Exogenous AHL signaling accelerates compost humification by reshaping DOM molecular profiles and strengthening microbe–DOM coupling

Microbial quorum sensing (QS) system can mediate metabolic coordination during composting, nevertheless, how QS modulates the coupling between community assembly and dissolved organic matter (DOM) molecular transformation remains poorly understood. This study supplemented exogenous N-hexanoyl- L -homoserine lactone (C6-HSL) to explore its regulatory functions in sewage sludge‑corn stover co‑composting. Results showed that addition C6-HSL increased the maximum composting temperature by approximately 5 °C and extended the duration of thermophilic phase. Likewise, a higher value of humic acid content (36.4 g kg⁻¹), HA/FA ratio (1.96) and germination index (96%) were observed in the C6-HSL group. Molecular‑level characterization illustrated that carbohydrate-, aliphatic-, and amino sugar-like molecular formulas were continuously consumed throughout composting, whereas lignin‑type DOM compounds accumulated. C6-HSL further promoted the conversion of lignin‑derived intermediates into condensed aromatic‑dominated molecules particularly in the late maturation period. Succession of microbial communities featured the decay of early-stage fermentative populations ( Clostridium , Turicibacter , and Romboutsia ), together with the proliferation of taxa ( Streptomyces , Thermobifida , and Thermobacillus ) that decompose recalcitrant organic substrates. Moreover, lower normalized stochasticity ratio (NST) values were detected for both bacteria and fungi under C6‑HSL addition. Ecological‑process decomposition further indicated that heterogeneous selection became more important in fungal assembly, while bacterial succession reflected combined contributions from selection, drift, and dispersal limitation. The microbe‑DOM utilization index (MDUI) maintained elevated levels in the C6‑HSL group and accounted for approximately 63% of compositional variation in microbial communities. Lignin‑like DOM components, molecular aromaticity and bio‑active molecular fractions exerted the strongest explanatory power for microbial succession. These findings demonstrate that C6-HSL promotes compost humification by coordinating stage-dependent microbial turnover with DOM molecular succession.