In Europe, food and agricultural waste amount to millions of tonnes annually. Effective management and valorisation of these residues result in environmental benefits and foster opportunities within the circular economy. Composting has emerged as a sustainable method to convert waste into fertiliser, enhancing soil fertility, water retention, and crop resilience against diseases. However, an adequate compost production process is vital to obtain a functional fertiliser. In this study, a controlled conditions self-produced compost from horticultural waste (C1) was compared against two other commercial composts, one of similar vegetable origin (C2) and another from chicken manure (C3). Physicochemical parameters and nutrient contents in the three compost types were analysed, and phytotoxicity and plant development tests were carried out on Lolium multiflorum Lam. seeds and Pistacia lentiscus L. seedlings. C1 presented fertility and germination parameters similar to C3 and showed the best seedling development. In contrast, C2 showed low levels of fertility, germination, and plant development because of impurities and possible substances inhibiting plant growth, suggesting inadequate compost formation processes. Finally, C3, although it presented germination data similar to C1, produced the worst results in the development of seedlings, probably because of high salinity values and low phosphorus content.

The restoration of unproductive abandoned agricultural soils under a semi-arid climate in southeastern Spain was evaluated. Four organic amendments from different composted wastes (greenhouse crop residues; worm compost from sheep–cow manure and plant remains; chicken manure; and sheep–chicken manure and plant remain) were applied, untreated control plots were installed, and natural unexploited soils were selected as a reference ecosystem. Through three sampling campaigns (initial, 3 months, and 12 months), significant changes were observed in soil physico-chemical properties, nutrient content (carbon -C-, nitrogen -N-, and phosphorus -P-), and bacterial composition of the restored soils with respect to control and natural soil, especially at 3 months. The increase of labile nutrients caused the proliferation of copiotrophic phyla at 3 months, which, after their consumption, were replaced by oligotrophic phyla at 12 months. Specific taxa involved in C, N, and P cycles were identified for each soil. For the soil bacterial composition of vermicompost, only chicken and sheep–chicken manures were more dissimilar to control and natural soils after 12 months, while greenhouse crop compost showed an intermediate position between them. Results indicated that composted greenhouse crop remains could be an optimal treatment for the short-term recovery of physico-chemical properties, nutrient content, and bacterial composition of agriculture-degraded soils in semi-arid areas.

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.