Selective transport of plant root-associated bacterial populations in agricultural soils upon snowmelt Dörte Dibbern a , Andreas Schmalwasser b , Tillmann Lueders a, * , Kai Uwe Totsche b a Institute of Groundwater Ecology, Helmholtz Zentrum München e German Research Center for Environmental Health, Ingolstädter Landstr. 1, 85764 Neuherberg, Germany b Institute of Geosciences, Department of Hydrogeology, Friedrich-Schiller-Universität Jena, Burgweg 11, 07749 Jena, Germany article info Article history: Received 19 August 2013 Received in revised form 16 October 2013 Accepted 17 October 2013 Available online 7 November 2013 Keywords: Bacterial transport Biocolloids Preferential flow Particulate organic matter Soil organic matter Lysimeter abstract Plants introduce abundant carbon into soils, where it is mineralised and sequestered. Proportions of this fresh organic carbon introduced to top soils can be relocated to deeper soil layers and even to groundwater by event-driven transport upon heavy rainfalls or after snowmelt. It is assumed that a significant fraction of this flux involves biocolloids and possibly microbial biomass itself. However, the nature of such transported microbes, their origin and the mechanisms of their mobilisation are still poorly understood. Here, we provide primary evidence that specific microbial populations are exported from top soils upon seepage events. At an experimental maize field, we have analysed the composition of mobilised bacterial communities collected in seepage water directly after snowmelt in winter at different depths (35 and 65 cm), and compared them to the corresponding bulk soil microbiota. Using T-RFLP fingerprinting and pyrotag sequencing, we reveal that mostly members of the Betaproteobacteria (Methylophilaceae, Oxalobacteraceae, Comamonadaceae), the Alphaproteobacteria (Sphingomonadaceae, Bradyrhizobiaceae), the Gammaproteobacteria (Legionellaceae) and the Bacteroidetes (Sphingobacteriaceae) were mobilised, all characteristic taxa for the rhizoplane. This highlights the importance of preferential flow along root channels for the vertical mobilisation and transport of microbes. Although the estimated quantitative fluxes of bacterial biomass carbon appeared low, our study allows for an improved under- standing of the links between top soil, subsoil, and groundwater microbiota, as well as carbon fluxes between soil compartments. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Soil organic matter (SOM) is the largest active carbon pool in terrestrial environments (Kindler et al., 2009) and it is known that carbon fluxes in soil can be considerable (Giardina et al., 2005). At the same time, many central mechanisms of carbon fluxes in soils are still poorly understood (Litton and Giardina, 2008). Quantity of SOM and carbon inputs is mostly determined by plants (Kögel- Knabner, 2002), however, SOM properties like stability, aggrega- tion and reactivity are determined by the transforming organisms, most prominently soil microbes (Deckmyn et al., 2011; Kindler et al., 2009). Besides sequestration and mineralisation, transport of SOM by seepage water from top soils to deeper zones is an important factor contributing to carbon fluxes in soils (Kindler et al., 2011). These vertical fluxes represent a significant supply of fresh carbon to deeper soil layers and to the groundwater. The mobile organic matter pool in soils not only comprises dissolved and colloidal organic carbon, but also biocolloids like bacteria, fungi and their fragments as well as viruses (Totsche et al., 2007). The translocation of colloids and particles, frequently along preferential flow paths including biopores can mediate fast and considerable mass transfer into deeper zones. It has been specu- lated that translocated microbes from the top soil could be an important source of biomass for subsoils, where they may signifi- cantly contribute to microbial activities (Jaesche et al., 2006). The general understanding of the physical factors controlling vertical carbon transport through soil has improved over the last years (Bolan et al., 2011; Kalbitz and Kaiser, 2008). Already now, there is a basic grasp of bacterial transport mechanisms in soils, mostly focused on the transport of potential pathogens to groundwater (Natsch et al., 1996). Important factors inhibiting bacterial mobilisation are retention at airewater- and soilewater- interfaces, attachment and growth in biofilms, straining and also active adhesion (Sen, 2011). Soil bacteria can move actively in soils guided by chemotaxis (Sen, 2011) or may be mobilised and * Corresponding author. Tel.: þ49 89 3187 3687. E-mail address: tillmann.lueders@helmholtz-muenchen.de (T. Lueders). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soilbio.2013.10.040 Soil Biology & Biochemistry 69 (2014) 187e196