Elevated atmospheric CO2 reshapes soil metal(loid) mobility and stress responses in Enchytraeus crypticus: a multi-level analysis from molecular to population endpoints

Projected increases in atmospheric CO 2 may alter soil properties and modify the ecotoxicity of metal(loid)-contaminated soils, yet their combined effects on invertebrates remain poorly understood. This study evaluated how elevated atmospheric CO 2 (600–1000 ppm), encompassing levels projected under future IPCC scenarios by 2100, influences soil physicochemistry, metal(loid) availability, and the biological responses of Enchytraeus crypticus in control and metal(loid)-contaminated soils, relative to standard CO 2 conditions (400–500 ppm). A multi-level approach integrated soil parameters, metal(loid) bioaccumulation, molecular and biochemical responses, and survival and reproduction following 2- and 21-d exposures. Elevated CO 2 was associated with higher soil pH and element-specific shifts in metal(loid) mobility, particularly higher water-extractable Pb and Zn after 2 d. In contaminated soil, higher water-extractable Pb under elevated CO 2 was accompanied by higher body Pb, reaching significance at 1000 ppm CO 2 . Elevated CO 2 was also associated with lower total RNA content, higher MT1 and CAT1 expression at 1000 ppm CO 2 , and lower Hsp70 expression, while ETS activity showed limited variation and lipid peroxidation remained stable. Adult survival remained high across CO 2 conditions, whereas reproduction was strongly constrained by soil contamination. In control soil, juvenile production was lower at 600 ppm CO 2 than under standard conditions, but not at 800 or 1000 ppm, indicating no consistent concentration-dependent reproductive response. Overall, elevated atmospheric CO 2 modified soil metal(loid) mobility and elicited pathway-specific molecular and biochemical responses without a consistent effect on population-level endpoints. These findings highlight the importance of considering climate-driven changes in contaminant exposure and biological responses under future atmospheric conditions.