Microbial load, carbon utilization potential, and immobilization efficiency vary across biochar types: implications for bioinoculant formulation.

BACKGROUND

Biochar is a carbon-rich, porous material produced through controlled pyrolysis and widely applied in environmental and agricultural systems. It is increasingly used as a carrier material for microbial bioinoculant formulations and to stabilize anaerobic digestion processes by supporting microbial growth, colonization, and immobilization. However, the use of unsterilized biochar to reduce time, labor, and cost can lead to interference from native microbiota, potentially suppressing desired microbial strains and reducing process efficiency. Therefore, evaluating native microbial populations, growth-supporting ability, and immobilization potential is essential prior to its application.

METHODS

Six biochar samples obtained from the UK Biochar Research Center (UKBRC) were analyzed. Native and recoverable culturable microbial loads were enumerated for each biochar type. Growth-promoting potential was assessed using E. coli as a model organism. Additionally, microbial immobilization capacity was evaluated using both Gram-positive and Gram-negative bacterial strains. Scanning electron microscopy (SEM) was employed to further validate immobilization characteristics.

RESULTS

Distinct variations in native microbial loads were observed among the biochar samples, even under similar storage conditions. None of the biochars served as a carbon or energy source for E. coli ATCC 25922, but some delayed bacterial cell death, depending on the biochar type, indicating a possible viability preserving effect that warrants further investigation in formulated bioinoculant systems. Immobilization capacity varied significantly across biochars and was influenced by both the biomass source and pyrolysis temperature. All biochars showed statistically significant values (P < 0.05) for the immobilization of both the studied strains. SEM analysis confirmed differences in microbial attachment and surface characteristics.

CONCLUSION

The study concludes that biochar potential to support growth and immobilization may vary according to production parameters and the type of biomass. Compatibility between biochar and microbial strains must be carefully evaluated before its use in bioinoculant formulations or other applications to ensure optimal performance and stability.