Impact of Soil Micromorphology on Stability of Soil Structure and Soil Hydraulic Properties.
Radka Kodesová, Czech University of Agriculture in Prague, Kamýcká 129, Prague, Czech Republic, Anna Zigová, Academy of Sciences of the Czech Republic, Rozvojová 135, Prague, Czech Republic, Marcela Rohosková, Czech Univ of Agriculture in Prague, Kamýcká 129, Prague, Czech Republic, Vit Kodes, Czech Hydrometeorological Institute, Na Sabatce 17, Prague, Czech Republic, and Miroslav Kutílek, /, Nad Patankou 34, Prague, Czech Republic.
Micromorphological properties, stability of soil structure, physical, chemical and hydraulic properties were studied for following soil types on different type of parent materials: Chernozem (loess), Luvisols (loess and loess loam), Cambisols (orthogneiss and paragneiss) and Phaeozem (loess). Undisturbed large soil aggregates, undisturbed 100 m3 soil samples and disturbed soil samples were taken from each horizon that were specified for each soil type. Micromorphological properties characterizing soil porous structure were studied on soil thin sections prepared from the large soil aggregates, consisting of primary, secondary and higher orders peds. Additional soil thin sections were made from the 100 cm3 soil samples after the analysis of physical and hydraulic properties. Depending on type of pedogenesis, the soil porous systems exhibit different shape factors (regular, irregular and elongated pores), different size classes, and different pore-size distributions, that appeared to be mono-, bi- or multi-modal. In general structured soils are characterized by a multimodal pore-size distribution with a hierarchical distribution of pore-sizes. Structureless soils are characterized by a tendency to mono-modal pore-size distribution and pores are randomly distributed in the space. Soil structure, as result of soil genesis, depends on a soil particle size distribution, presence of organic matter, transport processes within the soil profile, influence of roots and living organism and so on. Irregularity of prior soil pores, pore sizes and modality of pore-size distributions decreases with the depth in studied soil profiles. The sizes and shapes of pores affecting soil hydraulic functions are affected by coatings and infillings. Pores in the top ploughed horizons of all soil types apart from Phaeozem are not affected by coatings. Pores in subsurface horizons are affected by clay coatings in both Luvisols and Phaeozem and also by coatings of amorphous forms of CaCO3, calcite needles and calcite rhombohedras in deeper horizons of Luvisols (loess), and Phaeozem. Coatings and infillings of amorphous forms of CaCO3, calcite needles and calcite rhombohedras affect pores in subsurface horizons in Chernozem. Pores in both Cambisols are not affected by any coatings. The stability of the soil structure was studied using two different tests that are usually used to study aggregate stability under the different destruction mechanisms. Stability of soil structure initially increases and then decreases with the depth in Chernozem, Luvisol (loess) and Phaeozem. The increase of soil structure stability in horizon below the top one was caused by combination of relatively high organic matter content (that was lower then that in the top horizon), presence of clay coatings and no effect of tillage. Stability of soil structure decreases with depth in the other soil types. The soil structure stability for Cambisols appeared to be greater then that for other soils in spite of purely developed soil aggregates in the Cambisols probably due to presence of iron oxides. Soil hydraulic properties were studied in the laboratory. Soil water retention curves were determined using the sand tank and pressure plate apparatus. The saturated hydraulic conductivities were measured using the constant head test. The multi-step outflow method was applied to estimate soil water retention curves and unsaturated hydraulic conductivity curves via numerical inversion assuming the single-porosity, dual-porosity and dual-permeability model. The ratios of different pore domains were estimated based on micromorphological studies. The points of the soil water retention curves were also obtained from water volume balance in the soil sample. The soil water properties reflect the pore-size distribution studied using the images analysis as well as the stability of the soil structure.