Effect of Surfactants on the Transport and Availability of Plant Growth-Promoting Bacteria in Soil.

Plant growth-promoting bacteria (PGPB) are vital for sustainable agriculture as they mobilize key nutrients, boost stress tolerance, and encourage plant growth. Despite their benefits, delivering PGPB effectively into soil is challenging due to strong bacterial adsorption, limited movement, and variable soil conditions that hinder bacterial mobility and decrease bioavailability in the rhizosphere. Consequently, inoculated bacteria often stay near the application site and struggle to colonize roots effectively. Surfactants have shown potential in improving microbial transport through porous media by altering bacterial–soil and water–soil surface interactions. They lower surface tension and modify electrostatic and hydrophobic forces, reducing bacterial attachment to soil particles and facilitating cell detachment and movement. This research investigates how surfactants (Triton-100, rhamnolipid, and Tween-80) influence the mobility of two model PGPB, Azospirillum brasilense and Bacillus subtilis, in soil columns. It also assesses surfactants toxicity through standardized growth inhibition tests. Toxicity testing revealed that Tween-80 is non-inhibitory. Bacterial transport experiments were conducted in packed soil columns under controlled hydraulic conditions, both with and without surfactant, and bacterial breakthrough curves (BTCs) were generated by continuously monitoring bacterial concentrations in the column influent and effluent as a function of pore volumes to quantify transport behavior. Tween-80 improved bacterial breakthrough and decreased the bacteria deposition rate compared to controls, demonstrating enhanced bacterial transport. These findings suggest that non-toxic surfactants can significantly improve PGPB mobility, offering a promising approach for effective microbial inoculation in sustainable farming.