Elaborating the microbial mechanism of humic substance formation during lignocellulosic biomass composting by inoculation with different functional microbes

This study explored the microbial mechanism of humic substance (HS) formation in the lignocellulosic biomass composting by inoculation with different functional microbes. Typical types of functional bacterial and fungal strains with higher lignocellulose degradation characteristics were inoculated into compost. Three treatment groups were set up: uninoculated microbes (CK treatment), inoculation of Bacillus subtilis (B4 treatment) and Aspergillus fumigatus (Z1 treatment). Results showed that the HS concentration for inoculation B. subtilis and A. fumigatus increased by 15.42% and 56.31%, respectively. B. subtilis could produce phenolic metabolites, cellobiose and maltose, and then enter the tricarboxylic acid cycle, thereby releasing CO2, while A. fumigatus could promote HS formation by producing phenolic metabolites, 2,4-Di-tert-butylphenol, cellobiose, maltose, glucose, galactose, xylose, arabinose, mannose, ribose and deoxyribose. In addition, inoculating A. fumigatus enhanced the correlation among lignocellulose-degrading metabolites and HS, and increased the relative abundance and numbers of microbial hosts of lignocellulose-degrading genes, promoting the HS formation. In addition, cellulose-degrading genes and lignin-degrading genes differences were the main drivers of HS formation in the inoculated treatment groups. Soluble sugar, organic matter, lignin and pH were key drivers in influencing the lignocellulose-degrading genes. This study was important for analyzing microbial biological mechanisms of HS formation during composting.