Differences in distribution of functional microorganism at DNA and cDNA levels in cow manure composting

Denitrification and nitrification processes are the two prominent pathways of nitrogen (N) transformation in composting matrix. This study explored the dynamics of denitrifying and nitrifying bacteria at different composting stages of cow manure and corn straw using functional gene sequencing at DNA and cDNA levels. Corresponding agreement among OTUs, NMDS, mental test and network analyses revealed that functional bacteria community compositions and responses to physicochemical factors were different at DNA and cDNA levels. Specifically, some OTUs were detected at the DNA level but were not observed at cDNA level, differences were also found in the distribution patterns of nitrifying and denitrifying bacteria communities at both levels. Furthermore, co-occurrence network analysis indicated that Pseudomonas, Paracoccus and Nitrosomonas were identified as the keystone OTUs at the DNA level, while Paracoccus, Agrobacterium and Nitrosospira were keystone OTUs at the cDNA level. Mantel test revealed that TN, C/N and moisture content significantly influenced both the denitrifying bacteria and ammonia-oxidizing bacteria (AOB) communities at the DNA level. NO₃^--N, NH₄⁺-N, TN, C/N, and moisture content only registered significant correlation with the nosZ-type denitrifiers and ammonia-oxidizing bacteria (AOB) communities at the cDNA level. Structural equation model (SEM) showed that TN, NH₄⁺-N, and pH were direct and significantly influenced the gene abundance of denitrifying bacteria. Howbeit, TN, NH₄⁺-N, and NO₃^--N had significant direct effects on amoA gene abundance.