A chemotaxis-haptotaxis coupled mechanism reducing bacterial mobility in disturbed and intact soils

Contaminant influences microbial movement in soils toward contaminant-concentrated zones in mobile liquid phase via chemotaxis and/or along immobile soil surfaces via haptotaxis. This study investigated the influence of naphthalene on the transport of a bacterium (Pseudomonas fluorescens 5RL, chemotactic to naphthalene) through intact and disturbed soils collected at surface soil (0–5 cm) and subsurface soil (15–20 cm) depths of field. The results demonstrated that naphthalene adsorption on soils significantly reduced bacterial transport, with the extent of the effect subject to soil chemical and pore structure properties. The maximum relative effluent concentration (max C/C₀) of P. fluorescens 5RL from the 0–5 cm intact soil decreased from 0.93±0.01–0.76±0.01 when naphthalene co-existed with the bacteria. This decrease became more obvious in the disturbed soils than in the intact soils at the same depth (e.g., from 0.90±0.02–0.62±0.04 in the 0–5 cm disturbed soil) due to the destruction of macropores (preferential pathways) after the soil disturbation. The macropore destruction increased the diffusion of naphthalene in micropores and its adsorption on soil surfaces, leading to overall stronger toward-surface chemotaxis and haptotaxis effects in the disturbed soils than in the intact soils. In addition, the higher content of organic matter in the surface soils than in the subsurface soils favored naphthalene adsorption. As a result, the toward-surface chemotaxis and haptotaxis effects increased by ∼50 % in the surface soils than in the subsurface soils under both disturbed and intact conditions as indicated by the values of max C/C₀. This study suggests that tillage and organic amendments might be effective measures for in-situ bacterial degradation of organic contaminants brought into soils by applications of animal manures and reclaimed water.