Nano-selenium improves the humification process of cattle dung during composting by regulating carbohydrate-active enzymes and carbohydrate metabolism

Nano-selenium, as an efficient additive, has been shown to reduce greenhouse gas emissions and the risk of antibiotic resistance genes during composting. However, its impact on the humification process remains unclear. This study investigated the regulatory effects of different concentrations of nano-selenium (CK: 0 μg/kg, Se_75: 75 μg/kg, Se_150: 150 μg/kg, and Se_300: 300 μg/kg) on carbohydrate metabolism and carbohydrate-active enzymes (CAZymes) through a 28-day aerobic composting and a 3-month maturation experiment. The results indicated that the humic acid content in the CK, Se_75, Se_150, and Se_300 groups increased to 100.79 mg/g, 99.14 mg/g, 113.87 mg/g, and 93.86 mg/g during composting, respectively. A moderate concentration of nano-selenium (150 μg/kg) promoted the growth of Pseudomonadota, represented by Luteimonas , and upregulated the abundance of AA3, GT41, and CE4. Network analysis showed that the synergy among CAZymes was strengthened in the Se_150 group. At the end of composting, the contents of cellobiose, stachyose, galactinol, and raffinose in the Se_150 group were the highest among all groups. Among these, cellobiose was the most abundant metabolite, and the Mantel test results indicated its significant correlation with all CAZymes (p < 0.01). In conclusion, the regulation of the structure and collaborative ability of CAZymes by a moderate concentration of nano-selenium provided more precursor substances for humus formation and effectively increased the humic acid content in compost products.