Enzyme activities in soil treated with sulfite- or sulfate-based flue gas desulfurization products

A study was conducted to examine the impact of synthetic calcium sulfite (CaSO3 · 0.5H2O) and calcium sulfate (CaSO4 · 2H2O, i.e., gypsum) from flue gas desulfurization (FGD) on soil enzyme activities, used as soil quality indicators, following land application. We used application rates of 0, 1.12 Mg ha−1 of FGD-CaSO3 (CaS-1.12T), 3.36 Mg ha−1 of FGD-CaSO3 (CaS-3.36T), and 1.12 Mg ha−1 of FGD-gypsum (Gyp-1.12T). The field experiments were conducted on a silt loam soil with 1.6 % total C and a pH of 6.5. No crop was grown during the experiment. The concentration of water soluble SO4 2−-S and the associated electrical conductivity (EC) significantly increased during the first 2 weeks following gypsum application but decreased later on (2–4 weeks) due to leaching. There was no significant difference between the Gyp-1.12T and CaS-1.12T treatment after approximately 2 weeks indicating all of the sulfite in the CaS-1.12T treatment was completely oxidized to sulfate. In contrast, the sulfite treated with highest application rate (CaS-3.36T) was not completely oxidized within the 12-week experimental period. Application of FGD-CaSO3 and FGD-CaSO4 did not negatively affect C and N mineralization because β-glucosidase and β-d-glucosaminidase activities did not show any negative change compared with the control. Arylsulfatase activity of the CaS-1.12T treatment markedly declined compared with Gyp-1.12T and control, but then increased after 2 weeks. The arylsulfatase activity reduction, however, persisted for the duration of the experiment in the CaS-3.36T treatment. These activity changes were similar to the observed oxidation pattern of sulfite to sulfate, which indicated that arylsulfatase activity could be used as an indicator for the extent of sulfite oxidation in soil.