution from stable to labile phases. These findings demonstrate that Cd pollution represents a major environmental concern in karstic areas and underscore the importance of considering geological context and pedogenic processes when assessing and managing heavy metal risks in agricultural soils under different climatic conditions.