Mechanistic insights into nitrogen loss during food waste composting revealed by metagenomic and qPCR analyses under varying substrate C/N ratios

Nitrogen loss during composting can be substantial; however, it can be reduced by applying new insights to better control the substrate C/N ratio and optimise overall composting performance. This study provides mechanistic insights into how substrate C/N governs nitrogen loss during kitchen waste composting. By combining nitrogen speciation analysis, qPCR, and metagenomics analyses, this study explored the potential biochemical mechanisms of nitrogen loss. The results showed that a high substrate C/N ratio significantly reduced nitrogen loss by approximately 37 % (C/N of 25) and 47 % (C/N of 30) compared to the baseline C/N of 20. A higher substrate C/N ratio enhanced nitrogen fixation and assimilation processes while suppressing ammonification and denitrification related potential. The relative abundance of key ammonification-related genera (e.g. Thermobifida and Leuconostoc ) and denitrification-related genera (e.g. Pseudomonas and Geobacillus ) were decreased at a high substrate C/N ratio, resulting in synergistic mitigation of NH 3 and N 2 O emissions. A small reduction in germination index was observed at substrate C/N ratio of 30 compared with 25. Overall, the results suggest the need to optimize substrate C/N ratio for nitrogen conservation while maintaining overall composting performance.