Hydrosorption and Microstructural Characteristics of Soil Horizons of Different Geneses and Their Change in Constructozems

Soil substrates used in model constructozems (artificially constructed soils) of different fabrics created on the territory of Lomonosov Moscow State University in 2012 were investigated. A layered soil construction consisting of the Ap–peat–sand–Ap horizons with a total thickness of 30 cm and a construction made of a mixture of these horizons (18 cm in thickness) underlain by a 12-cm-thick Ap horizon were studied. As a control, a variant with homogenized Ap horizon was chosen. The isotherms of water desorption from the initial soil substrates and from the samples taken from the constructions in 2016 were obtained by the method of hygroscopic equilibrium over saturated solutions of salts. We calculated approximation parameters of the isotherms and values of the total specific surface area of soil substrates according to the BET theory. A shift of water desorption isotherms towards lower values of the water content in the Ap horizon depending on the depth of its location in the profile of constructozems and on the properties of the boundary layers was revealed. It caused a decrease in the total specific surface area by 20–50% in comparison with the initial values in 2012. A sharp decrease in water sorption was noted for the peat of the layered variant of constructozems, in which the median value of the total specific surface area decreased from 560 to 162 cm²/g, and the maximum hygroscopicity decreased from 57.8 to 20.96%. By means of scanning electron microscopy, a distinct close-space microaggregation was detected in the Ap horizon that acquired a loose consistence by 2016 on the surface of variant 2. The surface of the initial peat substrate was smoothed and represented an amorphous mass of light color attesting to its organic nature. By 2016, peat samples virtually lost their amorphous coatings, so that a porous cellular surface was exposed, and its topography corresponded to the surface of plant residues. A decrease in the sorption capacity against the background of stronger roughness and complicated surface topography in most of the studied substrates attested to the high sorption capacity of surface formations of organic nature that covered the initial microaggregates, elementary mineral soil particles, and peat particles.