Advances in manganese-based additives for optimizing composting processes and enhancing compost quality

The application of exogenous additives has become a crucial strategy to promote compost maturation and improve the quality of compost products. Among various additives, manganese-based additives, due to their unique multivalent redox properties, catalytic functions, and regulatory effects on microbial metabolic networks, have emerged as a research hotspot in the composting field for enhancing the composting process through multidimensional mechanisms. This review summarizes the research progress on the application of manganese-based additives in composting systems, with a comprehensive analysis of their effects on the transformation of organic components, greenhouse gas mitigation, humification processes, and the safety and quality of compost products. Through a comparative analysis of the efficacy of various manganese-based additives, the study systematically elucidates the intrinsic relationships between types, functions, and outcomes, as well as the underlying mechanisms. Evidence indicates that the rational application of manganese-based additives can effectively enhance organic component transformation, mitigate emissions of greenhouse gases and malodorous compounds, strengthen abiotic and biotic humification pathways, and improve the quality and safety of compost products. The effectiveness of these additives is influenced by several factors, including the type of manganese additive (e.g. MnO₂, MnSO₄, or manganese-loaded biochar), dosages and the nature of the composting feedstock. These factors significantly influence the transformation of humification precursors, functional microbial community structure, and key enzyme activities, thereby regulating humification processes, gas emissions, and quality of compost products. Despite these advances, several critical knowledge gaps remain. The long-term environmental impact and potential ecological risks associated with the land application of compost products amended with manganese-based additives require further systematic evaluation. The efficiency variations among different manganese additives and their precise dose-response relationships also need in-depth exploration. Furthermore, the mechanisms governing the migration, transformation, and detoxification of co-existing pollutants, such as heavy metals, antibiotic resistance genes, and persistent organic contaminants, during manganese-facilitated composting are not yet fully understood. Future research should focus on integrating multi-omics approaches to elucidate microbial metabolic pathways, assessing the synergy between manganese additives and other amendments, and evaluating the agronomic and environmental performance of the final compost under field conditions. This review aims to consolidate existing knowledge and provide a comprehensive reference to guide further research, technological optimization, and widespread implementation of manganese-based additives in composting, thereby promoting the transformation of organic solid waste recycling toward high-efficiency, low-carbon, and sustainable development.