Mineralisation of 2,4-dichlorophenol and glucose placed into the same or different hydrological domains as a bacterial inoculant

Abstract The spatial location of microorganisms in the soil three-dimensional structure with respect to their substrates plays an important role in the persistence and turnover of natural and xenobiotic organic compounds. To study the effect of spatial location on the mineralisation of 14 C-2,4-dichlorophenol (2,4-DCP, 0.15 or 0.31 μmol g −1 ) and 14 C-glucose (2.77 μmol g −1 ), columns packed with autoclaved soil aggregates (2–5 mm) were used. Using a chloride tracer of water movement, the existence of ‘immobile’ water, which was by-passed by preferentially flowing ‘mobile’ water, was demonstrated. By manipulation of the soil moisture content, the substrates were putatively placed to these conceptual hydrological domains (immobile and mobile water). Leaching studies revealed that approximately 1.7 (glucose) and 3.4 (2,4-DCP) times the amount of substrate placed in mobile water was recovered in the first 4 fractions of leachate when compared to substrate placed in immobile water. The marked difference in the breakthrough curves was taken as evidence of successful substrate placement. The 2,4-DCP degrading bacterium, Burkholderia sp. RASCc2, was inoculated in mobile water (1.8–5.2×10 7  cells g −1 soil) and parameters (asymptote, time at maximum rate, calculated maximum rate) describing the mineralisation kinetics of 2,4-DCP and glucose previously added to immobile or mobile water domains were compared. For glucose, there was no significant effect ( P >0.1) of substrate placement on any of the mineralisation parameters. However, substrate placement had a significant effect ( P