Animal-Waste Valorisation for Soil Health: Processes, Products, Agricultural Benefits and Potential Risks

Abstract

The global livestock sector produces vast quantities of organic waste that, if managed poorly, represent a significant environmental liability. If appropriately valorised, however, animal waste constitutes a renewable feedstock capable of rebuilding soil organic matter, improving soil biological activity, and supplying essential plant nutrients. This review synthesises current evidence on the principal valorisation processes—composting, vermicomposting, anaerobic digestion with digestate application, and pyrolysis to produce biochar—examining how each pathway transforms raw animal waste into soil amendments with measurable agricultural benefits. The literature search was conducted across multiple bibliographic databases, including Web of Science, Scopus, PubMed, and Google Scholar, which were selected for their broad disciplinary coverage and comprehensive indexing of peer-reviewed journals. Evidence drawn from meta-analyses, field experiments, and process-level studies demonstrates that manure-derived amendments consistently increase soil organic carbon stocks, stimulate microbial biomass, improve soil aggregate stability, and support crop productivity, though the magnitude of these effects varies markedly with amendment type, application rate, soil characteristics, and climate. The review also addresses the principal risks associated with valorised animal wastes, namely the dissemination of antibiotic resistance genes and antimicrobial-resistant bacteria, accumulation of potentially toxic metals, and the potential for elevated greenhouse gas emissions from poorly managed applications. These risks are contextualised within the circular bioeconomy framework, which positions animal-waste valorisation as a mechanism for closing nutrient loops and reducing dependence on energy-intensive synthetic fertilisers. The review concludes by identifying critical knowledge gaps and emerging research priorities, particularly in the governance of digestate quality, struvite production at scale, and the long-term fate of recalcitrant biochar carbon in diverse soil types.