Humification and greenhouse gas emissions divergently regulated during black soldier fly manure composting system by biochar, humic acid, and tea residue.

Chicken manure following black soldier fly (BSF) biotransformation retains high fermentability potential. Direct application as an organic fertilizer negatively impacts soil quality. This study evaluated the effects of three additives (biochar, humic acid, and tea residue) on humification, greenhouse gas emissions, and microbial mechanisms during BSF manure composting. Research indicates that humic acid accelerates compost humification by increasing substituents on aromatic rings. Biochar and tea residue promoted humification by increasing the proportion of highly aromatic structures. Compared to the control, humic acid addition increased the amino acids by 4.19% to 57.35% and reducing sugars by 2.02% to 5.45%, further promoting the increase in humic substances (HS) (29.71%) and humic acid (HA) (9.52%). Biochar and humic acid reduced CH4 emissions by 40.67% and 48.28% respectively compared to the control. Biochar and tea residue continuously activate the activity of C cycle related enzymes (BG, MCR and CDH). Microbial analysis shows that the unique microbial species are most abundant in the tea residue addition. Planktosalinus and Pelagibacterium exhibit higher abundance in the biochar addition. Coprinopsis and Mycothermus have higher abundance in the control and humic acid addition, while the opposite is true in the tea residue treatment. Partial Least Squares structural equation modeling showed that CO2 emissions were significantly associated with processes mediated by fungal metabolism and functional genes. This study establishes a theoretical foundation for the efficient composting and precise application of BSF manure, providing scientific insights for the development of low-carbon organic fertilizer technologies.