Geochemical and microbial functional responses of surface soil to simulated low-concentration CO₂ leakage from...

Geological CO₂ storage may pose environmental risks if leaked CO₂ migrates into near-surface soils. To evaluate early surface-soil responses to low-concentration CO₂ exposure, a 42-day aerated soil microcosm experiment was conducted using a control group and two CO₂ treatment levels of 2000 and 10,000 ppm. Soil physicochemical properties, dissolved cations, metal fractions, enzyme activities, bacterial community composition, and metagenomic functional profiles were analyzed. Sustained CO₂ exposure increased electrical conductivity and HCO₃^- concentrations, whereas soil pH remained within a narrow weakly alkaline range. Sequential extraction showed limited redistribution of selected metals among operationally defined fractions, but no evidence of extensive metal mobilization was observed. Among microbial indicators, FDA hydrolase activity responded significantly to CO₂ exposure, whereas microbial community structure, alpha diversity, and overall KEGG and CAZy functional profiles remained largely stable. Representative carbon- and nitrogen-cycling genes were influenced mainly by incubation time rather than CO₂ concentration. Under the tested short-term, low-concentration, aerated microcosm conditions, the soil system exhibited considerable buffering capacity and resistance to CO₂ exposure. The observed effects were mainly expressed as minor changes in soil solution chemistry and selected functional indicators rather than pronounced geochemical deterioration or microbial community restructuring. These findings provide experimental evidence and insights into the geochemical buffering capacity and microbial response mechanisms of surface soil systems under potential leakage scenarios of underground CO₂ storage. The findings offer scientific references for environmental risk assessment of CO₂ geological sequestration and the selection and interpretation of sensitive monitoring indicators.