Red mud conserved compost nitrogen by enhancing nitrogen fixation and inhibiting denitrification revealed via metagenomic analysis

The objective of this study was to investigate the effects of adding red mud (RM) on denitrification and nitrogen fixation in composting. The results revealed that the retentions of NH4+-N and NO3--N in experimental group (T) with RM were 16.20% and 7.27% higher than that in control group (CK) at the mature stage, respectively. The composition and structure of RM can effectively inhibit denitrification and enhance nitrogen fixation. Moreover, metagenomic analysis revealed that Actinobacteria and Proteobacteria were the main microorganisms in denitrification process, while Firmicutes were the main microorganisms in nitrogen fixation process. In T, denitrifying genes nirK and nosZ were 11% and 18% lower than those in CK, respectively, while nitrogen-fixing genes nifK and nifD were 18% and 34% higher than those in control group, respectively. Therefore, adding RM could reduce nitrogen loss and improve the quality of compost via enhancing nitrogen fixation and inhibiting denitrification process.