Bacillus subtilis immobilization on biochars produced from different feedstocks and pyrolysis temperatures: performance, mechanisms, and salt tolerance

Using biochar as a carrier to enhance the adaptability and survival of inoculated microorganisms under harsh environmental conditions is considered as a promising strategy. However, the immobilization performance and underlying mechanisms of microorganisms on different biochars remain insufficiently understood. In this study, a series of biochars were prepared from various feedstocks and at different temperatures, and their capacities to immobilize Bacillus subtilis (B.subtilis) were evaluated. The results demonstrated that high-temperature (700 °C) biochars exhibited 14.40%-60.00% greater loading capacity for B.subtilis, compared to low-temperature (400 °C) biochars. Analyses of surface morphology, functional groups, hydrophobicity, and Zeta potential before and after immobilization indicated that B.subtilis loading significantly altered the surface properties of biochar, including increases in C and N contents, enhanced richness and diversity of functional groups, and a transformation from hydrophobicity to hydrophilicity. Adsorption kinetics and isotherm modeling revealed that the immobilization process was dominated by chemical adsorption, characterized by monolayer adsorption on a homogeneous surface. Interface interaction analysis further confirmed that the electrostatic interaction, hydrogen bonding, and hydrophobic forces between the functional groups of biochar and those on B.subtilis cells synergistically facilitated microbial immobilization. Based on these findings, a two-stage adsorption process of B.subtilis on biochar was proposed: initial surface adhesion and pore filling, followed by extracellular polymer and surface functional group complexation. Biochar properties, including specific surface area, pore volume, Zeta potential, hydrophobicity, C/N ratio, and surface functional groups, were further identified as key factors influencing immobilization. Finally, the salt tolerance of B.subtilis was significantly enhanced when immobilized on biochars, particularly corn straw biochars with large specific surface areas and developed pore structures. This improved salt tolerance suggests that the biochar-based microbial fertilizers could serve as a promising approach for the amelioration of salt-alkaline soil.