Physiological responses of dandelion and orchard grass leaves to experimentally released upwelling soil CO2

Carbon sequestration is an important means of reducing the concentration of atmospheric CO2 by injecting CO2 into subterranean geological reservoirs known as sequestration fields. These deep sequestration fields must be monitored for structural integrity to ensure their long-term efficacy. Stress responses of plants to super-elevated soil CO2 are signatures that we evaluated as potential tools for surface leak detection of CO2 at the Zero Emission Research and Technology (ZERT) site in Bozeman, MT. To mimic a compromised field, CO2 was deliberately released at a rate of 0.15 ton/day through a 100 m long and 2.0-2.3 m deep Horizontal Injection Well (HIW) with intentional leaks, with injections of CO2 taking place on these dates: 7/19-8/15/2010 and 7/18-8/15/2011. Periodically over two years, we measured stomatal conductance, chlorophyll content, and specific leaf area in Taraxacum officinale Wigg. (dandelion) and Dactylis glomerata L. (orchard grass) along a 20 m transect perpendicular to a hot spot. Hot spots were visible as circular zones of leaf dieback where T. officinale leaves turned from green to red and became brown and desiccated. CO2 concentrations reached 29% by volume at the edges of hot spots. During CO2 injection, chlorophyll content (as measured with a Hansatech CL-O1 chlorophyll content meter to obtain a unitless measure) decreased significantly (p < 0.001) in T. officinale from 9.486 to 1.912 (80%) in 2010 and from 8.288 to 0.000 (99%) in 2011. The less pronounced decreases in chlorophyll content with CO2 injection in D. glomerata were 34% in 2010 and 37% in 2011. Average stomatal conductance (gs) rates in T. officinale were highest at the hot spot (22.41 mmol m−2 s−1 in 2010, 46.10 mmol m−2 s−1 in 2011) and lowest 20 m distally (11.76 mmol m−2 s−1 in 2010 and 3.69 mmol m−2 s−1 in 2011). Average gs rates in D. glomerata were highest at the hot spot (53.91 mmol m−2 s−1 in 2010 and 65.96 mmol m−2 s−1 in 2011), and lowest 20 m distally (38.52 mmol m−2 s−1 in 2010 and 24.83 mmol m−2 s−1 in 2011). These results suggest T. officinale leaves are more sensitive to super-elevated soil CO2 concentration than D. glomerata leaves and generally, that species-specific physiological responses of these leaves to varying concentrations of soil CO2 may be useful in monitoring CO2 leakage in vegetated areas above CO2 storage sites.