Tailoring composting with ionic liquids: glycine/cysteine cations and chloride/nitrate anions synergistically dictate microbial pathways and humification

This study decouples cationic and anionic effects using [Gly][Cl], [Cys][Cl], and [Cys][NO3]. Gly+ imposes the strongest initial selective pressure, streamlining the consortium toward a Kernia-dominated fungal community (57.6%) and Bacillus–Thermobifida bacterial synergy, which directly channels carbon into humic acid via an efficient direct conversion pathway with low fulvic acid accumulation. Cl− acts as a nitrogen-conservation switch via urease inhibition, reducing ammonia volatilization. NO3− serves as a stage-specific metabolic signal, initially activating denitrification then promoting nitrification, establishing a high-turnover dynamic pathway with rapid fulvic acid generation followed by conversion to humic acid. Cys+ expands the rare biosphere but buffers metabolic intensity. The synergy of ion-specific selective pressure and functionally partitioned microbial consortia (bacteria for degradation, fungi for direct humification) mechanistically drives enhanced humification. These observations suggest a structure-associated pattern that warrants further mechanistic validation before establishing an ion-by-design framework.