Microbial diversity and biosafety judgment of digestates derived from different biogas plants for agricultural applications

The composition of microbial communities is the key to effective anaerobic digestion (AD). The microbiome driving the AD process has been extensively researched, whereas the influence of specific substrates on the microbiome of digestate remains insufficiently investigated. Digestate has considerable potential for use in soil fertilization and bioremediation, therefore its biological safety should be monitored. Moreover, the knowledge about the composition of microbial communities and their interconnections in digestate should be extended, due to the impact on soil microbiota and its functionality. The aim of this study was a comprehensive assessment of the (1) sanitary quality, (2) core microbiome, and (3) microbial interactions in digestates collected from three full-scale agricultural biogas plants, with particular emphasis on their applicability from the perspective of the resident microbiota. Analyzed samples of digestate were derived from various substrates used for AD, including plant- and animal-based materials, and industrial waste. The study demonstrated that the phyla Bacillota, Bacteroidota, and Cloacimonadota were the most dominant in digestates regardless of the composition of the processed substrates, however, member composition at the genus level differed significantly between samples. In addition, we observed that microbial genera belonging to the less prevalent phyla play an integral role in the forming of microbial community interactions. Dominant microbial taxa with broad metabolic capabilities, potentially improving soil quality and functionality, have been identified. Moreover, we confirmed, that digestate samples were free of analyzed pathogenic bacteria and parasites. The study results indicate that digestate may have an immense fertilizing and bioremediation potential that has not been fully availed of to date.