Soil Inorganic Carbon (SIC)

2.1. Calcium Carbonate — The Main Player

The most common and important carbonate mineral in soils is calcite (CaCO₃). It is calcite that is usually meant when soil carbonates are mentioned. Calcite is the lime carbonate that forms limestones, chalk, marls, and many other sedimentary rocks. In soil, it can occur in a variety of forms:

• Finely dispersed calcite — tiny particles evenly distributed throughout the soil mass. It is virtually invisible to the eye, but it determines the overall carbonate content and the soil reaction (pH). Such calcite is often called “active” or “free” carbonate.

• Concretions and nodules — rounded or irregular accumulations of calcite formed by the redistribution of carbonates within the soil profile. In Russian soil literature, these are often called “beloglazka” (white‑eye) or, in the case of larger formations, “zhuravchiki” (little cranes) in loess soils. Sizes can range from fractions of a millimetre to several centimetres.

• Coatings and efflorescences — calcite may be deposited on the surfaces of peds, on plant roots, or on the faces of skeletal particles (gravel, rock fragments). Carbonate coatings on the lower sides of rock fragments in desert and semi‑desert soils are particularly characteristic — these are the so‑called “carbonate crusts” or “stalactites” (Foth, 1990; Scheffer et al., 2018).

• Pseudomycelium — very fine, cobweb‑like threads of calcite that penetrate the soil mass. They form when carbonates crystallise from solutions in capillaries and are often found in loess soils of southern Russia and Ukraine.

• Cemented horizons — at high concentrations, calcite may fill pores and cement soil material, forming hard, dense layers — calcretes (or “crusts”) and petrocalcic horizons. Such horizons are nearly impermeable to roots and water and serve as important diagnostic features in soil classification systems (more on this in Section 6).

Calcite is a moderately soluble mineral. Its solubility depends on the presence of carbon dioxide in the soil air and on pH. At low CO₂ concentrations, it is practically insoluble, but when the partial pressure of CO₂ increases (and in soil it is always higher than in the atmosphere due to root and microbial respiration), the solubility of calcite increases sharply. This is a key property that underlies carbonate equilibrium (to which we will return in Section 4).

2.2. Magnesium Carbonate — Second in Importance

Magnesium in carbonate form occurs less frequently than calcium, but in some soil types its role is very significant. The main magnesium‑bearing carbonates are:

• Dolomite (CaMg(CO₃)₂) — a double salt in which calcium and magnesium ions alternate in the crystal lattice. Dolomite occurs in nature as an independent rock — dolomites and dolomitised limestones. It usually enters soils from such parent materials. Dolomite is less soluble than calcite and dissolves more slowly, so in soils developed on dolomites the carbonate equilibrium is established differently (Scheffer et al., 2018).

• Magnesite (MgCO₃) — pure magnesium carbonate. It occurs in soils much less frequently than dolomite, mainly under conditions of strong salinisation and at high pH (e.g., in soda solonchaks).

It is important to note that in soil practice, when determining carbonate content, the indicator “calcium and magnesium carbonates” is often used together, since in natural objects they almost always accompany each other. However, to understand the buffering properties of soil, one must distinguish: calcium carbonate provides a “sharper” buffering in the alkaline range, while dolomite provides a “smoother” one due to its lower solubility.

2.3. Other Carbonate Minerals

Apart from calcite and dolomite, other carbonates may occur in soils (most often under arid climates or in specific geochemical settings):

• Siderite (FeCO₃) — iron carbonate. It forms under reducing conditions, for example, in waterlogged soils with oxygen deficiency. It gives soils grey‑greenish tints. In normal well‑drained soils, siderite is unstable and oxidises quickly.

• Rhodochrosite (MnCO₃) — manganese carbonate. It is rare, usually found in concretions or in reduction zones.

• Natron (Na₂CO₃·10H₂O) and trona (Na₃H(CO₃)₂·2H₂O) — soda minerals characteristic of soda solonchaks and strongly saline soils. Their presence leads to very high pH (up to 10–11), which drastically changes the living conditions for plants and microorganisms.

However, these minerals have local distribution. The bulk of SIC in the world’s soils is calcite and (to a lesser extent) dolomite.

2.4. Geographic Distribution of Carbonate Forms

It is important to understand that the forms and content of SIC strongly depend on climate. In humid regions, carbonates are actively leached from the soil profile and carried away with groundwater. Here SIC is practically absent — soils are “non‑calcareous”. In arid and semi‑arid regions, where evaporation exceeds precipitation, carbonates, on the contrary, accumulate. Moreover, accumulation follows a certain pattern: with depth, starting from a certain level, carbonate content increases, and then may decrease again if at great depth the water becomes aggressive towards carbonates. This distribution is a direct result of carbonate equilibrium, which we will discuss later.

Thus, the forms of inorganic carbon in soil are not just a set of minerals. They are a whole system that reflects the history of soil formation, climatic conditions, and modern hydrological regimes. And many of the functions of SIC in soil depend on the form in which it occurs.

In the next section, we will analyse where these carbonates come from in soil — what are their sources, and how lithogenic and pedogenic carbonates differ.

3.1. Lithogenic Carbonates

Lithogenic (from Greek lithos — stone, genos — birth) carbonates are those that were present in the parent material from the very beginning. They were formed long before soil formation began, during geological processes: sedimentation, diagenesis, metamorphism.

Which rocks are the main sources of lithogenic carbonates?

• Limestones and chalks — almost pure calcite. Weathering of such rocks produces soils initially rich in carbonates — rendzinas, typical of karst landscapes (White, 2006).
• Dolomites and dolomitised limestones — contain calcium and magnesium as a double salt. Soils on such rocks often have higher magnesium content and a slower response to acid loads.
• Marls — a mixture of carbonates with clay material. These are transitional rocks that give soils with moderate carbonate content.
• Loesses and loess‑like loams — aeolian deposits often containing a significant amount of finely dispersed calcium carbonate (sometimes up to 20–30%). Loesses are widespread in the chernozem and chestnut soil zones, and it is the presence of carbonates in them that largely determines the high fertility and alkaline reaction of these soils (Scheffer et al., 2018).
• Moraine deposits — in areas of ancient glaciation where the glacier has processed limestone rocks, carbonates may also be present.

It is important to understand: lithogenic carbonates are often unevenly distributed in the soil profile. In fresh, unweathered soils, they may be present throughout the profile. However, during soil formation under the action of moisture and carbonic acid, they may be leached from the upper horizons and reprecipitated lower down — this is already a transition to pedogenic processes.

The main characteristic of lithogenic carbonates is that they are a “ready‑made” mineral reserve that the soil inherits. Their further fate depends on climate: in humid conditions they will gradually dissolve and be removed from the profile, while in arid conditions they may persist and even accumulate.

3.2. Pedogenic Carbonates

Pedogenic (from Greek pedon — soil) carbonates are those formed directly within the soil thickness as a result of chemical reactions occurring during soil formation. They are not inherited from the parent rock but arise anew from components entering the soil with atmospheric precipitation, dust, biogenic weathering, or released during the mineralisation of organic matter.

How and under what conditions are pedogenic carbonates formed?

This happens when the soil solution reaches a high concentration of calcium (Ca²⁺), magnesium (Mg²⁺) and carbonate (CO₃²⁻) or bicarbonate (HCO₃⁻) ions, and conditions favour their precipitation. Let us consider the key processes:

1. Supply of calcium and magnesium:

• Weathering of primary minerals: In soils rich in calcium‑bearing silicates (feldspars, amphiboles, pyroxenes), chemical weathering releases Ca²⁺ and Mg²⁺ ions, which migrate with solutions (White, 2006).
• Atmospheric precipitation and dust: Rainwater and dry dust, especially in arid regions, bring significant amounts of soluble calcium and magnesium salts. In some areas, the contribution of atmospheric dust (including carbonate dust) to the calcium balance can be comparable to, or even exceed, that from the parent rock (Huang et al., 2012; Foth, 1990).
• Biogenic source: Calcium and magnesium can come from plant litter and animal remains during decomposition. This is especially noticeable in forests on poor parent materials, where the biological “pump” function of plants accumulates elements in the upper horizon.

2. Source of carbonate (or bicarbonate) ions:

• Dissolution of carbon dioxide: The main source is the dissolution of CO₂ in soil moisture, which is released during root and microbial respiration. The higher the partial pressure of CO₂ in the soil air, the more carbonic acid (H₂CO₃) is formed in the solution, and the more actively carbonates dissolve or, under certain conditions, precipitate.

3. Conditions for precipitation:

For Ca²⁺ and HCO₃⁻ to combine into CaCO₃ and precipitate, the solution must be supersaturated with respect to calcite. Supersaturation occurs when:

• Evaporation of water: At high temperature and wind, water evaporates, ion concentrations rise, and carbonate precipitates (Foth, 1990).
• Increase in pH: When CO₂ is removed from the solution (for example, when roots or microorganisms consume CO₂, or when the gas escapes to the atmosphere during soil drying), the equilibrium H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻ shifts towards carbonate ion formation, which reduces the solubility of CaCO₃.
• Biogenic precipitation: It is important to emphasise that in nature, carbonate precipitation almost never proceeds purely chemically. Microorganisms (bacteria, fungi) actively participate in the process — they excrete metabolites, create a microenvironment, and initiate crystallisation. Moreover, it has been experimentally shown that under sterile conditions carbonates precipitate extremely slowly or not at all, whereas in the presence of microflora the process proceeds much faster (Huang et al., 2012). It is important to remember: pedogenic carbonates are to a large extent biogenic formations.

Morphological manifestations of pedogenic carbonates we have already partly listed in Section 2: finely dispersed calcite, carbonate coatings and films, concretions, pseudomycelium, as well as calcretes and petrocalcic horizons. Importantly, all these forms are the result of redistribution and neoformation of carbonates within the profile, not simple inheritance.

3.3. Lithogenic and Pedogenic Carbonates: Dialectics of Interaction

In practice, it is often very difficult to draw a clear line between lithogenic and pedogenic carbonates. They may coexist in the same profile, overlapping each other. For example, in loess chernozems, carbonates inherited from the parent material are gradually leached from the upper horizons, but in the middle part of the profile they are re‑precipitated as pedogenic formations (concretions, pseudomycelium). Thus, the same horizon may contain both relict lithogenic carbonates and newly formed pedogenic ones.

In soil classification systems (e.g., in Soil Taxonomy and WRB), the presence and nature of carbonate neoformations serve as important diagnostic features. For example, a horizon with accumulation of secondary carbonates (a calcareous horizon, denoted Bk or Ck) is a direct indication of pedogenic processes and specific climatic conditions (arid or semi‑arid). The presence of a strongly cemented carbonate layer (petrocalcic horizon) indicates a long period of soil formation and a considerable age of the soil.

Thus, the origin of carbonates is not just a question of genesis, but also a key to understanding many soil processes: substance migration, water regime, structure formation, and, of course, buffering properties, which we now turn to.

In the next section, we will discuss carbonate equilibrium — the basis that links together the solubility, movement, and precipitation of carbonates, and explains why soil pH is so sensitive to the presence of CO₂.

4.1. Three Components of the Equilibrium

Carbonate equilibrium in soil includes three interrelated components:

1. Gaseous CO₂ — present in the soil air. Its concentration (more precisely, partial pressure) in soil is always higher than in the atmosphere, due to the respiration of plant roots and soil microorganisms. This “extra” carbon dioxide is the main driver of carbonate transformations.

2. Dissolved forms of carbonic acid — when CO₂ dissolves in soil water, it reacts with water to form carbonic acid (H₂CO₃). This is a weak acid that partially dissociates into hydrogen ions (H⁺) and bicarbonate (HCO₃⁻), and then, under certain conditions, into carbonate ions (CO₃²⁻). In soil chemistry, when speaking of “dissolved carbon dioxide”, one often refers to the sum of all these forms, denoted as H₂CO₃* (the asterisk means that it includes both physically dissolved CO₂ and carbonic acid itself and its dissociation products).

3. Solid carbonates — calcite (CaCO₃), dolomite (CaMg(CO₃)₂) and others that we discussed in Section 2. These minerals have a certain solubility, depending on the conditions in the solution.

All three components are in dynamic equilibrium, which can be represented by a simplified scheme (Foth, 1990; Huang et al., 2012):

The meaning of this scheme is as follows: carbon dioxide from the soil air constantly passes into water, forms carbonic acid, which supplies hydrogen ions and carbonate ions to the solution. These ions, in turn, interact with solid calcite, determining whether it dissolves or precipitates.

4.2. What Controls the Solubility of Carbonates?

The main factor governing the equilibrium is the partial pressure of CO₂ in the soil air. The higher the P(CO₂), the more carbonic acid is formed in the solution, the more H⁺ and HCO₃⁻ ions are produced. At the same time, the solubility of calcite increases sharply. This can be expressed by a simple rule:

Increased CO₂ → dissolution of carbonates → removal of calcium and magnesium into solution.

Conversely, when P(CO₂) decreases (for example, when the soil dries out and gases freely escape to the atmosphere), the equilibrium shifts towards the formation of solid carbonate:

Decreased CO₂ → supersaturation of the solution → precipitation of carbonates.

This is why carbonates behave like a “geochemical thermostat”. Over the course of a year, as wet and dry periods alternate, they can partially dissolve and reprecipitate, moving within the profile.

4.3. Relationship with Soil pH

Another very important aspect of carbonate equilibrium is its effect on pH. In a system containing solid calcite, the pH of the solution is set at a certain level, which depends on P(CO₂). At atmospheric P(CO₂) (about 0.0004 atm), the pH of a saturated calcite solution is about 8.3. However, in soil, where P(CO₂) can be tens or hundreds of times higher (due to respiration), the pH of the equilibrium solution is significantly lower — in the range of 7.5–8.0, and sometimes even down to 7.0 at very high CO₂ concentrations (Huang et al., 2012; Foth, 1990). Importantly, the pH still remains in the alkaline range, but its specific value is set precisely by the CO₂ balance, not by the amount of carbonate as such.

This property makes calcareous soils powerful buffering systems. An attempt to acidify such a soil (for example, by acid rain) leads to H⁺ being neutralised by carbonates, and the pH remains practically unchanged until all available calcite is dissolved. On the other hand, an attempt to make such a soil more alkaline (for example, by adding soda) encounters the buffering action of the same system — the excess OH⁻ will shift the equilibrium, and carbonate will begin to precipitate, maintaining the pH at the previous level.

4.4. The Role of Biota in Carbonate Equilibrium

Biota is not a passive observer. Plant roots and microorganisms actively influence P(CO₂) through respiration. In addition, they excrete organic acids that can dissolve carbonates and consume calcium, shifting the equilibrium. In the rhizosphere (the zone adjacent to roots), CO₂ is often higher than in the bulk soil, so carbonates can be actively dissolved there, while away from the roots they may precipitate. This creates spatial heterogeneity in the distribution of SIC, which we observe in the field (Foth, 1990).

4.5. Practical Implications for Soil Science

Knowledge of carbonate equilibrium allows us to explain the observed patterns:

• Vertical distribution of carbonates in the profile: in the upper, biologically active horizons, P(CO₂) is high, and carbonates are dissolved; below, where respiration is weaker, they precipitate. This creates a characteristic carbonate “illuvial” zone (horizon Bk or Ck). With sufficient moisture, carbonates may be completely leached, and the soil becomes non‑calcareous (as seen in humid zones).

• Dependence of the carbonate horizon on climate: in arid regions, evaporation prevails, solutions become concentrated, and carbonates precipitate closer to the surface; in semi‑arid regions — deeper; in humid regions — they are leached out of the profile altogether (Foth, 1990; White, 2006).

• Diagnostic value: the presence and depth of the carbonate horizon is one of the key features for soil classification (we will discuss this further).

Thus, carbonate equilibrium is not abstract chemistry but a living mechanism linking gas exchange in the soil, water regime, and the mineral component. It explains why SIC is so closely intertwined with biological activity, even though it is not itself organic matter.

In the next section, we will move on to the role of SIC in soil — how these equilibria and carbonate reserves affect buffering, structure, the carbon cycle, and other key functions.

5.1. Buffering and pH: The Soil’s “Shield” Against Acidification

Perhaps the most well‑known and practically significant function of SIC is its ability to resist soil acidification. Recall carbonate equilibrium: solid calcium carbonate (CaCO₃) is in equilibrium with calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻) in solution. When additional hydrogen ions (H⁺) enter the soil — for example, from acid rain, from physiologically acidic fertilisers, or as a result of nitrification — they react with carbonate ions:

What happens? The protons (H⁺) are “consumed” by carbonate and neutralised. Carbon dioxide is released and escapes to the atmosphere, while calcium ions and water remain in solution. Calcium, in turn, may replenish the exchange complex or be leached. But the main point is: the soil pH does not drop (or drops only very slightly) until all available carbonate is consumed.

This phenomenon is called carbonate buffering. Soils containing even a small amount of active calcite have enormous resistance to acidification compared to non‑calcareous soils. It is no coincidence that in classical chernozems, which contain carbonates in the lower part of the profile, the pH of the upper horizons remains neutral or slightly alkaline for a long time, even with prolonged application of acid fertilisers. In sod‑podzolic soils, where carbonates have long been leached, even slight acidification immediately affects pH and nutrient availability (Foth, 1990; White, 2006).

It should be noted: the buffering capacity of the carbonate system is not infinite. It is determined by the total stock of carbonates in the soil. When this stock is exhausted (carbonates completely dissolve and are removed), the soil passes into the category of non‑calcareous and begins to acidify much faster. That is why, on calcareous soils, periodic application of lime (or chalk) is not just a “calcium supplement”, but maintenance of the buffering mechanism itself, preventing the loss of this “shield” (Huang et al., 2012).

What does this mean for plants and microorganisms? Many agricultural crops (legumes, cereals) and most soil microorganisms prefer a neutral or slightly alkaline reaction. Carbonate buffering ensures pH stability, which directly affects the availability of macro‑ and microelements (phosphorus, molybdenum, zinc, manganese, etc.), the activity of nitrogen fixation, and the mineralisation of organic matter. Thus, SIC, through pH control, is a powerful regulator of soil fertility (White, 2006).

5.2. Influence on Soil Structure and Physical Properties

The second important role of SIC is its participation in the formation of soil structure and aggregate state. This is especially noticeable in arid and semi‑arid regions.

Calcium ions (Ca²⁺), which are released when carbonates dissolve, are excellent structure‑formers. They promote flocculation (coagulation) of clay particles, binding them into aggregates. Unlike sodium ions, which cause dispersion and structure destruction, calcium acts as a “bridge” between negatively charged surfaces of clay particles and organic colloids. As a result, a water‑stable granular or crumb structure is formed, which is so valued in chernozems and chestnut soils (Scheffer et al., 2018; White, 2006).

Moreover, in soils with high carbonate content, carbonate films and cementing interlayers often form. This can be both beneficial and detrimental. On the one hand, carbonate crusts on the surface of peds (structural units) strengthen them, increasing resistance to destruction and water erosion. On the other hand, with excessive accumulation of carbonates in the illuvial horizon (horizon Bk or Bkm — cemented by calcite), a petrocalcic horizon (the so‑called “calcite pan”) may form, which becomes almost impermeable to roots and water. This is already a diagnostic feature of extreme stages of carbonate accumulation (Foth, 1990).

Thus, SIC actively participates in shaping the architecture of the soil profile, affecting water permeability, aeration, and mechanical strength of the soil.

5.3. Carbon Accumulation: A Long‑Term Reservoir

The third, and increasingly relevant in the context of global climate change, role of SIC is long‑term sequestration and storage of carbon in inorganic form.

We are accustomed to talking about carbon accumulation in soil in the context of organic matter. But carbonates are also a form of bound carbon. Globally, the stocks of inorganic carbon in soils range from 700 to 1700 petagrams (Pg) — roughly comparable to the stock of organic carbon (2300–2500 Pg) (Huang et al., 2012). Most of this carbon is concentrated in arid and semi‑arid regions, where carbonates are not leached but accumulate over millennia.

What is special about this reservoir? First, the turnover time of inorganic carbon in soil is enormous — it amounts to tens and hundreds of thousands of years. This is orders of magnitude longer than the turnover time of organic carbon (usually from a few years to a few centuries). Therefore, SIC can be considered as a geological archive of carbon, stable on the scale of human life.

Second, the processes of accumulation and release of inorganic carbon are closely linked to carbonate equilibrium and depend on climatic and hydrological conditions. When climate changes (for example, when moisture increases), carbonates may dissolve and be removed, leading to the release of CO₂ into the atmosphere. Conversely, when the climate dries (or when irrigation with high‑alkalinity waters is used), carbonates may precipitate, binding additional carbon.

Thus, SIC is not passive ballast, but an active participant in the global carbon cycle, especially on geological timescales. Understanding this reservoir is crucial for assessing the carbon balance of large regions and for developing strategies to mitigate climate change (Huang et al., 2012).

5.4. Interaction with the Biological Cycle

Although SIC is an inorganic form, it interacts closely with living matter. First, microorganisms and plant roots release CO₂, which, as we have seen, governs carbonate equilibrium. Second, biogenic organic acids can dissolve carbonates, mobilising calcium and, indirectly, phosphorus (which is often bound to calcium in insoluble phosphates). Thus, SIC participates in “root nutrition” of plants, although indirectly. In addition, during decomposition of organic matter, CO₂ is released, which can react with carbonates, accelerating their dissolution — this is the so‑called “greenhouse” link between organic and inorganic cycles.

Consequently, ignoring SIC when studying soil fertility and ecological functions would be a serious mistake.

In the next section, we will discuss how carbonates are used as diagnostic features in soil classification systems — an important practical aspect that helps soil scientists correctly identify soil types and predict their properties.

6.1. Carbonate Horizons in Soil Classification

What is a carbonate horizon from the point of view of classifications? It is a horizon in which a significant accumulation of secondary calcium or magnesium carbonates is observed. Accumulation occurs through the processes described in Sections 3 and 4: dissolution of carbonates in the upper parts of the profile, their migration with downward water flow, and subsequent precipitation when conditions change (evaporation, decrease in PCO₂, increase in pH). Such a horizon receives the index Bk (in the American and many other systems) or Cca (in the Russian school, where “ca” stands for carbonate accumulations).

Criteria for identifying a carbonate horizon usually include:

• Minimum carbonate content — most often 5–10% CaCO₃ in the fine earth (although thresholds may vary between systems).
• Morphological features — visible neoformations: pseudomycelium, concretions (“beloglazka”), coatings, films, carbonate impregnation (Foth, 1990).
• Thickness — the horizon must have a certain thickness (most often at least 5–10 cm) to be diagnostically significant.

However, carbonate accumulation can be so intense that the horizon becomes cemented. Such a horizon is called petrocalcic (or calcic pan). It is designated as Bkm or Ckm (in some systems — Bca with an indication of cementation). Petrocalcic horizons are almost impermeable to roots and water, and can be as hard as rock (hence the prefix “petro‑” from Greek petra — rock) (Foth, 1990; Huang et al., 2012).

6.2. Carbonate Horizons as Indicators of Climate and Age

The most obvious link is with climate. In humid zones, carbonates are leached from the soil. In arid and semi‑arid zones, they accumulate. At the same time, the depth of the carbonate horizon is directly related to the amount of precipitation:

• At very low precipitation (<250–300 mm/yr), carbonates may accumulate at the surface or in the upper 20–30 cm of soil.
• With increasing precipitation to 400–500 mm/yr, the carbonate horizon descends to a depth of 40–80 cm.
• At precipitation >600–700 mm/yr (depending on temperature and evaporation), carbonates are usually completely leached beyond the soil profile (Foth, 1990; White, 2006).

This is a classic pattern observed along a moisture gradient, for example, in the prairies of North America or in southern Russia: from chestnut soils to chernozems and further to grey forest soils, the depth of carbonates increases, and then they disappear.

Soil age also leaves its mark. On young soils (Holocene, Pleistocene), carbonate horizons may be weakly expressed. On older, stable surfaces (Pleistocene and older), carbonates may accumulate over decades and centuries, forming thick, well‑expressed horizons, and with prolonged accumulation — petrocalcic pans (Huang et al., 2012). Thus, the degree of development of the carbonate horizon allows soil scientists to estimate the relative age of the soil even without detailed radioisotope studies.

6.3. Association with Other Diagnostic Features

Carbonate horizons rarely occur in isolation. They are often combined with other features, giving a more complete diagnostic picture:

• Gypsum horizons (Bgy or Cca + gy): In very arid conditions (where moisture is insufficient for sulphate removal), carbonates may be associated with gypsum (CaSO₄·2H₂O). This indicates an even more arid climate than just carbonate (Scheffer et al., 2018).
• Solonetz and solonchak: In soils with high exchangeable sodium content (solonetz), carbonates are often present as soda (Na₂CO₃) or bicarbonate, which is associated with salinisation processes. Such soils have very high pH and are diagnosed separately (White, 2006).
• Gleyic features: If the carbonate horizon is in a zone of variable moisture (for example, with a high groundwater table), it may be combined with ochreous or bluish shades (indicative of reducing conditions).

The presence of a carbonate horizon often serves as a boundary for the root zone — the roots of many plants penetrate only as far as this layer, which is important for assessing soil suitability for different crops (Foth, 1990).

6.4. Practical Value of Diagnosis

For a practising soil scientist, agronomist, or ecologist, knowledge of the presence and nature of carbonate horizons provides valuable information:

• Prediction of water regime: A petrocalcic horizon is a barrier for water and roots, which can lead to waterlogging of overlying layers during heavy rainfall or, conversely, to desiccation due to capillary rise.
• Prediction of chemical properties: The presence of active carbonates guarantees high buffering and alkaline reaction, but may also indicate a risk of micronutrient deficiencies (iron, zinc, manganese) due to their low availability at high pH (Scheffer et al., 2018).
• Mechanical tillage: Soils with well‑developed carbonate horizons may require special tillage practices (deep loosening to break the pan, combating compaction).
• Reclamation: In the presence of soda salinisation or excess sodium, gypsum application or leaching is required, not just liming (White, 2006).

Thus, carbonates as a diagnostic feature are not just an academic tool for classification. They are a powerful predictive tool that allows one to judge many practical soil properties even before full laboratory analyses.

6.5. Carbonates in the Russian School and International Systems

In the Russian soil classification (Classification and Diagnostics of Soils of Russia, 2004), carbonate horizons are also distinguished, but they are more often designated as Cca (for illuvial accumulations) or k for carbonate parent materials. Features such as “beloglazka”, “zhuravchiki”, “pseudomycelium” are well known to field soil scientists. In international systems, such as WRB (World Reference Base for Soil Resources) and US Soil Taxonomy, carbonate horizons are used as one of the key criteria for distinguishing reference soil groups. For example, in WRB, carbonate soils include Calcisols (soils with a thick carbonate horizon) and Gypsisols (with gypsum), while in Soil Taxonomy — Aridisols (arid soils), which often contain carbonate horizons.

Knowledge of these criteria allows specialists from different countries to “speak the same language” when describing soils and exchanging data.

In the next, concluding section of our lecture, we will make a comparison of organic and inorganic carbon and summarise why it is important to distinguish these two forms and how they complement each other in the unified soil system.

7.1. Nature and Chemical Composition

Organic carbon (SOC) is carbon that is part of organic compounds: humic substances, plant and animal residues, microorganisms. It is based on covalent C–C and C–H bonds that form complex polymeric structures. SOC is a product of biological synthesis, the result of photosynthesis and subsequent transformations of organic matter in soil (Foth, 1990).

Inorganic carbon (SIC) is carbon in minerals, mainly carbonates (CaCO₃, MgCO₃, double salts). Here, carbon is bonded to oxygen in the carbonate ion (CO₃²⁻) and is part of the crystal lattice of minerals. SIC is a product of geochemical processes: precipitation, crystallisation, weathering (Huang et al., 2012; White, 2006).

Even at this level, a fundamental difference is visible: SOC is the “living” carbon of the biosphere, while SIC is the “rocky” carbon of the lithosphere, involved in soil processes.

7.2. Dynamics and Turnover Time

This is perhaps the most striking difference.

SOC is a highly dynamic form. It constantly participates in the cycle: it enters with plant litter, is mineralised by microorganisms, humified, and may be leached as soluble organic substances. The turnover time of SOC depends on climate, organic composition, aggregate protection, and can range from several months (labile pools) to hundreds and thousands of years (stable humus pools associated with minerals). But even the most stable SOC, on a geological time scale, turns over relatively quickly (hundreds to a few thousand years) (Huang et al., 2012).

SIC is a geologically stable reservoir. The turnover time of carbonates in soil is measured in tens and hundreds of thousands of years, and in the case of cemented horizons — millions of years. Changes in SIC occur only with significant climatic shifts (for example, when transitioning from arid to humid conditions, when active carbonate dissolution begins) or under long‑term anthropogenic influence (for example, with irrigation or liming) (Foth, 1990; Huang et al., 2012).

Conclusion: SOC is “fast” carbon, reflecting the current state of the ecosystem; SIC is “slow” carbon, storing the memory of geological and climatic epochs.

7.3. Role in Soil Processes

Here the differences are no less fundamental.

• SOC is the basis of biological activity of soil. It serves as a source of energy and carbon for microorganisms, determines enzymatic activity, participates in the formation of humus status, affects water permeability, water‑holding capacity, and structural condition. Soil fertility and buffering against toxicants depend directly on SOC (White, 2006). The dynamics of SOC are closely linked to the management of nitrogen, phosphorus, and sulphur cycles.

• SIC is primarily responsible for the geochemical buffering of the soil system. It controls pH in the alkaline range, neutralises acid loads, participates in calcium exchange, and affects the availability of many elements (especially phosphorus and micronutrients). SIC is an important structure‑forming agent (Ca²⁺ ions), especially in arid conditions, and can serve as a barrier to substance migration (petrocalcic horizons). However, SIC itself is not a source of energy for microorganisms (with some exceptions related to carbonate buffering) (Scheffer et al., 2018).

Key difference: SOC is the energy and structural‑biological centre of the soil, while SIC is the geochemical and buffering centre. They operate on different time scales and by different mechanisms, but within a unified soil system they are closely interconnected.

7.4. Interrelationship of SOC and SIC: Inseparable Unity

Although we have separated them for understanding, in real soil SOC and SIC constantly interact. Let us give just a few important examples:

1. Soil respiration and carbonate equilibrium: The carbon dioxide (CO₂) released during SOC mineralisation directly controls the solubility of carbonates and, consequently, the state of SIC. Without biogenic CO₂, many dissolution/precipitation processes of carbonates would be greatly slowed. Thus, the biological cycle “triggers” the geochemical cycle of carbonates (Foth, 1990).

2. Organic acids and carbonate dissolution: Plants and microorganisms excrete organic acids (oxalic, citric, malic, etc.) into the rhizosphere, which actively dissolve carbonates, releasing calcium and phosphorus. This is one of the mechanisms of mobilising plant nutrition even on calcareous soils (White, 2006).

3. Formation of aggregates: Calcium released from carbonates participates in the formation of calcium bridges between organic matter and clay particles, which stabilises soil aggregates and protects SOC from rapid mineralisation (Scheffer et al., 2018). This is a striking example of how SIC contributes to the preservation of SOC.

4. Anthropogenic impact: Application of organic fertilisers on calcareous soils can enhance carbonate dissolution due to additional CO₂, while liming of acid soils, on the contrary, increases SIC stocks (provided sufficient calcium input). Thus, fertility management always affects both forms of carbon.

7.5. Significance for the Global Carbon Cycle

On a global scale, SOC and SIC are the two largest carbon reservoirs in the pedosphere (after the ocean and geological deposits). Their total stocks are comparable. SOC actively participates in the modern carbon cycle, its changes are strongly dependent on land use and climate. SIC represents a giant stable reserve that contributes to long‑term climate regulation, but its changes occur slowly and are mainly linked to geological processes (weathering, karst formation, sedimentation) (Huang et al., 2012).

Therefore, when assessing the carbon balance of soils, one cannot limit oneself to organic carbon alone. In arid and semi‑arid regions, ignoring SIC will lead to an underestimation of the total carbon stock in the soil and an incorrect assessment of its potential role as a sink or source of greenhouse gases.

7.6. Summary Comparison

For clarity, we summarise the key differences in a table:

Conclusion

Now we can answer the main question of the lecture: Why does carbon in soil exist not only as part of organic matter?

The answer: because soil is not only a biological but also a mineral system. Carbon, as one of the most common elements, inevitably occurs in minerals — carbonates — which are either inherited from parent rocks or formed as a result of soil‑forming processes. These two forms of carbon — organic and inorganic — coexist and interact, but they perform fundamentally different ecological and agronomic functions. SOC is the living, dynamic, energy‑rich part of the soil, the basis of its biological cycle. SIC is the stable, buffering, geochemical part, ensuring the long‑term stability of the soil system.

Understanding this distinction is not just academic knowledge. It is necessary for a proper assessment of soil fertility, for predicting soil responses to climate and land‑use changes, and for developing effective fertilisation and reclamation systems. I hope this lecture has helped you form a comprehensive view of inorganic carbon and its place in the soil system. In the next lecture, we will move on to other soil components, but now you have a solid foundation for understanding how carbon — in all its forms — governs the life and functions of soil.