Efficiency and mechanisms of microbial metabolic regulator addition on carbon and nitrogen retention in biochar-assisted distilled grain waste composting

The efficiency of biochar in enhancing carbon and nitrogen retention during distilled grain waste (DGW) composting is often limited. Microbial metabolic regulators, like malonic acid (MA), provide a potential strategy to overcome these limitations. This study investigated the effects of MA addition at different composting stages on biochar-assisted DGW composting and elucidated the underlying mechanisms. MA addition reduced carbon and nitrogen losses by 9.3%‒12.8% and 6.3%‒29.0%, respectively, owing to suppressed emissions of carbon- and nitrogen-containing gases and increased formation of NO₃^-, organic carbon, and nitrogen. Notably, MA addition during the thermophilic stage exhibited excellent efficiency. Bacterial community analysis revealed that MA addition regulated the abundance of genera (such as Bacillus, Ureibacillus, Filomicrobium, Thermobifida, Luteimonas, Nitrosomonas, etc.) involved in carbon and nitrogen conversions and promoted cooperative interactions among bacterial populations. Moreover, MA addition modified the activities of key functional enzymes in carbon and nitrogen metabolic pathways. Particularly, MA addition reduced the early phase of carbon- and nitrogen-containing gases emissions, and facilitated the later phase of carbon and nitrogen fixation, aromatic amino acid synthesis, nitrification, and ammonia assimilation. This study highlights the effectiveness and underlying mechanisms of MA addition in promoting carbon and nitrogen retention in biochar-assisted composting. These findings provided a novel strategy to improve compost quality and support carbon-neutral DGW management.