Impact of flue gas desulfurization-calcium sulfite and gypsum on soil microbial activity and wheat growth

Calcium sulfite (CaSO3·0.5H2O), a common by-product of coal combustion and flue gas desulfurization (FGD), spontaneously converts to gypsum (CaSO4·2H2O) with exposure to air and water. Although gypsum is a well-known soil amendment, the oxidation of SO3 to SO4 consumes oxygen and may have detrimental effects on plant root growth and soil microbial activity if FGD-CaSO3 is land applied. Because the oxidation rate is pH dependent, we conducted greenhouse tests to evaluate the effects of FGD-CaSO3 applied at rates of 0, 2.2, 4.4, and 8.8 Mg ha on wheat growth, soil enzyme activities, and the chemical properties of two soils with differing pH (4.0 vs. 6.2). A gypsum treatment applied at the rate of 2.2 Mg ha was used as a positive control. Exchangeable Ca and water-extractable Ca and SO4 increased significantly with increasing FGD-CaSO3 application rates in both soils, indicating rapid oxidation of SO3 to SO4 when neither water nor oxygen was limiting. No changes in soil pH were measured. Applications of 2.2, 4.4, or 8.8 Mg CaSO3 ha to the pH 6.2 soil produced no effect on wheat growth or the uptake of N, P, Ca, and Mg. The uptake of SO4-S increased, whereas K uptake decreased. No significant differences in the activities of urease, β-glucosidase, alkaline phosphatase, or arylsulfatase were observed relative to a control. In the acid soil, an application of 2.2 Mg ha FGD-CaSO3 increased wheat root growth and dry matter yield compared with an untreated control. The uptake of N, P, Ca, and K also increased presumably because of enhanced root development resulting from decreases in exchangeable Al and increases in soluble Ca. Wheat growth and alkaline phosphatase and arylsulfatase activities were significantly inhibited by addition of 8.8 Mg ha of FGD-CaSO3 compared with the untreated control or the same soil receiving 2.2 Mg ha gypsum. We conclude that surface applications of FGD-CaSO3 may be as effective as gypsum for inhibiting soil crusting, improving water infiltration, and promoting the movement of Ca into acid subsoils. Moreover, application rates of equal to or less than 4.4 Mg ha should have no negative impact on soil microbial activities or plant growth.