Insights into the interfacial dynamics and interaction mechanisms between phosphate-solubilizing bacteria and straw-derived biochar

Abstract To alleviate soil phosphorus deficiency, integrating straw-derived biochar with phosphate-solubilizing bacteria (PSB) has been recognized as a promising solution and is gaining growing attention. However, the mechanisms of bacterial immobilization and the influences of the physicochemical attributes of biochar remain unclear. In this study, we investigated the single-cell interactions of gram-negative Acinetobacter pittii and gram-positive Bacillus subtilis with cotton straw-derived biochars, subjected to progressively increasing pyrolysis temperatures, to understand the attributes of gradually modified biochar properties. The results revealed the correlations between adhesion forces and biochar properties (e.g., surface area and surface charge), and the strongest adhesion for both strains for the biochar pyrolyzed at 700 °C. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) model, structured to predict interaction energy, was subsequently compared with experimental observations made using atomic force microscopy (AFM). Discrepancies between the predicted high adhesion barriers and the observed attraction suggested that forces beyond Lifshitz-van der Waals also influenced the immobilization of PSB. Adhesion-distance spectroscopy and XDLVO theory jointly revealed four distinct phases in the immobilization process by biochar: planktonic interaction, secondary minimum entrapment, primary barrier transcendence, and initial reversible adherence, collectively facilitating biofilm formation. Notably, initial reversible adhesion positively correlated with increased protein and polysaccharide levels in extracellular polymeric substances (EPS) (R 2 > 0.67), highlighting its importance in biofilm formation. Unraveling PSB–biochar interactions can improve the effectiveness of soil inoculants, thereby enhancing phosphorus availability in soil, a crucial factor for promoting plant growth and supporting environmental sustainability. Graphical Abstract