The combination of quarry restoration strategies in semiarid climate induces different responses in biochemical and microbiological soil properties

Mining activities generate loss of environmental and landscape quality, especially in arid and semiarid Mediterranean regions. A precondition for ecosystem reclamation in such highly disturbed areas is the development of functional soils with sufficient amount of organic matter. In a restoration experiment in limestone quarries in the Sierra de Gádor (Almería), SE Spain, several combinations of organic amendments (sewage sludge and compost from domestic organic waste) and mulches (gravel and woodchip) were tested and native plants (Anthyllis cytisoides, A. terniflora and Macrochloa tenacissima) were planted. After five years, the effect of each treatment on the soil chemical properties, basal respiration, four enzyme activities (dehydrogenase, urease, β-glucosidase and alkaline phosphatase) and the microbial community composition was analysed. Analysis of phospholipid fatty acids (PLFAs) and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprinting were used to analyse the microbial communities (bacteria and fungi). Undisturbed natural soils adjacent to the mining area were used as soil quality references. Organic amendments, particularly compost, improved soil chemical and biochemical properties as well as microbial biomass. However, the effects of mulch application did not show a clear trend with respect to soil functionality and did not increase the microbial biomass. Soils treated with sewage sludge and compost showed bacterial PLFA concentrations similar to those of reference soils, but compost treatments presented fungal PLFA concentrations that were much higher. Each combination of organic amendment and mulch was selective for a proper microbial community. Nevertheless, increases in soil functionality and microbial biomass were not related to changes in microbial diversity. After five years, the microbial properties of restored soils had not yet converged to values recorded in the reference soils. However, the combination of mulches and organic amendments, particularly compost treatment, is suggested to be beneficial for restoring degraded soils from quarrying areas because they stimulate microbial growth and activity, with positive implications for the increase in both soil fertility and quality.