Red mud mitigates NH3 and greenhouse gas emissions during composting: functional gene and microbial community regulation.

The composting of agricultural wastes is accompanied by NH3 and greenhouse gas (GHG) emissions. Red mud, as a novel amendment, has been shown to accelerate humification of composting materials; however, its mechanisms of influencing NH3 and GHG emissions during composting remain unclear. This study investigated how the addition of 5% (w/w) red mud mitigated GHG and NH3 emissions during composting. The results showed that red mud addition effectively mitigated NH3 and GHG emissions during straw and horse manure co-composting, reducing NH3, CO2, and N2O emissions by 7.34%, 25.20%, and 28.90% compared with control, respectively. The emission mitigation was associated with two aspects. First, red mud addition promoted nitrification, as evidenced by lower NH4+-N accumulation, more complete NO2--N transformation and higher abundances of nitrification genes, while improving denitrification (lower nirS and norB; higher nosZ), thereby reducing NH3 and N2O emissions. Second, red mud addition affected the microbial community, altered the number of genera significantly associated with gas emissions, and promoted the dispersal of microbial species. The Mantel test suggested that N2O mitigation was associated with changes in Firmicutes abundance. Zi-Pi analysis identified 7 connector nodes and 4 module hub nodes in RM. The keystone taxa were potentially involved in humification and nitrogen transformation, which may have contributed to the reduction in gas emissions. This study provides evidence that the mitigation of NH3 and GHG emissions by red mud was associated with changes in functional gene abundances and keystone microbial populations, offering a promising approach for sustainable agricultural waste management.