Manganese slag promotes the breakdown of lignin into polyphenols to regulate humus formation in chicken manure composting

Lignin degradation produces a phenol-rich aromatic backbone, which serves as a link between various metabolic processes and is often referred to as "polyphenols"; it is central to humus formation via the polyphenol pathway. The incorporation of manganese slag into chicken manure and sugarcane bagasse composting produced conditions favorable for lignin breakdown and the formation of humic substances via the polyphenol route, manifested in accelerated heating and elevated temperatures. This resulted in a 329.49 %, 15.55 %, and 33.72 % increase in the relative abundances of Thermobifida, Bacillus and Truepera, When compared with the control group (CK), the T group exhibited a 16.95 % higher degradation rate of lignin, a 19.83 % higher content of humus (HS), and peak activity gains of 5.88 % for laccase and 12.36 % for polyphenol oxidase. In addition, the abundance of a set of polyphenol pathway-related genes (including ligM, lac, ppo, vanAB, and pobA) was also upregulated. Additionally, a significant positive correlation was observed between the relative abundance of polyphenol‑pathway genes and humification parameters, suggesting that the polyphenol pathway acts as a bridge connecting lignin degradation to humus formation. This study suggests that the addition of manganese slag to chicken manure and sugarcane bagasse compost primarily controls lignin degradation and humus formation by altering the composting environment and key microorganisms, thereby regulating enzymatic metabolic processes, which provides a theoretical basis for the relationship between polyphenol sources in compost and humus formation via the polyphenol pathway.