Changes in soil NO and CH emissions and related microbial functional groups in an artificial CO gassing experiment.

Potential CO leakage is a major concern for carbon capture and storage (CCS). The effects of high soil CO concentration on microbes is a major element of impact assessments of CO 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 CO conditions. A single-point injection gassing plot (2.5 m × 2.5 m in size), which had 52.2% CO in the center (radius = 0.5 m) and 5.5% in the edge (radius = 1.7 m) at 10 cm depth, was employed. NO and CH emissions increased after 1 day of injection because injected CO was instantly utilized by nitrifiers and methanogens. This suggests that the activities of the selected microbes could be stimulated by high soil CO concentrations. Prolonged CO injection has toxic effects on aerobic nitrifiers, but may favor anaerobic methanogens. However, the early stimulatory effects of high soil CO on NO and CH production did not last to the end of injection. These results imply that increased NO and CH emissions could be the minor side effects of high soil CO. Microbes responded faster than plants to high soil CO, with responses observed as late as 7 days after injection. The inhibition of plant absorption of soil water and nutrients by high soil CO concentrations may also influence microbial responses. Moreover, high soil water content could retard underground CO diffusion, which would magnify CO impacts on plants and microbes. Our results suggest that microbial response could be used as an early indicator of the impact assessments of CO leakage on soil ecosystems. An understanding of the interaction among soils, plants, and microbes would be helpful in assessing the biological risks of potential CO leakage.