Changes in bacterial communities during two agricultural solid wastes' co-composting processes

Microbes perform an important role in the solid-state fermentation (SSF) process, and bacterial communities are more or less abundant depending on the starting materials and the composting procedure. In this study, high-throughput sequencing was used to investigate the changes in bacterial communities in different composting piles containing spent pig litter and distiller grains in five dry weight ratios, i.e., 100% distiller grains (treatment 1), 75% distillers grains/25% spent pig litter (treatment 2), 50% distillers grains/50% spent pig litter (treatment 3), 25% distillers grains/75% spent pig litter (treatment 4), and 100% spent pig litter (treatment 5). The results showed that the fermentation time of the thermophilic stage was prolonged with an appropriate content of distiller grains. The alpha-diversity analysis showed that the variation in bacterial richness and diversity in the various treatments (except treatment 5) was greater in the high-temperature stage than in the inception stage and then was slightly lower in the stabilization stage than in the thermophilic stage. The relative abundance of the predominant bacterial communities differed during different composting stages except in treatment 5. The abundant bacterial communities were similar among the treatments with different proportions of distiller grains (treatments 2, 3, and 4) during the high-temperature stage but differed during the stabilization stage, as increasing proportions of distiller grains increased the relative abundance of the phyla Proteobacteria and Firmicutes, and decreased that of Actinobacteria. Additionally, principal coordinate analysis (PCoA) showed that the bacterial communities in treatments 1, 2, 3, and 4 during the initial stage (day 0) were different from those observed during two other stages (day 10 and day 53), while treatment 5 showed only slight variations in the bacterial community structure in response to changes in the composting process. The results indicated that spent pig litter is not suitable for single-material composting and the addition of an appropriate amount of distiller grains can improve the fermentation process. The understanding of the microbial community diversity at molecular level provided a theoretical basis for the optimization of spent pig litter and/or distiller grain fermentation.