Assessment on Zea diploperennis L. as bio-indicator of CO2 leakage from geological storage

Abstract: Carbon capture and storage (CCS) projects can sequester anthropogenic carbon dioxide (CO2) in deep geological formations and avoid emissions into the atmosphere while supporting coal use. Thus, the technology is an attractive way of controlling greenhouse gases in the countries whose economies heavily depend on coal energy, such as China, whose goal is to reach an emissions cap by 2030. Currently, more than 12 CCS demonstration projects are in development in China. The global first full-ranged demonstration project, Shenhua Ordos CCS project, which has captured CO2 from the first worldwide large-scale direct coal-to-liquid plant and injected cumulatively 300 000 t CO2 into the saline aquifer from 2010 to 2015, has developed long-term monitoring and early warning work flow, and the ground and above-ground monitoring schemes were designed for Shenhua CO2 project and have been proved to be safe so far. However, leakage of stored CO2, through gas permeable channels/faults in the caprock or failure of injection wells, has a significant adverse effect on plant growth and development. Thus, the risks of leakage and its subsequent effects on land degradation must be carefully evaluated. The most visible impact of CCS leakage is the degradation of plant cover. It is urged to select suitable bio-indicator for leakage of stored CO2. However, there is little knowledge about this. A carefully designed field experiment of CO2 shallow release of Zea diploperennis L. was put into effect to reveal the impacts of leaked CO2 in Yanqing, Beijing. In this study, we proposed a new index (leaking CO2 tolerance index, LCTI) to assess the tolerance of Zea diploperennis L. for leakage of stored CO2 through pot experiment. The application of the LCTI (higher values indicate more tolerance and lower values indicate more sensitivity) revealed that the plant height of Zea diploperennis L. decreased with the elevated CO2 leakage (500, 1 000, 1 500, and 2 000 g/(m2·d)). Measurement of maize leaf photosynthesis and transpiration parameters involved net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and water use efficiency (WUE), and the observational values of Pn, Gs, Tr, and WUE were reduced with the increasing of CO2 leakage. And the biomass of Zea diploperennis L. declined by more than 70%. The LCTI of Zea diploperennis L. was 0.26, which should be classed as less tolerant, and cannot be used as bio-indicator for CO2 leakage. The LCTI method is preliminary verified by our experiment which is suitable for selection of bio-indicator for leakage of stored CO2. In the future, for robustness of LCTI, we suggest to add plant management and growth cycle into LCTI assessment.