Bacterial dispersers along preferential flow paths of a clay till depth profile

Abstract

This study assessed the dispersal of five bacterial communities from contrasting compartments along a fractured clay till depth profile comprising plow layer soil, preferential flow paths (biopores and the tectonic fractures below) and matrix sediments, down to 350 cm below the surface. A recently developed expansion of the porous surface model (PSM) was used to capture bacterial communities dispersing under controlled hydration conditions on a soil-like surface. All five communities contained bacteria capable of active dispersal under relatively low hydration conditions (-3.1 kPa). Further testing of the plow layer community revealed active dispersal even at matric potentials of -6.3 to -8.4 kPa, previously thought to be too dry for dispersal on the PSM. Using 16S rRNA gene amplicon sequencing, the dispersing communities were found to be less diverse than their corresponding total communities. The dominant dispersers in most compartments belonged to the genus Pseudomonas and, in the plow layer soil, to Rahnella too. An exception to this was the dispersing community in the matrix at 350 cm below the surface, which was dominated by Pantoea. Hydrologically connected compartments shared proportionally more dispersing than non-dispersing amplicon sequence variants (ASVs), suggesting that active dispersal is important for colonizing these compartments. These results highlight the importance of including soil profile heterogeneity when assessing the role of active dispersal, and contribute to discerning the importance of active dispersal in the soil environment.

IMPORTANCE The ability to disperse is considered essential for soil bacteria colonization and survival, yet very little is known about the dispersal ability of communities from different, heterogeneous soil compartments. An important factor for dispersal is the thickness and connectivity of the liquid film between soil particles. The present results from a fractured clay till depth profile suggest that dispersal ability is common in various soil compartments and that most are dominated by a few dispersing taxa. Importantly, an increase in shared dispersers among the preferential flow paths of the clay till suggests that active dispersal plays a role in the successful colonization of these habitats.