Fluorescence imaging for mm-scale observation of macropore-matrix mass transfer: Calibration experiments

Abstract Macropores are important for gas, water, and heat movement in soils. Still, the transfer of water and solutes through the interface between macropore and matrix like the walls of biopores is less understood. For mass-exchange processes and water storage in soil aggregates, relatively small sizes and volumes limit the analysis of distributed interface properties including the permeability for water or air. Photo-based imaging techniques could perhaps be used to trace the movement of sorbing chemicals at such smaller scale similarly as for larger soil profiles. The objective was to test the suitability of Na-Fluorescein (i.e., fluorescein di sodium salt) for marking small-scaled flow and transport processes through surfaces of biopore walls and coated soil aggregates and for determining water and reactive solute mass exchange. Fluorescence imaging was carried out using fluorescence spectrometer (for aqueous solutions) and a uv/vis photo-based imaging technique for soil and intact biopore and aggregate surfaces of soil samples from the Bt-horizon of a haplic Luvisol. For calibration, a linear relation between fluorescence intensity and initial Na-Fluorescein concentrations could be used for aqueous solutions up to a concentration of 100 mg L−1 Na-Fluorescein. The fluorescence intensities of homogenized soil samples were used to determine the amount of sorbed dye at equilibrium. After calibration, the photo-based imaging technique allows determining mm-scale maps of dye concentrations at intact soil surfaces in both, the dissolved and sorbed fraction of the dye. Uncertainties are caused mainly by effects of fluorescent soil organic carbon that is heterogeneously distributed at biopore wall surfaces. The results suggest that sorption of the dye mostly occurs in the soil region next to the macropore surface. A few high concentration spots at greater distances could be an indication for rapid tracer movement along root channels suggesting occurrence of non-uniform flow and transport even at the scale of soil clods and aggregates. The method seems promising for a small-scale analysis of macropore-matrix exchange of reactive solutes.