Soil Microbial Responses to Artificial CO₂ Leakage in Controlled Field Experiments in South Korea

The potential environmental impacts of CO₂ leakage from carbon capture and storage (CCS) sites remain a critical concern for the long-term viability of CCS technologies. Soil microbial communities, which regulate essential biogeochemical processes, are particularly sensitive to changes in soil CO₂ concentrations. In this study, controlled CO₂ release experiments at two depths, 2.5 m (shallow subsurface) and 0.5 m (near-surface) in a field setting in Eumseong, South Korea, were conducted in 2016 and 2017, respectively, to evaluate soil microbial and biogeochemical responses to elevated CO₂ exposure. In the 2.5-m depth experiment, despite substantial CO₂ enrichment at 60-cm soil depth (peaking at 21.2%), minimal CO₂ accumulation occurred at the surface (15-cm depth: <2.6%), and no significant changes were observed in microbial enzyme activities, inorganic nitrogen concentrations, or microbial biomass carbon. These results suggest that seasonal factors such as rainfall exerted stronger influence than the CO₂ exposure. In contrast, the 0.5-m depth experiment produced highly localized CO₂ accumulation at the surface (center: 52.2%), resulting in hypoxic conditions (O₂: 10.5%). Under these conditions, N-acetyl-glucosaminidase activity declined significantly, and ammonium and nitrate concentrations showed spatial and temporal shifts likely driven by inhibited plant uptake and altered microbial nitrogen cycling. These findings demonstrate that microbial responses to CO₂ leakage are highly depth- and exposure-dependent. Monitoring microbial functional indicators, particularly N-acetyl-glucosaminidase activity and nitrogen metrics, along with gas flux measurements, can improve early detection strategies for the assessment of CCS site integrity.