Regulation mechanism of N2O metabolic footprint during the black soldier fly manure composting system amended with biochar, humic acid, and tea residue.

Black soldier fly (BSF) manure is typically accompanied by incomplete decomposition, requiring further composting treatment. The effects of biochar, humic acid, and tea residue on elemental N conversion, N2O emission, and associated enzyme activities and functional genes throughout the BSF manure composting process were examined. The factors influencing N2O emission were further explored. The study found that all three additives significantly suppressed N2O emissions, with tea residue showing the highest suppression rate (88.25 %), followed by biochar (66.89 %) and humic acid (54.11 %). Tea residue increased the narG gene abundance, promoted N2O reduction, and enhanced the nitrite reductase (NiR) activity. During the early composting stage, biochar, humic acid, and tea residue enhanced nitrous oxide reductase (N2OR) activity. Tea residue and biochar significantly increased hydroxylamine oxidoreductase (HAO) activity. Pearson correlation analysis indicates a strong positive correlation between N2O emission and both NO2--N and NO3--N. RDA analysis shows nirK, amoA and amoB have significant effects on N2O emissions and N transformation. Structural equation model analysis showed that denitrifying genes and HAO enzymes positively influence N2O emission, whereas nitrifying genes, denitrifying enzymes, and various N forms negatively affect N2O emission. This research will provide a theoretical safety basis for of the application of biochar, humic acid and tea residue during BSF manure composting, and promote their ecological and sustainable management strategy during agricultural using.