Torrefaction as a new Post-Composting strategy to obtain stabilized organic amendments with liming potential

Intensive agricultural and livestock production generates large volumes of by-products, and managing these wastes while maintaining soil fertility remains a major challenge. Composting provides a sustainable solution, but the process is often slow and produces heterogeneous material, particularly when feedstocks are complex, such as olive mill pomace (OMP). This study investigates a combined strategy of OMP partial composting followed by torrefaction (1  h, 275 °C) to produce a homogenized and stabilized organic amendment (thermocompost). The effects of thermal treatment at different composting stages were evaluated using Fourier transform infrared (FTIR) and fluorescence spectroscopies, together with key physicochemical and agronomic indicators. Torrefaction reduced the C/N ratio and water-soluble carbon while preserving the cation exchange capacity (CEC ≈ 150 cmol(+) kg^-1) developed during composting, with potassium as the predominant exchangeable base. Thermocompost exhibited elevated pH ( 11.0) and high lime potential, linked to carboxylate formation and carbon dioxide fixation in the form of carbonates, indicating suitability for acidic soil remediation. The material was non-phytotoxic, highly humified, and resistant to biological degradation, even after biofortification with vermicompost tea. These results demonstrate that combining partial composting with torrefaction enhances both the stability and agronomic performance of organic amendments, allows valorization of immature or suboptimal composts, and can reduce the time required to obtain high-quality soil conditioners. This approach is particularly relevant for compost batches that have not reached full maturity or have been poorly processed, providing a practical pathway to improve their agronomic properties and suitability for acidic soils.