Evaluation of Bacterial Diversity Dynamics During the Composting of Microbial Fertilizer with Inoculant Addition

In recent years, microbial biofertilizer has been widely applied as a crucial tool for enhancing soil fertility, promoting plant growth, and increasing crop yields and plays an increasingly prominent role in sustainable agricultural practices. The microbial communities in microbial biofertilizers modulate the efficiency of the composting process and the quality of the final product via diverse ecological functions, including organic matter degradation, nitrogen cycling, and carbon fixation. However, the dynamic shifts in microbial communities during composting and their intricate interactions with environmental factors remain insufficiently investigated. This study aimed to assess the dynamic shifts and diversity profiles of bacterial communities during the composting of microbial biofertilizers. The abundance, diversity, and community structural dynamics of bacteria across distinct composting stages were analyzed using PCR-DGGE and Illumina MiSeq sequencing technologies. The compositional variations in bacterial communities across different composting stages were compared using alpha diversity indices (e.g., Chao1, Shannon, Simpson), beta diversity analysis, and principal coordinate analysis (PCoA). The composting feedstocks included cow manure, straw, and a liquid inoculant of Bacillus sphaericus thermophilus (a thermophilic spherical urea-degrading bacillus). These materials were mixed at a specific ratio and subjected to aerobic composting. Samples were collected on days 10, 20, 30, 40, and 50 of composting (designated groups S1 to S5) to characterize the temporal succession dynamics of the microbial communities. The results demonstrated that the number of bacterial operational taxonomic units (OTUs) changed significantly over the composting period: the S1 group exhibited the highest OTU richness, while the S5 group had the lowest. Additionally, across different composting stages, the phylum Firmicutes dominated the initial phase but gradually decreased in relative abundance, whereas the abundance of phyla such as Proteobacteria progressively increased in the later stages. Taxonomic analysis revealed that Ureibacillus thermophilus was relatively abundant during the early composting stage, whereas the abundance of Sporosarcina and Sphaerobacter peaked during the middle stage. In contrast, the abundance of genera such as Taibaiella increased during the late composting stage. Principal coordinate analysis (PCoA) further revealed that bacterial communities differed significantly between samples from distinct composting stages. Microbial co-occurrence network analysis revealed that the microbial community exhibited small-world and modular properties during composting. The 17 identified keystone genera were primarily affiliated with five phyla, including Proteobacteria. Despite their low relative abundances, these keystone genera may play potentially critical roles in mediating the composting process. This study offers novel insights into the succession of microbial communities during composting and further elucidates the associations between shifts in microbial community diversity, composting efficiency, and the quality of the final compost product. The findings of this study provide a theoretical foundation for optimizing composting processes and enhancing composting efficiency while also offering valuable insights for the application and development of microbial inoculants. Future research may further investigate the specific contributions of distinct functional microbial taxa to the composting process, as well as strategies for optimizing microbial community assembly via environmental modulation to enhance composting performance and end-product quality.