Characterizing land use impact on multi-tracer displacement and soil structure

Summary Leaching of solutes below the root zone has been identified as a main source of potential groundwater pollution. The occurrence of preferential flow paths in structured soils can enhance rapid leaching of solutes below the root zone. There is evidence that the actual land use can affect solute displacement by altering soil structure and the abundance of preferential flow paths. In the present study, a field experiment was conducted to assess the impacts of land use (grassland vs. no-till cropland) on profile-scale displacement of bromine (Br) and Brilliant Blue FCF. The objectives were (i) to study both solutes displacement patterns, (ii) to analyze the spatial variation and anisotropic variance structures of the solutes and controlling physical soil properties, and (iii) to analyze soil structure development as a result of the land use system and possible implications for solute displacement. Two ponding infiltration experiments with Potassiumbromide (KBr) and Brilliant Blue FCF were performed on a silt loam soil in Lexington, KY. A total of 30 mm multi-tracer solution was infiltrated on an area of 1.20 × 0.70 m. Eleven vertical profile sections (width: 1.10 m, depth: 0.80 m) were excavated in steps of 0.05 m and sampled. Dye stained areas were mapped based on digital image analysis. Small soil samples were taken for Br concentrations, soil texture, and volumetric soil water content at regular intervals along a vertical 0.10 × 0.10 m raster. Vane shear resistance was measured as a proxy for mechanical soil strength. X-ray fluorescence analysis was used to determine total Br contents and the relative SiO 2 signal intensity, the latter being used as proxy for soil particle size distribution. Although both experimental sites were under the same land use until some 10 years ago before the current land uses were established, solutes displacement differed between both land uses. The dye-stained patterns revealed a high proportion of non-equilibrium flow through vertically orientated macropores and a less permeable soil matrix at the grassland site. Continuous biological activity since transversion into grassland resulted in these macropores and the absence of any compaction in the subsoil. Large proportions of Br also infiltrated directly into the loose, densely rooted soil matrix close to the surface. Soil structure development at the no-till cropland site was mainly controlled by agricultural operations. The homogeneous Br distribution in the topsoil reflected a less dense soil matrix with a network of well-connected inter-aggregate pores. A residual plough pan restricted solute displacement to deeper soil layers or to groundwater bodies. Although ponding infiltration was applied in this study, the leaching risk for both applied solutes – Br and Brilliant Blue FCF – was rather small.