Effects of adding exogenous microorganisms on nitrogen conversion and its mechanism of action in pig carcass composting

This study aimed to reveal mechanisms of nitrogen changes in a mixed compost of diseased pig carcasses and sawdust after adding exogenous microbial agents. It was found that adding exogenous microbial agents significantly increased the content of various types of nitrogen and the germination index of plants. Microbial dominance of phyla was significantly correlated with indicators of physicochemical properties and copy number of nitrogen-functional genes, with simultaneous nitrogen fixation, nitrification, denitrification, ammonification, and glutamate synthesis during composting. At the end of composting, TKN, NH4+-N, and NO3−-N contents were increased by 1.914–3.082 mg/g, 0.047–0.42 mg/g, and 0.247–0.303 mg/g, respectively, in the experimental groups compared to the control group. Proteobacteria, Bacteroidetes, Firmicutes, Actinobacteria, and Planctomycetes were the five predominant phyla, while Sphingobacterium, and Paenalcaligenes were the two special genera in this experiment. After inoculation with exogenous microbial agents, the levels of nitrate nitrogen were significantly and positively correlated (p < 0.05) with the number of Bacteroidetes, the glnA gene was positively related (p < 0.01) with Planctomycetes, and the amoA, GDH2, and GLUD1-2 genes were positively related (p < 0.05) with Paenalcaligenes. The combined analysis has shown that amoA gene was positively related (p < 0.001) with genes such as nxrA, GDH2 and GLUD1-2, as well as indicators of physicochemical properties such as temperature, organic nitrogen and nitrate nitrogen, while the correlation was negative with genes such as nirB and nirS. Inoculation with exogenous microbial agents can significantly prolong the high-temperature period, which was extended by 5, 6, and 5 days in each experimental group, and the roles of nitrogen-functional genes can also be changed. Meanwhile, we found that the addition of exogenous microbial agents could promote the growth and alteration of microorganisms, which regulated the stability and symbiosis of the nitrogen transformation process.