Gas phase and regulation of the soil air regime

Soil, deprived of gases, loses its biological properties and ceases to be a living medium. The air regime of the arable layer directly determines the respiration of plant roots, the activity of beneficial microbiota, and the rate of chemical processes. The amount of air in the soil is not constant and depends on its structure, type, and current humidity. Given the same level of moisture, a well-structured soil with large pores will always contain more air than a structureless and compacted one.

How the soil air regime is structured and what it depends on

The air regime encompasses all processes of intake, movement, and composition changes of gases within the soil profile. Gases constantly circulate between the soil, the atmosphere, the solid phase, and soil solution. The formation of soil air occurs in three main ways:

• intake of air from the surface layer of the atmosphere into free pores;
• diffusion of gases due to the difference in partial pressure between the soil and the atmosphere;
• release of gases as a result of respiration of soil organisms and chemical reactions.

Soil air exists in four physical states: free, trapped, adsorbed, and dissolved. Free air fills large non-capillary pores and dry capillaries, providing the main aeration and gas exchange with the atmosphere. Dissolved air is found in soil moisture; its diffusion is slow, but it is indispensable for the vital activity of microorganisms, soil animals, and plant roots.

Trapped and adsorbed air: hidden features of aeration

Trapped air is isolated in pores by water plugs, so it practically does not participate in gas exchange and hinders moisture absorption. Its volume directly depends on the particle-size distribution and soil density. The heavier the soil in terms of mechanical composition and the more strongly it is compacted, the more trapped air it contains.

• Average volume of trapped air — 5–8 %
• Volume of trapped air in loams — more than 12 %
• Proportion of trapped air in loam pores — more than 1/4

Adsorbed air is held on the surface of solid soil particles, with this process being most active in dry soils. Water displaces adsorbed gases into general circulation only when soil humidity exceeds maximum hygroscopicity. Soil particles adsorb gases in a strict sequence, depending on the properties of their molecules:

1. Nitrogen
2. Oxygen
3. Carbon dioxide
4. Ammonia

The ability to absorb gases is distributed unevenly across soil components. Gases are held most strongly by humus, humus, and sesquioxides. In gypsum, lime, and quartz, these properties are significantly less pronounced.

Dissolved soil air has components that are characterized by different solubility in water. Ammonia, hydrogen sulfide, and carbon dioxide have the highest solubility. The solubility of gases in water increases with an increase in their partial pressure and a decrease in temperature. Furthermore, the solubility of gases in soil water decreases with an increase in its mineralization.

Gases in the soil solution significantly change the properties of the solution. Thus, with an increase in carbon dioxide content in the soil solution, the solubility of carbonates, gypsum, phosphates, and other salts increases. Dissolved oxygen, despite its relatively low solubility in water, is capable of maintaining the oxidative properties of the soil solution. At the same time, its amount in 1 liter of soil solution can range from 0 to 14 mg. The higher the biological activity of the soil, the higher the consumption of dissolved oxygen and the lower the saturation of the soil solution with this gas. The highest saturation of the soil solution with oxygen is observed in the spring, when biological activity of the soil is low due to low temperature, and the soil is saturated with water enriched with oxygen.

In its composition, soil air differs from atmospheric air by a lower oxygen content and a higher carbon dioxide content. A comparison of the composition of atmospheric and soil air is presented in the table:

In addition to the aforementioned gases, soil air contains insignificant amounts of methane, hydrogen, hydrogen sulfide, nitrous oxide, and phosphine, which are formed as a result of anaerobic decomposition of organic matter. Of all the gases, hydrogen is the lightest; it is 14.5 times lighter than air. Its molecules move faster than all other gases. It has the highest diffusion coefficient, and as a result, its concentration in the surface layers of the soil is approximately the same as in the atmosphere.

In addition to gases and water vapor, soil air may contain volatile or gaseous organic substances (hydrocarbons, alcohols, esters, aldehydes, etc.) formed as a result of the vital activity of soil organisms. Many of these compounds are absorbed and assimilated by soil microorganisms and plant roots.

The composition of soil air is continuously changing. The main reasons for this change are the activity of microorganisms, plant root respiration, and gas exchange with the atmosphere. During the decomposition of organic residues by microorganisms and root respiration, soil air is enriched with carbon dioxide and hydrogen, while gas exchange brings the composition of soil air closer to that of the atmosphere.

Nitrogen content in soil air does not differ much from that of the atmosphere, although changes are still possible, both in the direction of decrease and increase. Changes occur for the following reasons:

• Decrease in nitrogen amount: its fixation by free-living nitrogen-fixing soil microorganisms and nodule bacteria.
• Increase in nitrogen amount: protein breakdown and denitrification of nitrogen-containing substances under the action of denitrifying microorganisms.

The varying concentration of oxygen and carbon dioxide in soil air is explained by two opposing processes: the intensity of oxygen consumption and carbon dioxide production, on the one hand, and the capacity for gas exchange between the soil and the atmosphere, on the other. The content of oxygen and carbon dioxide in soil air largely depends on weather conditions, soil type, its biological activity, and its physical and chemical properties. It is also subject to seasonal changes. On arable land, the composition of soil air depends on the crop being grown and the agricultural practices applied.

Oxygen enters the soil air from the atmosphere as a result of diffusion. Part of the oxygen reaches the soil with water or through plant tissues. It is involved in the respiration of plants, soil fauna, and microorganisms. In the absence of oxygen, the vital activity of soil biota ceases. When its content in the soil is less than 2.5%, anaerobic processes begin to predominate, which are accompanied by the accumulation of ferrous compounds that inhibit the development of plants and soil biota. At the same time, reduced forms of nitrogen and sulfur compounds (NH3, H2S) may also accumulate in the soil. Optimal conditions for plant development are created when the oxygen content in soil air is about 20%.

The main source of carbon dioxide accumulation in soil air is the respiration of plants and soil fauna. Its replenishment also occurs due to the following processes:

• desorption from the solid and liquid phases of the soil;
• conversion of bicarbonates into carbonates during the evaporation of soil moisture;
• oxidation of organic matter;
• action of acids on soil carbonates;
• entry into the soil along with groundwater.

The biological significance of carbon dioxide is multifaceted. It supports the process of photosynthesis in plants. The soil-chemical and geochemical role of carbon dioxide is also significant. Soil solution saturated with carbon dioxide dissolves many poorly soluble minerals – calcite, dolomite, magnesite, and siderite. This causes the migration of carbonates in the soil profile: their leaching from the upper soil horizons or upward movement from the lower horizons.

Effect of carbon dioxide on chemical processes and plants

The accumulation of carbon dioxide in soil air and solution triggers the process of decarbonization — the leaching of carbonates. This phenomenon is associated with the shift of chemical equilibrium to the left in the reaction Ca(HCO₃)₂ ⇄ CaCO₃ + H₂O + CO₂. During the reverse process, calcium carbonate (CaCO₃) precipitates, causing the formation of accumulative horizons in the soil profile. In practice, agronomists notice this through new formations in the form of white eyes (beloglazka), nodules (zhuravchiki), or carbonate mold.

For the normal life activity of plants, it is necessary to maintain an optimal air regime. The limits of safe concentration of carbon dioxide in soil air are strictly defined. Exceeding these limits directly harms the harvest.

• Minimum CO₂ level in soil — 0.03 %
• Maximum CO₂ level in soil — 3.0 %
• Concentration gradient for D calculation — 1

Gas exchange mechanisms and oxidation-reduction potential

Soil air is constantly renewed through gas exchange with the atmosphere via pores free of water. Gases move by diffusion towards lower concentration: oxygen moves deeper, while carbon dioxide is released outward. The mass of the transported substance is determined by Fick's first law. The intensity of diffusion depends on the properties of the specific soil (porosity, humidity, temperature, humus content) and the gas diffusion coefficient (D), which is equal to the volume of gas in cm³ passing in 1 second through an area of 1 cm² with a layer thickness of 1 cm and a concentration gradient equal to 1.

The release of carbon dioxide from the soil surface is called soil respiration. This indicator reflects the overall biological activity of microflora and roots. Soil respiration should not be confused with air exchange, during which the entire mixture of gases moves into and out of the soil without changing the concentration of individual components. Such movement occurs primarily through non-capillary pores and depends on physical conditions.

Soil aeration and the intensity of air exchange are influenced by the following meteorological factors:

• fluctuations in atmospheric pressure;
• changes in soil temperature;
• changes in soil moisture (for example, precipitation or irrigation displace air, while drying draws it back in);
• wind speed (especially on structured soils without vegetative cover);
• fluctuations in the level of groundwater and perched water.

The soil is also capable of absorbing gases from the atmosphere through its solid and liquid phases. Water vapor and ammonia are absorbed most intensively, oxygen and carbon dioxide to a lesser extent, and nitrogen is adsorbed the least. The degree of aeration directly regulates the redox potential (Eh), which determines the chemical activity of elements in the soil solution.

In practice, the redox potential (Eh) helps monitor the air regime of the soil. Its value indicates how saturated the soil environment is with oxygen and which processes predominate in it. A decrease in this indicator signals a lack of air and a deterioration of conditions for the root system of plants.

How to optimize the air regime in cultivated areas

Oxygen demand varies greatly among different crops. Plants such as corn, sugar beet, potatoes, beans, peas, and many root crops especially need good soil aeration. At the same time, rice and certain grasses are capable of developing normally in flooded areas with minimal air access.

Gas exchange in field conditions can be regulated with the help of targeted agrotechnical interventions. These measures improve the structure of the plow horizon and facilitate oxygen access to the roots. To optimize the air regime, the following techniques are used:

• tillage;
• creation of a stable cloddy structure;
• deepening of the plow layer;
• land reclamation measures.

Additionally, the plant nutrition system affects the soil gas regime. The application of organic and mineral fertilizers significantly increases the intensity of carbon dioxide (CO2) release and increases its content in soil air. This is due to intensified microbiological activity and more active development of the plants themselves.