Studying the vegetation response to simulated leakage of sequestered CO2 using spectral vegetation indices

a r t i c l e i n f o Measurement of spectral reflectance provides a fast and nondestructive method of stress detection in vegetation. In this shallow subsurface CO 2 release experiment to simulate CO 2 leakage of geologically sequestered CO 2 , the radiometric responses of plants to elevated soil CO 2 concentration were monitored using a spectroradiometer. Spectral responses included increased reflectance in the visible spectral region and decreased reflectance in the near-infrared region and thus an altered spectral pattern of vegetation. Visible responses of vegetation include purple discoloration and eventual death of leaves at sites where the soil CO 2 concentration was very high. Derivative analysis identified two features (minimum and maximum) in the 575–580 nm and 720–723 nm spectral regions. The normalized difference first derivative index (NFDI) was defined based on the spectral derivative at the two bands. Four vegetation indices were analyzed with the accumulated soil CO 2 concentration to assess the accumulated impact of high soil CO 2 concentration on vegetation. Results show that with increased soil CO 2 concentration due to the surface CO 2 leakage, (1) the structural independent pigment index (SIPI) increased, indicating a high carotenoid to chlorophyll ratio; (2) the chlorophyll normalized difference vegetation index (Chl NDI) decreased, suggesting a decrease in chlorophyll content with time; (3) pigment specific simple ratios (both PSSRa and PSSRb) were reduced for stressed vegetation compared to that at the control site, indicating a reduction in both chlorophyll a and chlorophyll b; and (4) NFDI was low where plants were stressed. Changes in NFDI during the experiment were 36% and 1% for stressed and control plants, respectively. All four indices were found to be sensitive to stress in vegetation induced by high soil CO 2 concentration. One of the potential options to mitigate the contribution of fossil fuel emissions to the global warming problem is to capture and store CO 2 in deep geological formations such as depleted oil and gas fields, unminable coal beds, and deep saline formations (IPCC, 2005). However, associated with the safety of CO 2 storage, the extent of CO 2 leakage is one of the key questions related to the integrity of storage (Hepple and Benson, 2005). Leakage of sequestered CO 2 adds one more potential source of CO 2 leakage to the atmosphere (Holloway, 2005), along with various other biogenic and geological sources. It is imperative to …