Seepage-mediated export of bacteria from soil is taxon-specific and driven by seasonal infiltration regimes

The seepage-mediated translocation of soil microorganisms across the soil-regolith boundary plays a central role in connecting surface and subsurface habitats. However, field studies targeting the factors controlling the dynamics of this translocation are still rare. We aimed to disentangle the effects of taxonomic identity, soil type,
land use, and hydrological season on the translocation of bacteria over 1.5 years. Seepage-mediated bacterial transport was assessed using tension-controlled lysimeters in various soil types of a low-mountain groundwater recharge area under a temperate climate. Susceptibility to transport with seepage was primarily taxon-specific while influence by land use played only a minor role. The estimated total export of cells from soil ranged from 5.2 × 1011 to 1.3 × 1012 cells per m2 during a six-months-period and was 1.5 times higher during hydrological winter than in summer. In winter, low ionic strength seepage likely favored the export of bacterial
groups with cell sizes of less than 1 μm such as Cand. Patescibacteria or Bdellovibrionaceae, or extracellular stages
of potential pathogens of eukaryotes (Chlamydiales, Rickettsiales). Cand. Patescibacteria alone accounted for 40% of the cells exported in winter. Mobile cells contributed 1.3–2.1% to the total organic carbon exported from topsoil and subsoil. Our results demonstrate that substantial bacterial biomass is continuously exported from soils to subsurface environments. Export is strongly driven by the susceptibility of specific bacterial taxa to undergo translocation, and by seasonal infiltration regime. Enhanced introduction of bacterivorous or potential pathogenic taxa from soil to groundwater upon recharge may represent an important controlling factor for bacterial interaction networks or trophic interactions in groundwater food webs.