Unveiling nitrogen and sulfur cycling mechanisms of odor reduction in kitchen waste composting driven by exogenous bacterial consortia

Odor emissions, primarily ammonia (NH3) and hydrogen sulfide (H2S) restrict the application of kitchen waste (KW) composting. Microbial inoculation is a promising strategy, yet mechanisms underlying odor mitigation remain unclear. Here, KW composting with and without an immobilized bacterial consortium (IBC) was compared by monitoring physicochemical conditions, odor emissions, and microbial and functional profiles. IBC extended the thermophilic phase, improved composting efficiency, and reduced cumulative H2S and NH3 by 44% and 18%. IBC reshaped bacterial, fungal and archaeal communities, and strengthened microbial network connectivity. Metagenomic analysis showed enrichment of nitrogen-fixation genes and suppression of ammonification, nitrification, denitrification, and nitrate reduction. In sulfur cycling, IBC enhanced thiosulfate and sulfite oxidation while inhibiting reductive pathways linked to H2S formation. Partial least squares path model confirmed that odor mitigation resulted from coordinated shifts in environmental conditions, microbial structure, and metabolic pathways. These findings provide mechanistic insights for designing low-emission, high-efficiency composting systems.