Effects of thermally dried and composted sewage sludges on the fertility of residual soils from limestone quarries
Abstract Limestone quarrying reduces the land's capacity to support a complete functional ecosystem. Adding sewage sludge to mining residues facilitates the establishment of a vegetation cover and can stimulate C and N cycling. We aimed to evaluate the effects of three composted and three thermally dried sewage sludges, on some biological properties of two types of debris (extraction soil and trituration soil) from a limestone quarry. Lysimeters filled with debris-sludge mixtures and control soils were sampled immediately after preparation and after being left in the open for 13 months. Total carbohydrates (TCH), 0.5 M K2SO4 extractable (ECH) carbohydrates, 0.5 M K2SO4 extractable organic C (EOC), microbial biomass carbon (MBC), microbial respiration (MR), β-glucosidase activity and β-galactosidase activity were determined immediately after sampling. The treated soils were also analyzed for their more general physicochemical characteristics. Adding sewage sludge clearly improved the physicochemical and biological properties of the residual soil and the effect of the type of sludge was greater than that of the type of soil. The sludge effect was generally more durable over the trituration soil. The sludge effect decreased the most in MR and EOC followed by MBC and ECH. Total carbohydrates showed the least enhancement but the sludge effect on this endpoint had the smaller decrease with time. Root exudates and plant debris contributed to β-glucosidase and β-galactosidase activities in the treated soils. Activities present in mixtures partly corresponded to enzymes free in the soil aqueous face. β-Glucosidase was also partly associated with humified organic matter. Thirteen months after sludge addition a fraction of the organic matter present in soils was still moderately labile. Results observed in BMC and MR suggests the sludge did not cause major toxic effects on residual soils. The sludge effect differed with the pre and post treatments of the sludges; thermal drying made the sludge organic matter more easily decomposable.