Evaluation of the co-application of fly ash and sewage sludge on soil biological and biochemical quality

Disposal of sewage sludge (SS) and fly ash (FA) is a multifaceted problem, which can affect environmental quality. FA has the potential to stabilize SS by reducing metal availability and making the SS suitable for application in the agricultural sector. An experiment was performed to evaluate soil biological quality changes with the combined amendment of SS and FA (fluidized bed combustion ash (FBCA) and lignite fly ash (LFA)). SS was amended with 0, 10, 30, 50 and 100%, (w/w) of FA, and then the FA–SS mixtures were incubated with red soil at 1:1 (v/v). Soil quality parameters such as pH, electrical conductivity, N, soil enzyme activities such as dehydrogenase (DHA), urease (URE), and catalase (CAT), and microbial biomass carbon (MBC) were evaluated at 20, 30, and 60 days of incubation. pH and EC increased with FA–SS dose; however, N decreased. DHA and URE were found to be increased with 10% LFA amendment; thereafter it decreased. However, URE increased up to 30% of FBCA. CAT and MBC increased with both FA amendments, even up to addition of 50% FA. Bioavailable Zn, Cu, and Co contents were decreased by the addition of FA. Principal component analysis showed that pH is the most influential factor. MBC appears to be a sensitive soil indicator for the effects that result from the addition of FA–SS. Phytotoxicity studies with Zea mays showed optimum performance at 30% FA. Addition of 10–30% FBCA or LFA to SS has a positive advantage on soil biological quality