Objective  This aims of this study were to verify the role of laccase in enhancing the humification process of composts in the lignocellulosic waste composting system, and to explore the possibility of producing high-quality organic fertilizer.

  Method  Corn stalks were used as the main raw material and rapeseed residue was used as auxiliary materials, the corn stalks and rapeseed residue were fully mixed at a ratio of 2.85:1 to obtain an organic mixture with a carbon to nitrogen ratio (C/N) of 30, and the moisture content of the mixture was adjusted to 50% for primary fermentation. When the composting temperature was reduced to about 40℃ (at the 18^th day), the secondary fermentation was carried out, and then the wo treatments of T1 (without adding laccase) and T2 (adding laccase) were set. The humification process was evaluated by analysis of the contents of cellulose and lignin, the activities of key degrading enzymes, and the concentration of precursors during composting process.

  Result  The results showed that the two groups of treatments without adding laccase (T1) and adding laccase (T2) could meet the fermentation maturity standard (GI ≥ 80%) at the end of composting. The GI value in the T2 treatment was significantly higher than that in the T1 treatment (P < 0.01). In addition, the activities of key degrading enzymes, the degradation efficiency of cellulose and lignin, and the degree of humification were significantly enhanced after adding laccase (T2) (P < 0.05). At the end of composting, the contents of cellulose and lignin in T2 treatment were significantly decreased by 20% and 22.2% (P < 0.05), and the organic material degradation rate and humic acid content increased by 19.05% and 52.15% (P < 0.05), respectively. In terms of precursor substances of humic acid, the exogenous addition of laccase significantly increased the contents of amino acids, reducing sugars and polyphenols in the early stage of secondary fermentation for composting (43.07 mg kg⁻¹, 10.97 mg kg⁻¹ and 14.81 mg kg⁻¹ on the 21^st day, respectively). In terms of key degrading enzyme activities, exogenous addition of laccase increased the activities of compost laccase, lignin peroxidase (LiP) and manganese peroxidase (MnP) compaered with the T1 treatment in the whole composting secondary fermentation. The activities of LiP and MnP were 454.5 U L⁻¹ and 390.9 U L⁻¹ in the T2 treatment at the end of composting, respectively. The redundancy analysis showed that the pH value significantly associated with the humification process, followed by reducing sugar and LiP in T1 treatment, indicating that pH was an important environmental factor affecting the humification process. The effect of pH and reducing sugar on humic acid (HA) formation decreased in T2 treatment, and polyphenols became the main factor affecting HA formation, indicating that the addition of laccase can significantly increase the polymerization of phenols to HA.

  Conclusion  Therefore, in the secondary fermentation stage of lignin-based waste composting, the addition of laccase enhances the degrading enzyme activity as a "bridge", accelerates the continuous degradation of lignocellulose, and promotes the polymerization of precursors to HA, which is an important mechanism of enhancing the humification process of lignin-based waste composting. The findings provide a new method to solve the problem of lignocellulosic degradation in straw composting, which is an economical and efficient technical way to improve the quality of organic fertilizer.