Inorganic bulking agents reshape microbial co-occurrence networks and functional guilds to differentially mitigate NH3 and N2O emissions in sludge composting

NH 3 and N 2 O emitted during sewage sludge composting cause odor pollution and greenhouse effects. This study evaluated three inorganic bulking agents (volcanic rock, zeolite, and ceramsite) on composting efficiency and emissions of NH 3 and N 2 O and explored the underlying mechanisms through physicochemical analyses, microbial co-occurrence networks and functional genes. All three agents increased seed germination index and promoted organic matters degradation and humification. Zeolite reduced NH 3 and N 2 O emissions by 51.31% and 39.70%, respectively, while ceramsite by 11.84% and 41.85%. In contrast, volcanic rock increased both emissions. Zeolite reshaped microbial networks into a highly modular and decentralized architecture. Furthermore, zeolite selectively enriched nitrifiers, denitrifiers and heterotrophic nitrifying-aerobic denitrifying (HN-AD) bacteria and increased the gene abundances of nitrification and denitrification pathways, particularly nosZ gene. Therefore, appropriate bulking agents, especially zeolite, could improve compost efficiency and mitigate gaseous emissions by modulating microbial community structure and function, offering an eco-friendly strategy for sludge composting.