Testing Carbon Sequestration in Soil Through the Addition of Gypsum

In order to help control adverse effects of increased atmospheric concentrations of CO2, effective methods for fixing carbon need to be developed. Given the large C inventories and fluxes associated with soils, it is important to identify cost- and energy-effective means for increasing long-term C retention within soil profiles. This study investigates the alternative strategy of increasing carbon retention in soils through accelerating calcite (CaCO3) precipitation and promoting soil organic carbon (SOC) complexation on mineral surfaces. With the addition of calcium ion to soils with pH > 8 often found in arid and semi-arid regions, the slow process of calcite precipitation may be accelerated. Calcium also promotes SOC binding onto mineral surfaces, diminishing leaching of SOC. Addition of flue gas desulfurization gypsum (FGDG) represents an inexpensive source of calcium to natural, slightly alkaline soil surfaces which might promote the fixation of CO2 as calcite and decrease leaching losses of organic carbon. To test this hypothesis, we prepared laboratory soil columns (7.5 cm in diameter and 85 cm in height) with and without calcium sulfate-amended layers. The distribution of carbon in the columns was monitored in gaseous, aqueous and solid phases over a period of several months to test the effect of adding calcium ions. In some columns, a relatively high fraction of 13C-labeled bicarbonate was injected to differentiate the newly precipitated calcite from the initial calcite present in the soil. The potential for more distinct calcite precipitation within the soil root zone will be investigated in vegetated soil columns. Through obtaining C mass balances in soil profiles, this study is quantifying the efficiency of gypsum amendments for mitigating C losses to the atmosphere.