Rising temperature and atmospheric CO2 combine to antagonistically alter Cd mobility and biogeochemistry in an agricultural soil.

Soil cadmium (Cd) contamination threatens ecosystems and crop safety. Understanding how individual climate change factors influence soil Cd bioavailability is essential for mechanistic understanding and future risk assessments. This study examined individual and combined effects of elevated temperature (+4 °C) and doubled atmospheric CO2 (800 ppmv) on soil Cd bioavailability, biogeochemistry, and greenhouse gas emissions in agricultural soils with native (0.13 mg Cd kg-1) and high Cd (1.5 mg Cd kg-1). Elevated temperature increased porewater Cd up to 50 % relative to ambient, while doubled atmospheric CO2 did not alter porewater Cd. Combined future conditions increased porewater Cd by 30 % relative to ambient indicating an antagonistic interaction. Doubled atmospheric CO2 enhanced microbial nitrogen fixation and reduced ammonium oxidation, increasing ammonium concentrations up to 10-fold relative to ambient. Elevated temperature stimulated microbiome activity and ammonium oxidation, leading to 1.7-fold more CO2 and 5.5-fold more N2O compared to ambient, both exceeding levels observed under combined future climate. These contrasting single-factor responses highlight the non-additive nature of combined climate factor effects. Warming alone overestimated and CO2 alone underestimated the combined impact on Cd mobility and soil biogeochemistry. Simulating multiple climate drivers is therefore essential for accurate environmental prediction and sustainable Cd management under climate change.