A novel carbon-nitrogen coupled metabolic pathway promotes the recyclability of nitrogen in composting habitats

This study revealed a novel carbon-nitrogen coupled metabolic pathway. Results showed that the addition of inorganic carbon sources slowed down the decomposition of urea and conserved more nutrients in composting. Metagenomic analysis showed that the main bacteria involved in this new pathway were Actinobacteria, Proteobacteria and Firmicutes. During the late composting period, the dominant genus Microbacteium involved in denitrification accounted for 22.18% in control (CP) and only 0.12% in treatment group (T). Moreover, ureC, rocF, argF, argI, argG were key genes involved in urea cycle. The abundance of functional gene ureC and denitrification genes decreased in thermophilic and cooling phases, respectively. The genes hao, nosZ, ureA and nifH were more closely associated with Chloroflexi_bacterium and Bacillus_paralichenformis. In conclusion, composting habitats with additional inorganic carbon sources could not only weaken denitrification but also allow more nitrogen to be conserved through slow-release urea to improve resource utilization and decrease the environmental risk.