Linking pore network characteristics extracted from CT images to the transport of solute and colloid tracers in soils under different tillage managements

The understanding of relations between quantitative information of soil structure from X-ray computed tomography (CT) and soil functions is an important topic in agronomy and soil science. The influence of tillage on macroporosity (i.e., pores measured by CT > 240 mu m in all directions) could be manifested in their effects on solute and colloid transport properties. Tillage will also have crucial importance on preferential flow; i.e., a direct flow through root and earthworm channels. Increasing knowledge of the relationships between soil tillage, structure, and transport contributes to a deeper insight into the key factors of soil management influencing productivity, environmental quality and crop health. The aim of this work is the identification of relationships between soil management of the pore network and the influence of the characteristics of the paths identified by CT on the transport of solute and colloidal.tracers. In this work, we used CT to characterize the macropore network ( > 0.24 mm) of sixteen columns (100 height x 84 diameter, mm) of adjacent plots under different soil management as follows: conventional management with shallow tillage after sowing (4 samples), conventional management with no tillage after sowing (4 samples), and organic vegetables (8 samples). The soil samples were installed in columns under a dripper, and the transport behavior was examined during breakthrough of Br- and 1-mu m latex microspheres in samples near saturation. Transport of Br- and latex microspheres was modeled using a two-region physical non-equilibrium model (dual porosity). Preferential flow was higher under organic management, although the pore water velocities were, in general, lower. The preferential flow of Br- was correlated with the total volume of macropores extracted from each tomography, and the local increase in the Hounsfield value (i.e., CT matrix density, CTmatrix) surrounding the macropores. The denser lining, produced by the earthworms in the inner walls of the pores, was inversely correlated with the kinetic exchange coefficient between mobile and immobile zones of the dual-porosity model. The macropore roughness indicated by the CT-macropore surface area was correlated with the solute dispersion coefficient and with the solute travel time. Finally, we found that the overall CTmatrix density is inversely related to the preferential flow. The importance of this work lies in the improvement of the accuracy of predictions related to flow and transport through soils, especially those processes that include particles traveling through the soil.