Changes in soil N₂O and CH₄ emissions and related microbial functional groups in an artificial CO₂ gassing experiment

Potential CO₂ leakage is a major concern for carbon capture and storage (CCS). The effects of high soil CO₂ concentration on microbes is a major element of impact assessments of CO₂ leakage on terrestrial ecosystems. We conducted a field experiment to investigate the responses of microbial functional groups of ammonia-oxidizers, methanogens, and methanotrophs in high soil CO₂ conditions. A single-point injection gassing plot (2.5 m × 2.5 m in size), which had 52.2% CO₂ in the center (radius = 0.5 m) and 5.5% in the edge (radius = 1.7 m) at 10 cm depth, was employed. N₂O and CH₄ emissions increased after 1 day of injection because injected CO₂ was instantly utilized by nitrifiers and methanogens. This suggests that the activities of the selected microbes could be stimulated by high soil CO₂ concentrations. Prolonged CO₂ injection has toxic effects on aerobic nitrifiers, but may favor anaerobic methanogens. However, the early stimulatory effects of high soil CO₂ on N₂O and CH₄ production did not last to the end of injection. These results imply that increased N₂O and CH₄ emissions could be the minor side effects of high soil CO₂. Microbes responded faster than plants to high soil CO₂, with responses observed as late as 7 days after injection. The inhibition of plant absorption of soil water and nutrients by high soil CO₂ concentrations may also influence microbial responses. Moreover, high soil water content could retard underground CO₂ diffusion, which would magnify CO₂ impacts on plants and microbes. Our results suggest that microbial response could be used as an early indicator of the impact assessments of CO₂ leakage on soil ecosystems. An understanding of the interaction among soils, plants, and microbes would be helpful in assessing the biological risks of potential CO₂ leakage.