Laser-based 3D microscopic gauging of soil aggregate coating thickness and volume

In structured soils, clay-organic coatings are spatially distributed along macropore surfaces. Information on thickness and volume of coating material is essential for macropore-matrix mass exchange of water and solutes. However, their determination is difficult and fraught with uncertainty due to irregular shapes of macropore surfaces. The objective of this study was to test the use of a three-dimensional (3D) confocal laser scanning microscope for gauging soil macropore coatings. For this test, coating material was manually separated from the surface of intact soil clods (5 cm edge length) sampled from Bt horizons (with illuvial clay) of till-derived and loess-derived Haplic Luvisols. The resulting coating thicknesses ranged between approx. 0.1 and 0.5 mm and volumes between 0.057 and 2.9 mm3 with mean densities of 1.96 g cm−3 for loess-derived and 2.67 g cm−3 for till-derived coatings. The 3D laser microscopic method yielded data based on micro-topographic information. The key benefits of the 3D confocal laser scanning microscope are the information on the mm-scale spatial distribution of coating thickness and bulk density. These additional parameters of the clod surface or aggregate micro-topography can be useful for improved quantification of accessibility of sorption surfaces and for describing macropore-matrix mass transfer of reactive solutes.