Effects of natural non-volcanic CO₂ leakage on soil microbial community composition and diversity

Geological carbon capture and storage (CCS) can reduce anthropogenic CO₂ emissions, but questions exist about impacts at the surface if CO₂ leaks from deep storage reservoirs. To examine potential impacts on soils, previous studies have investigated the geochemistry and microbiology of volcanic soils hosting high fluxes of CO₂ rich gas. This study builds on those previous investigations by considering impacts of CO₂ leakage at a non-volcanic site, where deep geogenic CO₂ leaks from a cracked well casing. At the site, we collected 26 soil cores adjacent to soil gas monitoring wells. Based on measured CO₂ fluxes, the soil samples fall into two groups 1) high CO₂ (flux = 304.6 ± 272.1 g m^-2 d^-1, conc. = 29.1 ± 34 %) and 2) low CO₂ (flux = 15.8 ± 6.1 g m^-2 d^-1, conc. = 0.8 ± 0.9 %). Soil pH was significantly lower (p < 0.05) in high flux group samples (4.6 ± 0.3) than the low flux ones (5.3 ± 0.7). Beta diversity calculations using 16S rRNA gene sequences and redundancy analysis (RDA) revealed clear clustering of microbial communities relative to CO₂ flux and significant correlations of community composition with pH and organic carbon content. In the high flux soils, abundant microbial groups included Acidobacteriota, Ktedonobacteria, and SC-I-84 in the phylum Proteobacteria, as well as Nitrososphaeria, a genus of ammonia oxidizing archaea. Compared to volcanic sites described previously, our non-volcanic site had slight differences in soil geochemical properties and gradual shifts in community compositions between CO₂ hotspots and background locations. Moreover, the elevated abundance of SC-I-84 has not been reported in studies of volcanic sites. This study improves our ability to predict potential environmental impacts of geological CCS by expanding the range of conditions over which existing CO₂ leakage has been observed.