Unraveling the dose-dependent mechanisms of nicotine on humification during tobacco waste composting: Insights from microbial ecology and metabolic partitioning

The inherent toxicity and wide-ranging concentrations of nicotine severely impede the predictable and efficient valorization of tobacco waste via composting. This study reframes nicotine from a simple contaminant to a key ecological modulator, systematically investigating its dose-dependent effects on the humification process. Four nicotine concentrations (0 %, 0.5 %, 1.0 %, and 2.0 % w/w) were tested in controlled composting reactors. A distinct optimal window was identified at 0.5 % nicotine, which significantly enhanced the final humic acid (HA) content to 18.57 %, outperforming both the control and higher-concentration treatments. Conversely, concentrations ≥ 1.0 % triggered a complete process collapse, defining a clear inhibitory threshold. Metagenomic analysis revealed the underlying mechanism for this non-linear response: a sophisticated metabolic trade-off. In contrast to the control group, which showed the highest genetic potential for nicotine degradation, the 0.5 % treatment moderately suppressed these catabolic pathways. This suppression likely partitioned metabolic resources away from the costly mineralization of nicotine and towards the value-added synthesis of HA. This interpretation was supported by the concurrent enrichment of the lignin-degrading Brachybacterium - Bacillus functional hub. These findings provide a fundamental explanation for the process instability in tobacco waste composting and establish a scientific basis for “precision composting”, where targeting an optimal nicotine concentration via feedstock blending can transform a hazardous waste into a high-quality soil conditioner. • Nicotine dose dictates tobacco waste composting humification efficiency. • Low dose significantly enhances humification and maturity. • Optimal nicotine boosts synergistic microbial interactions & network modularity. • Nicotine’s dual role linked to microbial shifts and metabolic partitioning. • Provides 0.5 % threshold for targeted tobacco waste valorization via composting.