Restoring soil quality in semi-arid mining-degraded soils: Effects of different combinations of organic amendments on microbial nutrient cycling after 40 months of application

Medium-term effects of different organic amendments on the recovery of mining-degraded soils in a semi-arid limestone quarry were evaluated. Five organic amendments, including composts (garden pruning and greenhouse residues) and stabilised sewage sludge (alone and in mixtures), were compared to untreated soils and natural reference soils. After 40 months, different soil physico-chemical properties, total nutrient (organic carbon –C–, nitrogen –N– and phosphorus –P–) and labile P and N fractions were analysed together with bacterial functional groups catalysing major steps in P (phoD, appA, phnX, pstS) and N turnover (chiA, archaeal amoA, bacterial amoA, nirS, nirK, nosZ, nifH), as well as total bacterial biomass. Restoration altered soil properties, including decreasing pH by up to 10% and increasing total organic C (up to 3.54%), total N (up to 0.33%) and total P (up to 0.18%). Labile P- and N-fractions increased significantly, with ammonium and nitrate doubling in some cases. Microbial activity also rose significantly, with bacterial biomass and functional genes involved in P (phoD, pstS) and N turnover (chiA, nirS, nosZ) increasing 2–3000 times compared to non-restored soils. Sewage sludge had the most pronounced effect on physico-chemical properties, nutrient content and functional groups abundance, while greenhouse compost produced conditions resembling natural reference soils. These results demonstrated that organic amendments can rehabilitate degraded soils by enhancing nutrient content and bacterial community potential for N and P turnover. Organic amendments are thus a viable strategy for medium-term restoration of degraded soils in semi-arid climates.