Dynamics of microbial diversity during the composting of agricultural straw

The dynamic changes in microbial diversity during the aerobic composting of agricultural crop straw with additives were evaluated using high-throughput sequencing at four phases of composting (mesophilic, thermophilic, cooling and maturation phases).  In addition, the physicochemical parameters of the composting system were determined in this study.  The fermentation time of the thermophilic period was prolonged with the addition of urea or urea combined with a microbial agent.  The ratio of C/N and germination index variation indicated that the additives were favorable for composting, because the additives directly changed the physicochemical properties of the compost and had effects on the diversity and abundance of bacteria and fungi.  The abundance of operational taxonomic units (OTUs), diversity index (Shannon) and richness index (Chao1) of fungi and bacteria were found to significantly increase when urea+microbial agents  were added to straw in the thermophilic phase.  The relative abundance of the predominant bacteria and fungi at the phylum and genus levels differed during different composting phases.  The abundance of the phyla Firmicutes and Proteobacteria declined in the order of treatments SNW>SN>S (S is straw only compost; SN is straw+5 kg t^–1 urea compost; and SNW is straw+5 kg t^–1 urea+1 kg t^–1 microbial agent compost) in the thermophilic phase.  The abundance of the genera Staphylococcus, Bacillus and Thermobifida followed the same order in the mesophilic phase.  Ascomycota accounted for more than 92% of the total fungal sequences.  With the progression of the composting process, the abundance of Ascomycota decreased gradually.  The abundance of Ascomycota followed the order of S>SN>SNW during the thermophilic phase.  The abundance of Aspergillus accounted for 4–59% of the total abundance of fungi and increased during the first two sampling periods.  Aspergillus abundance followed the order of SNW>SN>S.  Additionally, principal component analysis (PCA) revealed that the community compositions in the straw and straw+urea treatments were similar, and that the bacterial communities in treatments S, SN and SNW in the mesophilic phase (at day 1) were different from those observed in three other phases (at days 5, 11, and 19, respectively), while the fungal communities showed only slight variations in their structure in response to changes in the composting process.  Canonical correlation analysis (CCA) and redundancy analysis (RDA) showed that total carbon (TC), NO₃^–-N (NN), electrical conductivity (EC) and pH were highly correlated with community composition.  Therefore, this study highlights that the additives are beneficial to straw composting and result in good quality compost.