• Bacillus subtilis enhances σ a of unsaturated soils via two state-dependent pathways. • In freshly inoculated soils, σ a increases due to microbially altered pore-water chemistry. • In bacteria-colonized soils, biofilms enhance water retention, sustaining conductive pathways. Bacteria present in saturated sediments significantly alter geoelectrical properties. But their role affecting the apparent electrical conductivity of unsaturated soils ( σ a ) remains insufficiently understood. This study elucidates the mechanisms by which Bacillus subtilis enhances σ a under unsaturated conditions. Using a comparative experimental design, we examined freshly inoculated soil (FIS), bacteria-colonized soil (BCS), and water-treated systems across four soil textures, ranging from quartz sand to clay loam. Time-domain reflectometry was employed for high-frequency electrical measurements, complemented by analyses of water content, water potential, pore structure, pore-water chemistry, and bacterial abundance. The results indicate dual mechanisms that enhance electrical conductivity induced by bacteria. In the FIS system, increases in σ a (averaging 72% in quartz sand and 35% in loamy sand) are driven by a soil solution chemistry role, i.e., elevated pore-water conductivity due to bacterial secretion of ions and extracellular polymeric substances. In the BCS system, σ a increase of 10–19% were observed across all textures during a drying period. This is attributed to a hydrological-chemical role: biofilm formation enhances water retention and stabilizes conductive pathways by inducing texture-dependent changes in pore structure observed by CT, and by releasing metabolic and cell-lysis products that increase the chemical pathway. These findings indicate that microbial activity influences soil σ a through direct pore-water chemistry modification and indirect biofilm-mediated hydraulic-chemistry control. This mechanistic duality underscores the necessity to integrate bacteria-related factors into models for the interpretation of soil electrical properties.
胡欢 et al. (Thu,) studied this question.