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Effect of Phenanthrene and Hexadecane on the Release and Transport of Mobile Organic Matter in Artificial Soil ‐ A Two Layer Column Study

Biogeochemical interfaces (BGIs) in soils are “hot spots” of microbial communities and turnover of organic substances. We hypothesize that the model compounds and additional carbon sources phenanthrene (PHE) and hexadecane (HEX) influence microbial communities, which in turn may affect the release of mobile organic matter (MOM), including biocolloids, and the properties of BGIs in pristine soils. We explored the release of PHE and HEX using a new experimental approach employing two‐layer columns (10x12 cm) filled with an artificial mineral soil mixture (quartz, illite, goethite). In this set‐up, the lower layer, further called “reception layer” (RL, 10 cm), contained the mineral soil material and the upper layer (2 cm) served as source layer (SL) and consisted of the mineral mixture, sterile manure (as OM source) and microbial community extracted from a Luvisol (Scheyern, Germany). SL was spiked with PHE or HEX (2.0 mg g‐1) and, to identify the active microbial degraders, two columns were additionally spiked with labeled 13C‐PHE or 13C‐HEX (2.0 mg g‐1). Un‐spiked columns served as controls. In general, five different variations of SL‐spiking were run in parallel with two replicates per treatment. The experiment was carried out under unsaturated flow conditions. All columns were irrigated with artificial rain water for around 35 days (d) (0.5 pore volume day‐1) with several flow interrupts (FI) of different durations (1‐30 d) to allow reactions on solid phases. Physicochemical and chemical parameters (pH, EC, turbidity, TOC/DOC, anions/cations, PHE, HEX and their metabolites) and microbial community composition were analysed in effluent samples. After the transport experiment, columns were sliced in to 1 cm thick layers and microbial community composition, morphology and topography of solid phases and contact angle were determined along the columns depths. The release of MOM from the columns was in general controlled by non‐equilibrium. The maximum release of organic carbon (after 0.7‐0.9 pore volumes) is delayed which is caused by longer flow path through the columns. PHE and HEX had no effect on MOM release. Turbidity and hydrodynamic diameter, two parameters that relate to suspended particles and colloids, were unaffected in all the treated columns. In all effluent fractions, diverse and heterogeneous bacterial communities were found. This proves the assumption that microorganisms are mobile in the studied artificial soil and under designed flow conditions. Denaturing gradient gel electrophoresis (DGGE) results showed high similarities in bacterial communities associated with the immobile solid phase among different depth layers and treatments. Few bacterial populations in DGGE fingerprints were depended on the distance to SL in PHE‐spiked columns.

Two‐layer columns ; artificial soils ; biogeochemical interfaces ; mobile organic matter, microbial communities

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