phenolic compounds promote humification by inducing functional focus in microbial communities.

Functional dispersion within microbial communities is a major constraint on lignin humification during composting. Lignin degradation relies on extracellular enzymes whose activities can be modulated by phenolic units released during lignin depolymerization. Accordingly, acetaminophen (APAP) and mature composting products (MC) were combined in the MCA treatment to evaluate their synergistic effects on microbial functional specialization and humification. The MC treatment modestly enhanced humification, increasing HD and final HA content by 30.3 % and 16.4 % relative to CK, respectively. APAP produced larger increases of 119.4 % and 24.2 %, respectively, but reduced community stability. By contrast, the combined MCA treatment produced the greatest response, with HD and final HA content increasing by 172.7 % and 40.9 %, respectively, at the end of composting. Activities of laccase, lignin peroxidase, and manganese peroxidase also increased, while pathways associated with aromatic compound degradation and energy metabolism were coordinately enriched. These responses suggest broader metabolic activation accompanied by greater functional focus. Together, the findings suggest that combining phenolic small molecules with mature composting products promoted microbial functional specialization. They also provide complementary support for a conceptual model linking substrate induction, microbial community assembly, and enzyme responses. Overall, the combined treatment enhanced humification, providing a useful basis for optimizing straw composting and resource utilization.