Determination of microbial numbers in anaerobically digested biofertilisers

This study aimed to quantity total numbers of bacteria, fungi and archaea in different types of commercial liquid anaerobic digestates, and to identify common patterns in their microbial numbers post-digestion and possible implications of their use as biofertiliser. Relationships between microbial numbers and physical–chemical traits of the digestates were also investigated. Quantification was performed using culturable and molecular (quantitative PCR) approaches. Bacterial and fungal CFUs ranged up to five orders of magnitude (10⁵–10¹⁰; 0–10⁵ g⁻¹ DW, respectively) between different types of anaerobic digestates. Bacterial, archaeal and fungal gene copy numbers (GCN) varied by two orders of magnitude (10⁸–10¹⁰; 10⁷–10⁹; 10⁴–10⁶ g⁻¹ DW, respectively) between digestates. All microbial variables analysed showed significant differences between the different types of anaerobic digestate investigated (p < 0.05). Culturable microbial numbers for fungi (6.43 × 10⁴ CFU g⁻¹ DW) were much lower than for bacteria (2.23 × 10⁹ CFU g⁻¹ DW). Gene copy numbers were highest for bacteria (16S) (1.09 × 10¹⁰ g⁻¹ DW), followed by archaea (16S) (5.87 × 10⁸ g⁻¹ DW), and fungi (18S) (1.77 × 10⁶ g⁻¹ DW). Liquid anaerobic digestates were predominantly dominated by bacteria, followed by archaeal and fungal populations. At 50% similarity level, the microbial profiles of the eleven anaerobic digestates tested separated into just two groups, indicating a broad relative degree of similarity in terms of microbial numbers. Higher bacterial (16S) GCN was associated with low OM and C/N ratio in digestates.