G. N. Fedotov

Colloid-chemical model for describing some soil processes

Arguments are presented for changing the approach to soil studies in order to consider the behavior of soils in terms of a colloid-chemical model rather than in terms of a physical model. Experimental studies of changes in the stickiness of an air-dry chernozem depending on the time elapsed since adding water, of the catalase activity of soils depending on the time elapsed since adding water and salt solutions, of the thermal diffusivity of soils depending on the time elapsed since adding water, and of the soil temperature showed that the soil solution is a structured colloidal system. Electron microscopic studies of soil solutions confirmed that colloidal particles of soil solutions interact with one another due to long-range aggregation to form periodic colloidal structures enclosing soil moisture. It was concluded that the colloidal structuring of the soil solution makes it similar to physiological fluids and makes the soils similar to biological systems; therefore, the consideration of soils in terms of a colloid-chemical model expands our knowledge of these systems and can be useful for soil scientists.

Possible Ways of Nanostructure Development in Soil Gels

It was shown that self-organization with the development of nano- and microstructures is typical for soil gels. Humic substances (HSs), which form the basis of soil gels, have a multilevel structural organization. They appear due to weak interactions during the aggregation of low-molecular-weight organic substances, which results in the formation of HS supermolecules several nanometers in size. The obtained results indicated that the aggregation of HS supermolecules produces fractal clusters of many tens to several hundreds of nanometers in size. These fractal clusters of HS supermolecules are the main elements forming soil gels. The resulting soil gels are suprapolymer systems. On the basis of the generalized experimental data, a conclusion was drawn that self-organization involving microsegregation with the formation of different-type structures is typical for the suprapolymer matrix of soil gels. A mechanism was proposed for the self-organization of the suprapolymer humic matrix with the formation of various nano- and microstructures in soil gels.

Gel structures in soils

The colloidal structures in soils were studied by scanning and transmission electron microscopy. Small-angle neutron scattering was used in the pioneering study of the colloidal soil structures and their rearrangements under the effect of different factors. It was found that colloidal particles are fixed apart in a gel matrix formed by organic molecules. The results obtained suggest that the organomineral gel is composed of soil humus occurring, at least in part, in a gel-like status and reinforced by organic and inorganic colloidal particles. In the interaction with water, the reinforced humus gel behaves as many polymers: it swells, absorbing water and increasing in volume; it shrinks under drying conditions. Different impacts on the soil affect the status of the reinforced humus gel, which results in the observed changes of the soil properties.

Current ideas on the possible pathways for the formation of humic substances in soils

In the current approach to the study of humic substances (HSs), they are usually considered as complex chemical polymeric compounds with a specific combination of properties. This concept of HSs cannot explain their role in soils as a depot of nutrients for microorganisms and some experimental data. The main problem is related to the low energy efficiency of this depot of nutrients, because microorganisms have to consume energy for synthesizing enzymes that destroy HS polymers (macromolecules). At the same time, the recently proposed consideration of HSs as a system of supramolecular compounds completely eliminates this contradiction. In this work, an attempt has been made to consider HSs from other positions based on their possible functional role in soils and entire ecosystems. A scheme has been proposed for the transformation of the litterfall that is based on the supramolecular nature of the HSs and confirmed by reported experimental results.

Structure and properties of soil organic-mineral gel

Changes in the fractal dimension and scattering intensity of colloidal structures in a chernozem, soddy-podzolic soil, and a krasnozem were studied by small-angle neutron scattering at different temperatures and soil water contents. The character of the neutron scattering by soil colloids indicated that the latter were mass fractals in all of the soils studied; i.e., the colloidal particles were located apart from one another even in dry soils. The obtained results confirmed the supposition about the distribution of colloidal particles in the humus gel matrix. The changes in the fractal parameters of the soddy-podzolic soil and chernozem with increasing water contents were nonmonotonic in character, which indicated complex structural rearrangements of the colloidal component in these soils. From the results obtained, a conclusion was drawn that the destruction of the molecular network of reinforced humus gel occurred upon heating the soils to high temperatures: colloidal particles reinforcing the humus gel began to move and coagulate with the formation of dense aggregates. The electron-microscopic study of gel films released from the predried and then capillary wetted aggregates in water showed that the gel films were nonhomogeneous and included zones of humus gel reinforced by colloidal particles and zones almost free from these particles.

Physicochemical principles of the fractal organization of soil colloids

An electron-microscopic study was conducted of gel films collected from aggregates from humus-accumulative horizons of chernozem and soddy-podzolic soils. The aggregates were dried and then capillary-wetted and immersed in water, Solutions obtained by pressing from these soils were also studied. Based on the results obtained, a hypothetical mechanism was proposed for the development of fractal organization of soil colloids, which involves the fixation of micron-size mineral particles in the humus gel and their transformation under the effect of aggressive substances with the formation of colloidal particles of reaction products diffusing in the humus gel. Humus macromolecules contain many polar groups; therefore, the colloidal particles pass some distance and are then fixed on these groups. The greater the distance from a coarse particle in the center of a cluster the smaller the number of colloidal particles capable of traversing it. Therefore, the concentration of colloidal particles decreases when going from the cluster center to its periphery according to an exponential law, which results in the development of the fractal organization in the colloidal soil component. Results of soil studies using the small-angle neutron scattering method were analyzed in terms of the hypothesis proposed.

Colloidal component of granulodensimetric soil fractions

Granulodensimetric soil fractions isolated from chernozem, gray forest soil, and soddy-podzolic soil have been studied using a scanning electron microscope. The studies confirmed that the light fractions with density <1.8 g/cm3 and particle size >53 μm mainly consists of plant residues; however, they also contain mineral particles covered with relatively thick organomineral films and carbonaceous substances. It has been shown that the clay fraction consists of a mixture of ultramicroaggregates of clay minerals ≤1 μm in size covered with a hydrophilic organomineral gel, and the organic matter of the residual fraction includes a stable highly aromatized (lignin-like) material, as well as coals and coal-like materials.

Soil yeasts and their role in seed germination

A collection of yeasts isolated from soils of different types (soddy-podzolic, gray forest, and chernozems) in European Russia was analyzed. Soil yeasts were found to accelerate the development of seeds of common crops (wheat, barley, and rye). Cultures of yeasts from different taxa (representatives of the Rhodotorula, Cystofilobasidium, Sporobolomyces, Metschnikowia, Saccharomyces, Aureobasidium, Debaryomyces, Cryptococcus genera) are shown to stimulate the seed germination. The impact of soil yeasts (and preparations made on their basis) on seed germination is determined by their penetration to seed endosperm (endophytic microflora) followed by the isolation of amylolytic enzymes and physiologically active substances stimulating seed germination. The method of seed treatment is shown to affect greatly the direction of the preparations’ action. When soaking seeds in solutions with the preparation, microorganisms multiply on the seed surface, and when treating seeds using a semi-dry method, seeds themselves and endophytic microorganisms inside them begin to develop. This study can be considered one of the new directions of soil microbiology related to studying the participation of soil microorganisms in the formation of endophytic communities and their role in the germination of vascular plant seeds.

Structural transition in the humic matrix of soil gels and its effect on the soil properties

The analysis of drying-wetting cycles in soils has shown that the existence of the humic matrix of soil gels and, hence, the soil structure is ensured by hydrophilic bonds in dry soils and hydrophobic bonds in wet soils. This suggests that the structural transition from one mechanism controlling the stability of the soil gels and the existence of the soil structure to another mechanism occurs in the humic matrix of soil gels in a specific range of water content. The experimental results have confirmed the effect of the structural transition on the water stability of the soil structure, the pHwater, the hydrophilicity of the soil particle surface, and the structural-mechanical properties of the soils.

Oscillatory processes in the interaction of air-dry soils with water

247 Currently, it is commonly accepted that soil coll loids in the form of gels cover and bind soil particles to one another providing for existence of soil as a system with a certain set of properties [1]. Note that soil gels are regarded as a humus jelly reinforced by various particles [2]. Interacting with water, the reinforced humus jelly behaves as many polymers: it swells absorbing water and increasing in its volume and shrinks when drying. Structural–mechanical characteristics are among the most important properties characterizing such syss tems [3, 4]; these characteristics provide the informaa tion about the structural transformations taking place in the systems. Similar methods have been used in the soil research in this country to get a deeper insight into soils [5–7]. Of special interest is the study on the structural rearrangements in soils during the interaction of airr dry soils with water. These experiments were conn ducted using a Rebinders conical plastometer [8]. The processes that take place after moistening of airr dry soils to the water content that corresponds to elass tic–brittle, plastic, and viscoplastic states have been studied. It has been demonstrated that the strength of a soil structure increases during several days after addd ing water to airrdry soils [8]. Initially, the shear stress changes rather rapidly and than the rate of change gradually slows down. This suggested that swelling of soil gels and transition of airrdry soils into a new steady state required rather a long period. However, study of the structural–mechanical properties with the use of a Rebinders plastometer, first, is insufficiently sensitive and accurate and, secc ond, cannot provide information in a continuous mode. This prevented from studying the structural rearrangements in soils during their interaction with water at the most important initial stage. The goal of this work was to study the changes in airrdry soils during their interaction with water using highly sensitive rotational viscometry [9]. Specimens of gray forest soil from the Vladimir high plains were examined. For preparation of samples, 20–25 g of airrdry soil was mixed for 2–3 min with water in the amount required to obtain the soil samples with specified humidity. Then, the soil sample was loaded into the cell of a Brookfield HBDVll+PRO (Brookfield, United States) viscometer equipped with an SSA small sample adapter and an SC4427 SSA spindle. The time period from water addition to the beginn ning of measurements …