Tracer, Dissolved Organic Carbon, and Colloid Leaching from Erosion‐Affected Arable Hillslope Soils

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Core Ideas Pedogenetic processes are spatially related to the slope positions in arable soil landscapes. The Regosol is affected by erosion‐induced CaCO 3 and reduced organic C contents. The leaching potential for colloids and DOC was reduced in the truncated Regosol soil. Soil erosion modifies transport properties in the form of the effective dispersivity. In a morainic arable soil landscape, tillage and water erosion created truncated soil profiles at exposed steep slope positions and colluvial soils in depressions. This study examined the effects of erosion‐induced pedogenetic dynamics on flow and transport properties, colloid mobilization, and dissolved organic C (DOC) leaching for an uneroded Luvisol (LV) at the plateau and a strongly eroded Regosol (RG) at the slope position. Unsaturated steady‐flow experiments were performed on intact soil columns (20‐cm diameter, 16–18 cm high) extracted from the Luvisol (Ap, E, and Bt horizons; three depths) and the Regosol (Ap and CBkl horizons; two depths) in three replicates. Together with a 3 H 2 O tracer, the leaching of colloids and DOC and the electrical conductivity were determined in the percolate. Colloid concentration was obtained from turbidity data. Effective dispersion coefficients, D eff , were determined by fitting the convection–dispersion equation and the mobile–immobile transport model (program CXTFIT) to 3 H 2 O breakthrough curves (BTCs). Values of D eff were lower for Luvisol (20.1 ± 0.3 cm 2 d −1 ; mean ± standard deviation) than for Regosol columns (30.2 ± 12.6 cm 2 d −1 ). The pore structure of the Regosol topsoil was influenced by incorporation of CaCO 3 from glacial till. The plow pan in the Luvisol caused an increased heterogeneity in the BTCs. Differences in transport properties and leaching potentials for colloids (LV: 98 ± 9 mg L −1 ; RG: 15 ± 4 mg L −1 ) and DOC (LV: 13.7 ± 3.7 mg L −1 ; RG: 6.9 ± 1.1 mg L −1 ) suggested a gradual change in the solid‐phase composition and structure of these contrasting soils.