Abstract
BACKGROUND: Arid and semi-arid regions cover approximately 41% of Earth's surface and their soils are highly vulnerable to degradation due to harsh climatic conditions and extractive activities, such as opencast mining. Organic amendments are widely used to restore degraded soils because they improve physical, chemical, and biological properties. However, little is known about how these amendments alter microbial communities and the relationship between microbial taxonomy and function, particularly in nitrogen and phosphorus cycling. To address this knowledge gap, the effects of different organic amendments (gardening compost, greenhouse horticultural compost, sewage sludge and two blends of the above) on soil properties, microbial communities and their contributions to nitrogen metabolism and phosphorus turnover in degraded soils from a limestone quarry in the Gádor Range (Almería, SE-Spain) six months after their application were investigated. RESULTS: Organic amendments increased nutrient content (total organic carbon, total nitrogen and available phosphorus), microbiological activity, and bacterial biomass compared to unamended soils, with the largest increases in sewage-sludge-treated soils. Shotgun metagenomic assays revealed that organic amendments modified bacterial community composition and differentially influenced potential function pathways, contributing more strongly to nitrogen metabolism than phosphorus turnover, particularly within the phosphonate pathway. Across soils, Pseudomonadota and Actinomycetota were the dominant phyla. Sludge-amended soil showed higher relative abundance of Pseudomonas, associated with denitrification processes (nirK, nosZ, norB) and phosphonate degradation via C-P lyase (phnJ). Genera such as Streptomyces were linked to ammonium assimilation (glnAd, gltBD) and phosphonate synthesis (pmmS), and were more abundant in soil with vegetable-compost and unamended soils. Both nitrogen and phosphorus metabolisms exhibited phylogenetically unrestricted functional patterns, indicating high functional redundancy at phylum and genus levels. CONCLUSIONS: This research establishes key relationships between taxonomy and function in restored soils and demonstrates how organic amendments rephase microbial communities and their potential roles in nutrient cycling. Although dominant taxa and functions were identified, many microorganisms involved in nitrogen and phosphorus turnover remain insufficiently characterized. Further research across restoration contexts is needed to compare nutrient-cycling responses and to deepen understanding of taxonomy-function linkages in soils amended with organic residues.