Preferential flow paths shape the structure of clay till bacterial communities

Preferential flow paths in subsurface soils serve as transport routes for water, dissolved organic matter and oxygen. Little is known about bacterial communities in flow paths or in subsoils below about four metres. We compared communities from preferential flow paths (biopores, fractures and sand lenses) with those in adjacent matrix sediments of clayey till from the plough layer to a depth of six metres. 16S rRNA gene-targeted community analysis showed bacterial communities of greater abundance and diversity in flow paths than in matrix sediments at all depths. Deep fracture communities contained a higher relative abundance of aerobic taxa and plant material decomposers such as Planctomyces, Nitrospirae and Acidobacteria than adjacent matrix sediments. Similarly, analyses of the relative abundances of bacterial amoA, nirK and dsrB genes indicated transition from aerobic to anaerobic nitrogen and sulphur cycling at greater depth in preferential flow paths than in matrix sediments. Interestingly, preferential flow paths contained more indicator OTUs from the plough layer community than the matrix sediments. This study indicates that the availability of oxygen and organic material and downward transport of bacteria shape bacterial communities in preferential flow paths, and suggests that their lifestyles differ from those of bacteria in matrix communities.