Thermodynamic system of soil, its homeostasis, and probable mechanism of structure formation

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Thermodynamic system of soil, its homeostasis, and probable mechanism of structure formation

S. Kolomiiets

Institute of Water Problems and Land Reclamation NAAS

37, Vasylkivska Str., Kyiv, 03022, Ukraine

Goal. To substantiate the dynamic functional model of the thermodynamic system of the soil in interaction with cyclic environmental factors, to disclose internal homeostatic processes, the result of which is the reproduction of the soil structure. Methods. System analysis, theoretical -- to generalize the specifics of soil structure and create its physical model; experimental -- for laboratory research. Results. Laboratory hydrophysical tests of soil samples of the intact structure revealed the occurrence of hysteresis of water holding capacity of the soil, the value of which can reach 10 - 20%. The main reason for this phenomenon is the compression of air by liquid membranes in the pore space. The results of the influence of heating the soil sample of constant moisture saturation are presented, which fix the self-oscillating character of the dynamics of capillary potential in the soil. That testifies to the intra-porous redistribution of moisture and the local transport mechanism of convective transport of the matter. Conclusions. The interaction of soil with thermodynamic environmental factors transforms it into a micro gradient dissipative structure with the emergence of many subordinate processes. The main results of their action are the periodic increase of local thermodynamic availability of nutrition for plants and the formation of non-uniform in space cementation of dispersed particles, which become the nuclei of structural units. The concept of soil homeostasis, combining subordination processes, characterizes the level of energy consumption of the external energy flow, due to which the basic properties and structural organization of the soil are reproduced.

Key words: pore space, heterogeneity, hyste-resis, thermodynamic system, synergetics, capillary potential, homeostatic processes.

Soil is a dynamic, living biosystem that is able to reproduce its properties under the action of natural and anthropogenic disturbances. Internal processes, due to which the basic properties and structural organization of the soil system are reproduced, and called homeostasis [1]. Identification of these internal (subordinate) processes in the soil will facilitate the transition to deterministic parametric models of soil interaction with environmental factors, which opens the way to conscious purposeful management of soil properties.

Insufficient attention has been paid to the study of soil interaction with the external environment related to soil ecology. Ideas of soil ecology, which were initiated simultaneously with the formation of genetic soil science by V.V. Dokuchaev, P.S. Kosovich, L.I. Prasolov, B.B. Polinov, V.G. Kornev, scientist school by V.R. Williams, and later developed by G.V. Dobrovolskyi, E.D. Nikitin, V.R. Volobuev, P.P. Nadtochii, and other researchers, unfortunately, have not found practical application [2 - 4]. At this stage of the nascent of the ecology of soil was formed on the relationship between soil and environment, on the simultaneous action of soil formation factors, the concept of paragenesis of soils genetic types in accordance with the spatial variability of their conditions, the ecological functions of soils are determined. However, the interaction of soil with environmental factors in real-time was not considered (aspect “soil-moment”) [5].

The term of general ecology involves the consideration of constant subject-object interaction on a quantitative level. By definition, the soil is an open thermodynamic system that exchanges matter, energy, and information with the environment [1,6]. The current system of soil fertility reproduction in accordance with the law of nutrients return is based on metabolism with the environment. However, without the other 2 components of the interaction, this system is not effective enough, so now there is a further loss of soil fertility around the world. The exchange of soil with energy for the environment determines the availability of nutrients for plants in the vector of soil-plant- atmosphere. And plants in the production process are guided by the rule of minimizing the cost of their own energy consumption. In fact, the study of the energy interaction of soil with the environment began at the beginning of the last century, with the formation of E. Buckingham of the concept of moisture potential [7], and the V.G. Korneev's invention of the equipment for measuring the force of moisture retention in unsaturated soil - tensiometer [8]. The soil water holding capacity determines the availability of moisture for the roots of plants as the main nutrient, through which all water-soluble compounds are consumed. The availability of moisture to plants can be equated with the availability of nutrients from the soil.

The exchange of soil with the environment is practically not studied, it is a kind of terra incognita, although scientists have paid attention to this aspect in the last century [9]. Only due to the formation in the 70-80 years of the last century of the young scientific discipline - synergetics formulated the conceptual foundations of information interaction of soil with the environment, in particular the transfer of related information due to soil regimes and its accumulation in the soil in the form of negentropy - negative entropy [10, 11]. Stability of the soil system due to the law of accumulation of information: the more information in the system, the more resistant it is to external factors.

Thus, the creation of the theory of soil agrogenic evolution and the theory of soil fertility management are impossible without considering the exchange of soil with the environment not only matter but also energy and information.

The purpose of the research is to substantiate the dynamic functional model of the thermodynamic system of the soil in interaction with cyclic environmental factors, the disclosure of internal homeostatic processes, the result of which is the reproduction of soil structure.

Research methods

Theoretical research, systems analysis and synthesis using synergetics tools, experimental laboratory experiments involving thermodynamic hydrophysical methods.

Research results

thermodynamic system soi

The priority in soil ecology is the need to establish a mechanism of external climatic conditions action on the energy state of the heterogeneous soil system. External thermodynamic parameters: temperature (T° C), atmospheric pressure (Ratm), and moisture (0) (precipitation) are distributed in the soil environment from the soil surface in the form of thermal and humidification waves namely have a structure gradient. The thermodynamic state (potential) of a three-phase heterogeneous system of soil can be determined at any stage. However, the best to determine it is in the liquid phase, due to the moisture potential [8], which depends on the configuration of the water body, bounded by the interface “solid particles--liquid” and “liquid-air” and determines the level of surface energy in the system. It is these interface surfaces that determine the fundamental property of the soil - its heterogeneity. Another fundamental property is its hysteresis, which is manifested in the ambiguity of the relationship between moisture saturation and the strength of the water holding capacity of the soil [12]. A clear explanation of the physical essence of the phenomenon of capillary hysteresis was obtained on the basis of the proposed model of the soil pore space in the form of corrugated equivalent capillary [13], which justifies the presence in the expansion of the pore space of compressed air, which may be in equilibrium. There is always trapped air in the soil, the amount of which depends on the structure of its pore space and moisture saturation and can be tens of percent of the soil volume. The most important thing is that the air concentrated in the entire volume of the soil environment and with the change of external thermodynamic parameters reacts by changing its volume and gas pressure. In fact, this compressed air acts like a micropump and it becomes a dispersed center of thermodynamics imbalance (CTI), from which moisture can be squeezed out or, conversely, enter the body of the pore. That is, radial gradients are periodically created in the CTI environment, which leads to a local centrifugal redistribution of moisture in the soil. In fact, when interacting with changing external climatic factors, the soil becomes a micro gradient structure with periodic local redistribution of matter, where the trapped air plays the role of a propellant, a kind of “heart” of the mobility of the pore solution. Synergism of interaction of these microzones can lead to changes in the thermodynamic state of the soil at the level of the macroparameter of the system.

Particular attention should be paid to the local redistribution of the pore solution in the environment of the CTI, which convectively transports various compounds. Such a transport mechanism was lacking in theories of humus formation [14] and concepts of structure formation [15]. Another feature of air compression is that with increasing gas pressure in the bubble, it begins the diffusion of gases that make up the soil atmosphere. Its content of carbon dioxide (CO2) is an order of magnitude higher than the open atmosphere up to 3-20 %. This gas has the highest solubility in the pore solution and, dissociating, increases the concentration of hydrogen ions, and namely increases the acidity of the pore solution (pH) in the environment of the trapped air in certain periods. That is, the CTI periodically becomes an acid center (AC), near which centrifugal acidity gradients (pH) are created. Acidity is a determining factor of phase equilibrium for compounds sensitive to this parameter. Thus, in the environment of CTI+AC the is locally disturbed the phase of equilibrium, which together with the local mobility of pore solutions creates conditions for a certain spatial selection and zoning of compounds centrifugally from compressed air and unique opportunities to change the volume of elementary pores due to relative movement of excess gas pressure which release from the cement of dispersed particles that make them consist. An ecotone of the soil microbiota is also formed around the bubbles of compressed air in accordance with the radial zonation of the ecological conditions of their soil life.

The concept of an active kinetic environment is key for synergetic [16], and the soil fully corresponds to it. It has a distributed source of energy and/or energy-rich substances; each elementary volume of the medium is in a state far from thermodynamic equilibrium; communication between adjacent elementary volumes is carried out through exchange processes.

The priority for the soil is the question of the availability of a distributed energy source. Organic matter is an energy reservoir, which is slowly released into the soil during its biochemical decomposition and actually determines its energy efficiency. However, the kinetic component of the energy balance of the soil is many times greater than the energy content of organic matter in it and depends on the level of dissipation of solar energy, which is determined by the intensity of energy-consuming subordinate soil processes. Such processes ensure the homeostasis of the soil, the result of which is the reproduction of a certain structure and high humus content.

Soil structure. In modern agrophysics, there is no concept of elementary volume as the level of organization of soil mass [17]. However, its presence is fundamental for thermodynamic and synergetic. In agrophysics, the structure of the soil is understood as a set of features, aggregates, different in size, shape, strength, and coherence [15], characteristics of each soil, and its horizons. However, this approach to assess the structure of the soil as a highly organized system is incorrect, because the ratio of fragments is a function of the method of violation and many other factors (temperature, humidity, etc.). The most important thing is that the synergy of the interaction of all components of dispersed soil elements is not taken into account. Plant interacts with the soil as a whole system, and the availability of food is due to the potential of moisture as an integrative concept of the three-dimensional structure of the soil. Therefore, thermodynamic methods should be used to characterize the structure of the soil.

The fundamental property of soil is its heterogeneity, which is integrally characterized by the surface areas of the section “solid particles--liquid” and “liquid-air”. An integral characteristic of the structure of soil composition is the structure of its pore space, which determines through the surface tension of the liquid surface energy and variable bond strength of moisture with the frame soil, namely its capillary potential. This functional dependence of the capillary potential on the moisture saturation P = f (0) is called the curve of the soil water holding capacity, or its main hydrophysical characteristic (MHC) [18]. In fact, the MHC is an integral curve of the distribution of the pore space of the soil by size - the radii of the inscribed sphere in the imaginary capillaries. However, taking into account the irregularity of the intersection of imaginary capillaries in a real dispersed medium, the pore space can be characterized by 2 characteristic dimensions - r,™ and rmax [19]. These radii correspond to the radii of the inscribed sphere at the narrowest point of the body of the pore rmin and at the extensions of the pore - rmax. It is these radii of curvature of the pore solution that determine the capillary pressure during drying and filling of the pore body during sorption and desorption, which is the cause of capillary hysteresis.

The proposed physical model of the pore space in the form of a corrugated equivalent capillary takes into account the irregularity intersection of the pore space and characterizes integrally the volume of all capillaries due to the curvature of the equivalent capillary [19]. In the expansions of the pore space, the trapped air can be in the equilibrium state, where the condition is fulfilled and where ra is the radius of the generalizing meniscus of the liquid in contact with the atmosphere [13]. In this case, the surface area of the interface is liquid-air, connected with the atmosphere, which is appropriate to call extraheterogeneity, the surface area of the liquid-air interface in the bubbles of compressed air is added, which can be called intraheterogeneity. That is, the structure of the pore space of soil unsaturated with moisture at any time is limited by the interface of extra - + intraheterogeneity. This is the relationship of 2 fundamental properties of the soil - heterogeneity, and hysteresis [4, 12].

In fig. 1 shows the results of a laboratory experiment on the effect of a thermal pulse on the dynamics of the capillary potential of an isolated sample of loess species of constant moisture saturation. It is the loess soil that is the main parent rock on which the most fertile of Chernozem soils of Ukraine are formed.

The experiment was as follows: a cylindrical sample of forest-like species of the disturbed structure of constant humidity 0 = const with a volume of 10 dm3, isolated from moisture exchange with the atmosphere (connected for gas exchange with it only through a labyrinth), was equipped with a tensiometer. Soil temperature was monitored by laboratory thermometers directly next to the tensiometer. The soil was heated in a water bath from 20 to 60° C, which corresponds to the range of natural variability of soil temperature, after which it was slowly cooled in the atmosphere for several days.

Fig. 1. Dynamics of redistribution of moisture in the pore space of the loess soil for sustainable moisture content (в = const) under the influence of temperature: a - chronological graphs of the samples, T° C = f (T); b - the capillary potential of moisture in soil P = f (t); с - a loop of temperature hysteresis of capillary moisture potential P = f (T° C)