Effects of Prior Thermal Exposure of a Soil–Compost System on Potentially Bioavailable Mercury and Its Accumulation in Rice Grown on Mining-Impacted Soils

Mercury (Hg) mobility in contaminated soils may be altered by solarization-induced thermal disturbance and organic amendments. This study evaluated whether a cover-induced prior solarization pretreatment of a mining-impacted soil–compost system modifies the operationally defined labile Hg fraction (F1 + F2, considered potentially bioavailable), rice productivity, grain Hg accumulation, and screening-level health risk. Mining soils from San Jorge River basin were amended with compost at a 1:9 ratio and solarized for one month before crop establishment, generating temperatures of 32, 40, 44, and 50 °C under different cover conditions. After pretreatment, covers were removed and rice was cultivated for four months under uniform flooded conditions. Total Hg in soils and grains was determined by EPA Method 7473, and the potentially bioavailable fraction by modified Bloom extraction. Prior thermal exposure increased this fraction, especially at 50 °C, whereas compost reduced total Hg and partially buffered this increase. Productivity depended on the compost × temperature interaction: compost improved yield at 40 and 44 °C, while no grain production occurred in substrates previously exposed to 50 °C. Compost-amended treatments showed lower grain Hg than unamended soils, although all values exceeded the 20 μg kg−1 reference limit. Hazard quotients remained below 1, with the highest value in unamended soil previously exposed to 44 °C.