Sustainable fertilizers from wastes: a strategy to enhance soil carbon, improve soil quality, and mitigate emissions

The transition to sustainable fertilization strategies is essential to reconcile agricultural productivity with soil health and climate change mitigation. This study assessed the agronomic and environmental performance of four waste-derived fertilizers -compost (C ), vermicompost (V), olive-based digestate (D), and sulphur-bentonite enriched with olive pomace (SBO) - through integrated soil analyses and life cycle assessment (LCA). Results showed that compost and vermicompost exhibited the highest degree of organic matter humification, improving total organic carbon (TOC), cation exchange capacity (CEC), soil microbial biomass C (MBC), and enzymatic activity. Vermicompost provided the greatest increase in humification rate and bacterial biomass, while compost maximized fungal-to-bacterial ratios, soil enzymatic activities, and stable carbon pools. Digestate enhanced soil organic carbon and fungal biomass, though with lower humification indices and higher water-soluble phenols. SBO strongly acidified the soil and shifted microbial communities toward fungal dominance but contributed the highest greenhouse gas emissions due to sulphur processing. LCA confirmed vermicompost as the most climate-friendly amendment (25 kg CO₂ eq ton^-1), followed by compost (43), digestate (110), and SBO (167). These findings indicate that waste-derived organic fertilizers can simultaneously improve soil quality, promote carbon sequestration, and reduce emissions compared to synthetic inputs, with vermicompost and compost offering the greatest agronomic and environmental benefits.