Testing a New Carbon Sequestration Strategy by Accelerating Calcite Precipitation in Soils

 

Tetsu Tokunaga, Jiamin Wan, and Young Soo Han—ESD

Effective methods for fixing carbon need to be developed o help control the adverse effects of increased atmospheric CO₂concentrations. Unlike very deep subsurface reservoirs now being developed for geologic CO₂ sequestration, surface soils are readily accessible, and hence are potentially more economical way of sequestering carbon. Soil, the major reservoir of carbon on the Earth’s surface, also contributes a significant CO₂ flux to the atmosphere through organic carbon oxidation from microbial and plant root respiration. Thus, the possibility of transforming some soils into long-term C sinks appears worthy of investigation. While numerous recent studies have highlighted the possibility of increasing soil organic carbon (SOC) retention in order to sequester C, SOC oxidation and CO₂ releases back into the atmosphere remain major components of the C cycle. Therefore, sustainable pathways for soil C sequestration have yet to be identified.

In their ongoing LDRD project, ESD’s Tetsu Tokunaga, Jiamin Wan, and recent ESD hire Young Soo Han are pursuing the alternative strategy of increasing C retention in soils through accelerating calcite precipitation and promoting SOC complexation (binding) on mineral surfaces. The long-term accumulation of calcite in some arid and semiarid soils suggests that increases in inorganic soil C can be large. When present, the soil carbonate mineral fraction typically takes the form of calcium carbonate (CaCO₃, as calcite), which often amounts to several percent of the total soil mass, making it the dominant fraction of the soil C inventory in many soils. Given the commonly elevated CO₂ partial pressures in soils, calcite precipitation is most favorable at and above neutral pH, if sufficiently high calcium ion concentrations are present. Calcium also promotes SOC binding onto mineral surfaces, and thus diminishes the leaching losses of SOC. Consequently, identifying an inexpensive supply of calcium is also an important requirement. We have selected gypsum, calcium sulfate dihydrate, as a candidate soil amendment for promoting C retention in soils, because of its low cost and long history of agricultural application for improving soil structure. Moreover, gypsum is a byproduct of the flue gas desulfurization process used to remove sulfur dioxide from exhaust gases in fossil fuel power plants.

The overall scientific goal of this study is to identify conditions in which inorganic and organic C sequestration is practical in semi-arid and arid soils. The possibility of sequestering C within and immediately below the rhizosphere (root zone) of biomass energy fuel crops, such as miscanthus and switchgrass, and in rangeland ecosystems is attractive—because it (1) employs pathways that have high CaCO₃ capacity in neutral to alkaline soils, (2) diminishes soil respiratory CO₂ fluxes back to the atmosphere, and (3) can have synergistic effects on both biomass production and SOC stability. Laboratory soil column experiments are currently being designed to obtain C mass balances under several different conditions, including rates of Ca (gypsum) supply, presence/absence of CaCO₃, and levels of bicarbonate/carbonate in soil pore waters. Macroscopic chemical analyses to be obtained include pore water and effluent chemistry (major ions and dissolve organic C), soil mineralogy, calcite and gypsum content profiles, SOC content profiles, and gas analyses. Modeling of these systems will include gypsum dissolution, calcite precipitation/dissolution, and SOC-mineral complexation reactions. Microspectroscopic analyses will be conducted for understanding basic mineral transformation processes.

The anticipated general results will enable the investigators to determine the extent of C fluxes in soils, as well as the extent to which C mass balances in soil profiles can be controlled through varying Ca supply.

Soil carbon cycle, with proposed increased C retention by calcite precipitation and by SOC binding onto soil mineral surfaces, with both processes driven by calcium released from gypsum dissolution.