Biochar drives humus formation during composting by regulating the specialized metabolic features of microbiome

Humus formation of composting is crucial for improving organic fertilizer quality. Biochar has been found to be a low-cost, eco-friendly and effective promoter for humus formation, while the mechanisms underlying the biochar-driven humus formation during composting remain unknown. The present study was aimed to explore the effect of biochar addition on humus formation by integrated analyses of fungal communities, functional genes and metabolic features during composting using the advanced techniques of fluorescence spectrometry, high throughput sequencing, metagenomics, and metabolomics. The efficiencies of fulvic acid and humic acid formation were increased by 17.4 % and 39.4 %, respectively, under biochar addition relative to the control at the mature phase. Biochar addition significantly increased temperature, lignocellulose degradation and the activities of urease, peroxidase, manganese peroxidase and lignin peroxidase, and altered the fungal community composition and diversity with the latter significantly correlated with humic acid content. The altered fungal community subsequently resulted in the stimulation of fungal function of Wood Saprotroph and expression of CAZyme genes at the thermophilic and mature phases. Ultimately, biochar addition significantly enhanced the Xenobiotics metabolism (Bisphenol metabolism and Quinone biosynthesis) and Amino acid metabolism (Tryptophan metabolism and Tyrosine metabolism), which were responsible for humus formation via producing metabolic compounds such as quinolines, phenolics and aromatic amino acids at the mature phase. Our findings suggest that biochar addition can effectively stimulate humus formation by selectively regulating fungal communities and metabolic features during composting, so that benefits the improvement of soil fertility and health upon the organic fertilizer application.