SOM in the Subsoil: Quality and Transport of Mobile Organic Matter Across the Topsoil-Subsoil Boundary and Beyond.

The fate of organic carbon in soils is strongly linked to the (trans-)formation, release and transport of its mobile fraction. Commonly, mobile organic carbon is considered as the dissolved organic carbon, an operationally defined fraction that passes a 0.45µm filter. Yet, under certain and favourable conditions even particles up to a size of some tenths of micrometers can be translocated in soils and beyond. Thus, a considerable amount of mobile organic matter that includes biocolloids, mobile genetic elements and microorganisms is vastly ignored. Mobile organic matter is formed and released at biogeochemical interfaces, the “hot spots” of microbial activity and turnover in soils. Release, composition, transport and transformation of mobile organic matter is studied with a combined theoretical and experimental approach, the latter comprises groundwater wells, field lysimeter and laboratory column studies. New to the field approach was a specific design of the groundwater wells to allow for collection of particles up to the size of some millimeters as well as the design of the tension-controlled lysimeters that sampled particles up to 10µm. The column studies were run at variable initial and boundary conditions, yet a specific feature was to mimic the topsoil-subsoil situation by using columns packed with an organic rich source layer (top-soil) and a mineral rich reception layer (subsoil). By doing so, the combined effects of release, transformation and reactive transport under conditions relevant for natural systems can be studied. The lysimeters used in this study were not packed or refilled, but installed below both topsoil and subsoil without disturbing the natural soil bedding. Release of mobile organic matter was in general controlled non-equilibrium processes with strong “biological” and “colloidal” control. The released material comprised a vast variety of organic materials of biotic origin and organo-mineral associations. Besides the operationally defined dissolved and colloidal fraction, at certain conditions also larger particles including competent cells and microorganisms are translocated. Retardation and redistribution does not exclusively on the presence of reactive surfaces. Yet, it is strongly linked to the dynamics of the soil solution (aqueous phase). Immobilization and transformation in the subsoil is thus controlled by physical (hydraulics; entrapment), physicochemical (sorption; colloidal interactions) and biological mechanisms.