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 SO₃^2- to SO₄^2- 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 SO₄^2- increased significantly with increasing FGD-CaSO₃ application rates in both soils, indicating rapid oxidation of SO₃^2- to SO₄^2- 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 SO₄^2--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.