Biochar-Amended Composting for Livestock Manure Management: A Critical Review of Microscale Mechanisms and Agronomic Applications

Aerobic composting is widely used to stabilize livestock and poultry manure and recover organic nutrients, but its performance and end-product safety depend on nitrogen conservation, gas emissions, aeration, contaminant fate, and compost maturity. This critical review synthesizes evidence from a structured search of the Web of Science Core Collection, Scopus, and PubMed, complemented by citation tracking. Of the 204 records identified, 58 publications were retained for the review. Among these, 45 empirical studies met the eligibility criteria and constituted the formal narrative synthesis, whereas 13 review articles or meta-analyses were used only for background information and backward citation tracking. The 45 empirical studies were classified according to their relevance to livestock-manure composting and the directness of the supporting evidence. The review evaluates relationships among biochar feedstock, pyrolysis conditions, particle size, surface properties, and amendment rate, with emphasis on physicochemical regulation, microbial responses, contaminant fate, and post-application agronomic implications. Across the selected studies, biochar was frequently associated with improved porosity and moisture regulation, lower nitrogen losses and gaseous emissions, increased humification, and reduced operationally defined labile fractions of Cu and Zn. However, these responses were highly context-dependent and were not consistently observed across manure types, biochar materials, particle sizes, amendment rates, or composting systems. Biochar may also increase pH or electrical conductivity, inhibit germination when soluble salts or water-extractable organic compounds are present, redistribute rather than permanently immobilize contaminants, and produce null or contrasting responses for antibiotic resistance genes. Evidence for heavy-metal passivation was generally stronger than evidence for suppression of horizontal gene transfer or long-term ARG attenuation. The frequently investigated amendment range of approximately 10–15% (w/w) should therefore be regarded only as a provisional, feedstock-specific starting range rather than a universally applicable recommendation. Similarly, the apparent advantages of moderate pyrolysis temperatures represent trends observed in selected studies, not quantitative optima. The review also highlights the need for process-scale validation, direct measurements of microbial activity and contaminant speciation, evaluation of biochar-derived organic contaminants, and long-term soil–plant experiments. The simple study-quality appraisal further indicates that the available evidence should be interpreted cautiously because reporting and methodological quality varied among studies. Function-oriented selection criteria should be developed to match biochar properties with manure characteristics, composting conditions, and intended agronomic use.