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. 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 (-44 %) and NH3 (-18 %). IBC reshaped bacterial, fungal and archaeal communities and strengthened microbial network connectivity. Metagenomic analysis showed IBC enriched nitrogen-fixation genes and suppressed ammonification, nitrification, denitrification, and nitrate reduction. IBC also enhanced thiosulfate and sulfite oxidation while inhibited reductive pathways linked to H2S formation. Partial least squares path model confirmed odor mitigation resulting from coordinated shifts in environmental conditions, microbial structure, and metabolic pathways. Overall, microbial inoculation effectively reduces odor emissions and enhances composting performance by redirecting nitrogen and sulfur transformations.