Biochar mitigates N2O and NH3 emissions in sheep manure composting by regulating microbial genes associated with nitrogen cycle

Improper treatment of livestock and poultry manure from animal husbandry could lead to substantial Ammonia (NH 3 ) and Nitrous oxide (N 2 O) emissions. In this study, rice husk biochar (RHB) and sawdust biochar (SDB) were investigated as additives to control NH 3 and N 2 O emissions during sheep manure composting. Metagenomic sequencing was conducted to characterize the microbiome during composting and functional genes associated with nitrogen cycle (NC genes), and correlations among properties of composting feedstocks, NH 3 and N 2 O emissions, NC genes were investigated. The results showed that RHB and SDB significantly reduced both NH 3 emission (42.0% and 61.3%, respectively) and N 2 O emission (17.5%, and 41.7%, respectively), yet the reduction exhibited only a weak correlation with the physical attributes of the RHB. Microbial co-occurrence network showed biochar enhanced the complexity of the microbial community and the positive-to-negative correlation ratios. RDA analysis elucidated that biochar additives improved both total organic carbon (TOC) and C/N by inhibiting denitrification-related narG/Z/nxrA, nirK and norB/C genes while promoting nosZ , and inhibiting mineralization-related GLUD1_2/gdhA, E3.5.1.49, E3.5.5.1, gudB/rocG genes. PLS-PM model further confirmed that TOC was negatively correlated to NC genes involved in N 2 O- and NH 3 - production. The study provided theoretical basis and a road map to develop efficient composting additives to regulate TOC and C/N and to reduce NH 3 and N 2 O emissions. • Biochar significantly reduced 17.5% - 42% NH 3 and 41.7% - 61.3% N 2 O emission during composting. • Biochar improved TOC and C/N and inhibited genes related denitrification and mineralization. • Biochar enhanced the complexity and the positive-to- negative correlation ratios of microbes. • Regulating TOC and C/N could be a controllable pathway to reduce NH 3 and N 2 O emission in composting.