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 SO32− to SO42− 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−1 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−1 was used as a positive control. Exchangeable Ca2+ and water-extractable Ca2+ and SO42− increased significantly with increasing FGD-CaSO3 application rates in both soils, indicating rapid oxidation of SO32− to SO42− 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−1 to the pH 6.2 soil produced no effect on wheat growth or the uptake of N, P, Ca2+, and Mg2+. The uptake of SO42−-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−1 FGD-CaSO3 increased wheat root growth and dry matter yield compared with an untreated control. The uptake of N, P, Ca2+, and K+ also increased presumably because of enhanced root development resulting from decreases in exchangeable Al3+ and increases in soluble Ca2+. Wheat growth and alkaline phosphatase and arylsulfatase activities were significantly inhibited by addition of 8.8 Mg ha−1 of FGD-CaSO3 compared with the untreated control or the same soil receiving 2.2 Mg ha−1 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 Ca2+ into acid subsoils. Moreover, application rates of equal to or less than 4.4 Mg ha−1 should have no negative impact on soil microbial activities or plant growth.