Soil Organic Carbon (SOC)

1.1. Definition and terminology

So, what is Soil Organic Carbon (SOC)? In the simplest and most precise formulation, SOC is the chemical element carbon that is part of all organic compounds present in the soil (Eash et al., 2016). It is not just some separate substance, but a common denominator for a huge variety of molecules—from simple sugars exuded by plant roots to complex and often difficult-to-identify macromolecules of humus.

It is very important to distinguish immediately between two related but not identical concepts: SOC and Soil Organic Matter (SOM).

• SOM – the entire mass of organic compounds, including carbon, hydrogen, oxygen, nitrogen, phosphorus, and other elements. It is the “weight” of all organic material.
• SOC – only the carbon component of that weight.

There is a simple quantitative relationship between them. On average, organic matter consists of approximately 50–58% carbon (Huang, 2012; Weil, 2017). Therefore, for rough estimation and conversion, a factor is used:

SOM ≈ SOC × 1.72 or 2.0

The factor 1.72 is traditionally used to convert carbon into organic matter, based on the assumption that it contains 58% carbon (White, 2006). The factor 2.0 (i.e., 50% carbon) is often applied in modern studies due to the higher variability in SOM composition (Weil, 2017).

In practice, in scientific literature and quantitative calculations (carbon balance, modelling), almost always the soil organic carbon content (SOC) is used, because it is a more accurate and reproducible value than the conventional “humus”.

1.2. What does organic carbon consist of?

Having understood what SOC is, we must answer the question: “What exactly does this carbon consist of?” It is important to recognise that it is heterogeneous. Within SOC, three main categories can be distinguished, which differ greatly in their fate in the soil:

1. Living biomass: This is carbon contained in the cells of living organisms—bacteria, fungi, protozoa, nematodes, earthworms, and living plant roots. This fraction usually accounts for only 1–5% of total SOC (Foth, 1990; White, 2006), but it is the “engine” of all biological processes in the soil.

2. Dead organic residues (detritus): These are fresh or partially decomposed plant and animal remains. This includes fallen leaves, stubble, roots, corpses, and animal excrement. In this fraction, plant tissues can often still be recognised. It is the main “fuel” for soil heterotrophs.

3. Humus: This is the stable, dark‑coloured, amorphous part of organic matter. Humus is the final product of long and complex transformations of organic residues. It is not a single substance but a complex mixture of stable organic compounds that have lost the features of the original tissues. It is humus that gives soil its dark colour, provides its high water absorption (up to 20 times its own weight), and is the main source of cation exchange capacity (Foth, 1990; Weil, 2017).

1.3. Global carbon reservoir

To appreciate the scale, let us look at the numbers. Soil is the largest reservoir of active organic carbon on land. The upper metre of the world’s soils stores about 1500–1600 Pg (petagrams, 10¹⁵ g) of organic carbon (Eash et al., 2016; Weil, 2017). For comparison, the atmosphere contains about 760 Pg C, and terrestrial vegetation about 560 Pg C (Huang, 2012). Thus, there is almost twice as much carbon in the soil as in the atmosphere and plants combined.

That is why soils play a key role in the global carbon cycle and climate change. Even a small change in soil SOC stocks can significantly affect the concentration of CO₂ in the atmosphere.

1.4. Why is SOC the main indicator of soil health?

To conclude this section, let us answer why SOC is considered the central indicator of soil health (Weil, 2017). This is because carbon is inextricably linked to all key soil functions:

• Fertility: SOC is the main reservoir and source of nitrogen, phosphorus, sulphur, and other nutrients. Its mineralisation supplies plants with nutrients.
• Physical properties: Humus binds mineral particles into aggregates, improving structure, aeration, water permeability, and resistance to erosion. It acts like a sponge, increasing soil water‑holding capacity.
• Chemical properties: Organic matter is the main source of negative charges (cation exchange capacity, CEC) in most soils, allowing them to retain cations (Ca²⁺, Mg²⁺, K⁺) and prevent their leaching.
• Biological activity: SOC is an energy source for heterotrophic microorganisms, forming the base of the soil food web. The more organic carbon, the higher the biological activity and biodiversity.

So, in this section we have defined that SOC is not a passive component but a dynamic system, consisting of parts that differ in origin and properties, and is the heart of soil fertility and the global carbon balance.

We will begin the next section, which is devoted to how this carbon is distributed among different “pools” depending on how quickly it decomposes.

Excellent, colleagues. We have established that soil organic carbon is not a uniform mass. Now we come to the most interesting and key question for understanding the whole dynamics: why can the same carbon stay in the soil from several days to several thousand years? The answer lies in the concept of carbon pools.

2.1. Active Pool

This is the fastest and most dynamic reservoir. It includes:

• Living microbial biomass: Bacteria, fungi, actinomycetes.
• Easily decomposable plant residues: Sugars, starches, simple proteins, organic acids, which are part of root exudates (rhizodeposits) and fresh litter.
• Dissolved organic matter (DOM) circulating in the soil solution (Weil, 2017; Foth, 1990).

Chemically, these compounds are simple and energy‑rich. Microorganisms do not need to synthesise complex enzymes to decompose them. Therefore, the turnover time of the active pool is extremely short—from a few days to several years (Huang, 2012; White, 2006).

In size, this pool is very small—usually no more than 5–10% of total SOC in arable soils and up to 20% in soils under perennial vegetation (Weil, 2017; Scheffer et al., 2018). However, despite its quantitative “lightness”, it is the active pool that is the engine of all soil life. It:

• Provides energy to the entire soil biota.
• Is the main source of mineral nutrients (primarily nitrogen) for plants during mineralisation (Foth, 1990; Scheffer et al., 2018).
• Is most sensitive to agricultural practices. Tillage, application of organic fertilisers, or, conversely, fallowing lead to rapid changes precisely in the size of the active pool (Weil, 2017).

2.2. Slow (or Intermediate) Pool

This is the “golden mean”. It includes organic compounds that have already passed through the first stage of decomposition but have not yet reached the state of stable humus. These include:

• Partially decomposed plant residues (POM – Particulate Organic Matter): Fragments of plant tissues in which the most readily available carbohydrates have already decomposed, but more resistant compounds (e.g., hemicellulose and cellulose) still remain.
• Microbial metabolites and breakdown products that were not immediately mineralised.
• Organic matter protected from decomposition by incorporation into macroaggregates (> 250 μm) (Weil, 2017; Huang, 2012).

The chemical composition of this pool is more complex, and its decomposition requires more energy from microorganisms. Therefore, the turnover time of the slow pool is measured in years and decades (from 10 to 50 years) (Eash et al., 2016; White, 2006).

The size of the slow pool varies considerably and depends on soil type and management. In well‑cultivated soils, it can range from 20 to 40% of total SOC (Weil, 2017; Scheffer et al., 2018). This is the part of organic matter that largely determines the physical properties of the soil—aggregation, water‑holding capacity, and resistance to erosion. It serves as a medium‑term reserve of nutrients and responds to changes in land use, but more slowly than the active pool.

2.3. Passive Pool

This is the most stable and long‑lived part of soil organic carbon—the so‑called stable humus (Foth, 1990; Weil, 2017). Carbon in this pool practically does not participate in the biological cycle on anthropogenic timescales. It includes carbon from:

• Chemically stable compounds: Modified lignin, high‑molecular‑weight polymers, and also black carbon (char, charcoal) formed during fires (Huang, 2012; Weil, 2017; Scheffer et al., 2018).
• Physically protected organic matter: Organic matter tightly bound to the surface of clay minerals and iron/aluminium oxides (organo‑mineral complexes). It can also be “walled in” inside tiny microaggregates (< 250 μm), into which microorganisms and their enzymes cannot penetrate (Huang, 2012; Foth, 1990).

The passive pool is dominant in mass, constituting 60–90% of total SOC in most soils (Weil, 2017). It provides high cation exchange capacity (CEC), soil buffering, and water‑holding capacity. The turnover time of this pool is hundreds and thousands of years (Huang, 2012; White, 2006). Because of its inertness, it responds weakly to everyday agronomic practices, but it can be degraded by long‑term intensive agriculture, erosion, or drainage of peatlands.

2.4. An important nuance: Continuum

It is important to understand that division into strictly separate pools is a useful but simplified model, used, for example, in computer models of the carbon cycle (RothC, CENTURY) (Eash et al., 2016; White, 2006). In real soil, these pools are not separated by clear boundaries. Organic matter exists as a continuum—from easily decomposable substances to increasingly stable forms. And the relative decomposition rate of a particular compound depends strongly on exactly where in the soil matrix it is located (we will discuss this in detail in the next lecture).

Nevertheless, the pool concept is a cornerstone for understanding organic carbon dynamics. It helps explain why:

1. Rapid loss of organic matter in the first years after ploughing occurs from the active pool (Foth, 1990).

2. Restoration of carbon stocks after a change in management is a slow process, as it requires the accumulation of more stable passive forms (Weil, 2017; Scheffer et al., 2018).

So, we see that the fate of carbon in the soil is determined not only by its chemical “palatability” to microorganisms, but also by how well it is protected from them. In the next part of our lecture, we will examine in detail what mechanisms provide this stabilisation and allow carbon to persist in the soil for millennia.

3.1. Chemical (molecular) recalcitrance

Although we speak of its limited role, chemical nature cannot be ignored. Some molecules do decompose more slowly than others:

• Lignin—a complex aromatic polymer found in plant cell walls. Its decomposition requires highly specialised enzymes (peroxidases, laccases), produced mainly by white‑rot fungi (basidiomycetes). That is why, in conditions unfavourable for fungi (e.g., under anaerobiosis), lignin accumulates (Foth, 1990; Scheffer et al., 2018).
• Aliphatic biopolymers (cutin, suberin)—resistant to hydrolysis because of ester bonds.
• Black carbon (char, biochar)—a product of pyrolysis of organic matter during fires. Its structure consists of condensed aromatic rings that are very resistant to enzymatic oxidation. In some soils (e.g., in steppes where fires are frequent), the proportion of black carbon can reach 40–50% of total SOC (Weil, 2017; Huang, 2012).

However, chemical stability alone does not guarantee long‑term preservation in soil. Experiments show that even “recalcitrant” compounds such as lignin can decompose within several years if they are in a microbially accessible location and the microbial community is adapted (Huang, 2012). That is why we shift the focus to more powerful protection mechanisms.

3.2. Physical protection (inaccessibility)

This is perhaps the most significant mechanism of long‑term stabilisation. It is based on spatial separation between substrate and microorganisms (Huang, 2012; Foth, 1990).

3.3. Chemical (sorptive) protection

This mechanism is associated with the formation of strong bonds between organic molecules and the surface of mineral particles, primarily clay minerals (illite, smectite, kaolinite) and iron and aluminium oxides/hydroxides (Huang, 2012; Scheffer et al., 2018).

3.4. Interaction of mechanisms: hierarchy of protection

It is important to understand that in real soil these mechanisms act together and in a hierarchical sequence:

1. Fresh plant residues are decomposed first in macroaggregates. Part of the undecomposed fragments, together with microbial metabolites, enters microaggregates—here physical protection begins.

2. Inside microaggregates, organic molecules are actively sorbed onto clay particles and oxides (chemical protection).

3. Over time, aggregates can be destroyed (e.g., by tillage), and then the seemingly protected matter becomes available, but its chemical nature is already such that it decomposes much more slowly than the original detritus. This demonstrates the aging effect—when even after release from the protective environment, the decomposition rate can be low because of chemical changes that occurred during its residence in the aggregate (Huang, 2012).

3.5. Brief conclusion

So, we can now formulate the main answer to the central question of the lecture: Carbon remains in the soil for millennia not because it is chemically “eternal”, but because it is reliably protected from microbes by physical barriers (aggregates, pores) and chemical bonds with minerals. It is precisely these stabilisation mechanisms that make soil the main carbon reservoir on the planet and allow organic matter stocks to be maintained even under intensive agriculture (though in smaller volumes).

In the next section, we will move on to SOC dynamics—we will see how organic carbon enters the soil, by what paths it is lost, and how equilibrium is established between these opposing fluxes.

4.1. Inputs of organic carbon

The main and practically the only source of new organic carbon to the soil is photosynthesis. Plants are the main “gateway” through which atmospheric carbon dioxide enters the soil system (Foth, 1990).

All carbon fixed by a plant sooner or later enters the soil. This happens through several pathways:

1. Aboveground plant residues (leaf litter, stubble): In agroecosystems, these are primarily crop residues of cereals, leaf litter in forests, or mown grass. Their input rates vary greatly: from 0.1–0.4 t C/ha/year in arctic forests to 4–5 t C/ha/year in tropical rainforests (White, 2006).

2. Belowground plant residues: These are roots and root exudates. For a long time, this pathway was underestimated, but modern research shows that belowground input can account for 20 to 70% of all photosynthetically fixed carbon (White, 2006; Huang, 2012). Root exudates (sugars, organic acids, amino acids) are particularly important because they are a readily available energy source for the active pool of microorganisms in the rhizosphere.

3. Application of organic fertilisers (compost, manure, green manures): In agroecosystems, humans can significantly increase carbon input by adding organic materials from outside (Eash et al., 2016).

It is important to note that not all incoming carbon becomes part of stable humus. A significant portion (up to 60–70% of fresh litter) is mineralised within the first year and returns to the atmosphere as CO₂ (White, 2006; Weil, 2017). It is this “metabolic” fraction that feeds the active pool.

4.2. Losses of organic carbon

Carbon leaves the soil through four main pathways:

1. Microbial mineralisation (main pathway): This is the main channel of losses. Heterotrophic microorganisms, oxidising organic matter, gain energy and release CO₂ into the atmosphere (Eash et al., 2016). The rate of this process is regulated by environmental conditions: temperature, moisture, and aeration. It is this process that makes soil a source of greenhouse gases.

2. Erosion (wind and water): This is the mechanical removal of the upper, most organic‑rich soil layer. Erosion is especially dangerous because it removes not only active but also passive humus that has accumulated over centuries (Eash et al., 2016; Weil, 2017).

3. Leaching (lessivage): Part of the soluble organic compounds (DOM — Dissolved Organic Matter) can be washed out from the upper horizons into deeper layers or into groundwater. Although this flux is much smaller in mass than mineralisation, it is important for pedogenesis (e.g., podzolisation) and can be significant in some ecosystems (Huang, 2012; Weil, 2017).

4. Removal with harvest: In agroecosystems, a significant portion of plant‑fixed carbon (grain, fruits) is irretrievably removed from the field, directly reducing the potential input of organic matter to the soil (Eash et al., 2016).

4.3. Equilibrium and the factors that determine it

SOC dynamics are described by a simple but fundamental balance equation (White, 2006; Weil, 2017):

ΔSOC = (Inputs) — (Losses)

If inputs exceed losses, carbon stocks increase (accumulation). If losses exceed inputs, stocks decrease (degradation). If inputs equal losses, a steady‑state equilibrium is established, in which SOC content does not change from year to year (Foth, 1990; Eash et al., 2016).

The position of this equilibrium is influenced by three main groups of factors (Foth, 1990; Weil, 2017; Scheffer et al., 2018):

1. Climate:

• Temperature: Decomposition processes are more sensitive to temperature than photosynthesis (Huang, 2012). Therefore, in warm and humid climates (tropics), decomposition proceeds very rapidly, and the equilibrium SOC level is low. In cold climates (taiga, tundra), conversely, decomposition is slowed, and organic matter accumulates (e.g., peatlands). Rising temperatures generally shift the balance towards losses (Eash et al., 2016).
• Moisture: In arid conditions (steppes, deserts), plant biomass input is limited, and SOC is low. In waterlogged conditions (wetlands), oxygen availability is limited, anaerobic decomposition is slow, and organic matter accumulates (Foth, 1990).

2. Soil texture and mineralogy (protective capacity): As we discussed in the previous section, clayey soils and soils with high Fe/Al oxide content have a greater capacity for carbon stabilisation. Therefore, their equilibrium SOC content will be higher than on light sandy soils, all other things being equal (Foth, 1990; Weil, 2017).

3. Land use management: This is the main factor that can change the equilibrium within a single generation:

• Ploughing of virgin land: Sharply accelerates mineralisation through aeration, destruction of aggregates, and mixing of organic matter. As a result, SOC content declines rapidly, reaching a new lower equilibrium level (usually within 30–50 years) (Foth, 1990; Weil, 2017).
• Application of organic fertilisers and green manures: Increases input, shifting equilibrium towards accumulation.
• Reduced and zero tillage: Reduces losses by preserving aggregates and a mulching layer, promoting carbon stabilisation (Eash et al., 2016).

Example from long‑term experiments: The famous Rothamsted experiment (England) shows that on plots that received manure, the organic carbon content remained stably high for 100 years. On plots where manure application ceased, SOC content began to decline slowly to a new, lower equilibrium level typical of mineral fertilisers (Weil, 2017; White, 2006).

4.4. Time scales: relationship with pools

Now we can link the equilibrium concept with the pool concept. The change in total SOC is the sum of changes in all three pools:

• Active pool responds quickly to changes in inputs or management (days–years).
• Slow pool responds on decadal timescales.
• Passive pool is practically inert on human lifespans.

That is why, when we plough virgin land, organic matter is lost rapidly in the first years (from the active pool), and then the process slows down. And that is why, when we try to restore carbon, we need decades to “fill” the slow and passive pools (Weil, 2017; Scheffer et al., 2018).

So, SOC dynamics are always a balance between inputs and outputs. This equilibrium responds sensitively to climate, soil properties, and, most importantly, our agronomic decisions. The goal of sustainable agriculture is to shift this equilibrium towards carbon accumulation, increasing inputs and reducing losses.

In the next section, we will talk about conscious management of this process—about carbon sequestration as a modern tool not only for increasing fertility but also for mitigating climate change.

Excellent, colleagues. We have reached the most relevant and practically significant part of our lecture—the question of carbon sequestration. This concept is widely used today in discussions about climate change, but for us as soil scientists, it is important to understand its precise scientific meaning and, above all, the soil mechanisms behind it.

5.1. How does sequestration work in the soil process?

From the perspective of soil dynamics, sequestration is a shift in the balance between carbon inputs and outputs towards a positive balance over a long period (Eash et al., 2016; Weil, 2017). This process is implemented through three key mechanisms operating at different levels:

1. Increasing inputs: The most obvious path. We must ensure that more organic material enters the soil than it loses. This is achieved through:

• Increasing the net primary productivity of agroecosystems (optimising plant nutrition, selecting varieties) (Weil, 2017).
• Growing cover crops (green manures) in the off‑season, when the main crop does not occupy the field (Eash et al., 2016).
• Applying organic fertilisers (compost, manure, biochar) (Weil, 2017).
• Important: increasing inputs is effective only if this carbon eventually moves into protected pools.

2. Reducing losses: The second critical path is to slow the rate of mineralisation:

• Transition to zero and minimum tillage (No‑Till, Minimum Tillage): This is a key agronomic technique. Avoiding ploughing: preserves aggregates, protecting organic matter from physical destruction; reduces aeration, slowing humus oxidation; creates a mulching layer, reducing erosion and soil overheating (Eash et al., 2016; Weil, 2017).
• Erosion control: Protecting the top horizon from washing away and blowing away (Eash et al., 2016).
• Optimising crop rotations: Including perennial grasses with a powerful root system that creates a deep carbon pool (Weil, 2017).

3. Enhancing stabilisation mechanisms: This is the most complex but also the most long‑term path. It aims to make incoming carbon move as quickly as possible from the active to the passive pool (Scheffer et al., 2018). This is promoted by:

• Creating conditions for aggregation: Root exudates and microbial exopolysaccharides glue particles into aggregates, within which carbon is physically protected (Weil, 2017).
• Applying materials with a high content of stable carbon forms: For example, biochar. Its aromatic structure makes it chemically recalcitrant, and its high porosity promotes sorption of other organic molecules (Huang, 2012; Weil, 2017).
• Maintaining optimal pH and providing nutrients: This stimulates plant growth and the activity of aggregate‑forming microorganisms, accelerating the formation of stable organo‑mineral complexes (Foth, 1990).

5.2. Why is this important now? (Link to the global cycle)

As we have already said, soil is the largest reservoir of active carbon on land. However, over the past 150 years, as a result of ploughing virgin lands, draining wetlands, and erosion, soils have lost about 50–70% of their original organic carbon stock (Eash et al., 2016; Weil, 2017). This “historical loss” has contributed significantly to the rise in atmospheric CO₂ concentration.

Carbon sequestration offers a “reverse gear” for this process. Theoretically, we can return part of the carbon back to the soil, turning it from a source of greenhouse gases into a sink (Eash et al., 2016). This is the concept of “climate‑smart” agriculture.

5.3. Potential and limitations

The potential for sequestration is huge. It is estimated that a global shift to resource‑conserving agricultural technologies could sequester between 0.4 and 1.2 Gt C per year (Weil, 2017; Eash et al., 2016). However, it is important to understand the limitations:

1. Saturation: Each soil has its own carbon saturation limit, determined by its texture and mineralogy (Huang, 2012; Weil, 2017). You cannot accumulate carbon indefinitely. Once all available mineral surfaces are occupied and all micropores are filled, the sequestration rate will drop to zero. This means that sequestration is a temporary solution that buys us time while we transition to renewable energy sources (Eash et al., 2016).

2. Reversibility: The carbon we have “sequestered” with great effort can be lost very quickly upon a change in land‑use regime (e.g., by ploughing a fallow). Therefore, sequestration requires continuous efforts to maintain favourable practices (Weil, 2017).

3. Management challenges: Switching to No‑Till, introducing complex crop rotations and cover crops require knowledge, new investments, and time for the positive effect to manifest (Eash et al., 2016).

5.4. Practical examples of sequestration measures

Here are key practices that have proven effective in increasing SOC stocks (Weil, 2017; Eash et al., 2016; Scheffer et al., 2018):

• Minimum/Zero tillage: Increases carbon in the top layer, reduces erosion and mineralisation.
• Cover crops: Provide a continuous flow of carbon to the soil (especially as root exudates), protect soil from erosion in winter.
• Application of organic fertilisers and biochar: Direct addition of stable and semi‑stable forms of carbon.
• Optimised pastures: Managing grazing to stimulate root growth and restore vegetation.
• Afforestation and agroforestry: Converting arable land to pastures or forests increases root carbon input.

So, sequestration is not an abstract idea but a concrete soil management task, based on fundamental knowledge of pools, stabilisation, and balance. We can turn soil from a source of greenhouse gases into a reliable sink, while simultaneously increasing its fertility. That is the essence of sustainable agriculture in the 21st century.

In the next section, we will rise above the field level and consider what role soil organic carbon plays in the global carbon cycle of the planet. This will help us finally understand the scale of the phenomenon.

6.1. Soil — the largest reservoir of active carbon on land

Let us turn again to the numbers, but now place them in a global context. Estimates show that the top metre of the world’s soils contains between 1500 and 1600 Pg (petagrams, 10¹⁵ g) of organic carbon (Weil, 2017; Huang, 2012). If deeper layers (up to 2–3 m) are included, this figure rises to 2300–2450 Pg (Huang, 2012; Scheffer et al., 2018).

For comparison:

• The atmosphere contains about 760 Pg of carbon (mostly as CO₂).
• Terrestrial vegetation (forests, grasses, crops) — about 550–560 Pg.
• The ocean contains a huge amount, but mostly in dissolved inorganic form.

Thus, soil contains almost twice as much carbon as the atmosphere and all vegetation combined (Weil, 2017; Eash et al., 2016). This means that even a small relative change in soil carbon stocks can cause a huge absolute change in atmospheric CO₂ concentration. For example, if climate change or improper land use caused us to lose just 5% of SOC stocks in the top soil layer, that would correspond to an additional CO₂ emission into the atmosphere comparable to annual emissions from fossil fuel burning (Huang, 2012; Eash et al., 2016).

6.2. Main carbon fluxes involving soil

The carbon cycle is not a static picture. It is a dynamic system with powerful fluxes:

1. Input (Photosynthesis): Plants annually remove a huge amount of CO₂ from the atmosphere (about 120 Pg C/year) and fix it in organic biomass (Weil, 2017; White, 2006). Part of this biomass (roots, litter) enters the soil, becoming a source of SOC.

2. Output (Soil respiration): This is the main return flux. Heterotrophic microorganisms (and to a lesser extent plant roots) oxidise soil organic matter, returning carbon dioxide to the atmosphere. This process releases about 60–62 Pg C/year, making soil respiration one of the largest carbon fluxes in the global cycle (Weil, 2017; Eash et al., 2016).

3. Output (Erosion and leaching): Smaller but still significant fluxes. Carbon carried away by erosion can be buried in river sediments or the ocean, temporarily leaving the active cycle. Dissolved organic carbon leaches into groundwater (Weil, 2017; Foth, 1990).

If these fluxes were in perfect equilibrium, atmospheric CO₂ concentration would not change. However, over the past 150 years, humanity has substantially disrupted this balance.

6.3. Anthropogenic disturbance of equilibrium

We are a geological force, and our impact on the global carbon cycle is enormous. The main anthropogenic factors related to soil are:

1. Ploughing of virgin land and agricultural development: When we plough virgin steppes or cut down forests, we sharply accelerate the mineralisation of organic matter. Through aeration, destruction of aggregates, and increased temperature of the topsoil, the soil begins to lose carbon, turning from a sink into a powerful source of CO₂ (Foth, 1990; Weil, 2017; Eash et al., 2016). Historically, this “soil emission” has contributed hugely to the rise in CO₂ since pre‑industrial times.

2. Drainage of wetlands and peatlands: Peat soils (Histosols) contain colossal carbon stocks that have accumulated for centuries under anaerobic conditions (Weil, 2017; Scheffer et al., 2018). Their drainage leads to oxygen influx, activation of microorganisms, and rapid decomposition of peat. As a result, a gigantic amount of CO₂ is released, and the soil surface can subside (so‑called “subsidence”) (Foth, 1990; Eash et al., 2016).

3. Erosion: Accelerates the removal of the top, most carbon‑rich horizon (Weil, 2017; Eash et al., 2016).

As a result of these processes, it is estimated that the world’s soils have lost between 40 and 60% of their original organic carbon in agricultural areas. This “historical loss” is one of the key causes of the current rise in greenhouse gas concentrations (Eash et al., 2016; Weil, 2017).

6.4. Soil as a sink and as a source of greenhouse gases

It should be clearly understood that soil can act in two roles:

• As a sink (sequestration): When undisturbed ecosystems, especially forests and wetlands, or well‑managed agroecosystems, where carbon input exceeds losses, soils accumulate carbon.
• As a source (emissions): When natural ecosystems are disturbed, or when the soil becomes waterlogged or, conversely, dried out, the balance is upset, and the soil begins to release carbon intensively.

It is important to add that soil is a source not only of CO₂ but also of other greenhouse gases:

• Methane (CH₄): Formed under anaerobic conditions (rice paddies, wetlands). Its greenhouse potential is 25–30 times higher than that of CO₂ (Eash et al., 2016; Weil, 2017).
• Nitrous oxide (N₂O): Formed during denitrification under oxygen deficiency and in the presence of nitrates. It is a powerful greenhouse gas with a potential 300 times greater than CO₂ (Weil, 2017).

6.5. Soil — the key to solving the climate problem

Understanding the global role of SOC changes our view of agronomy. The task of sustainable agriculture today is not only to “preserve fertility” but also to return carbon back to the soil. This is called climate‑smart agriculture.

By managing the SOC balance through organic amendments, reduced tillage, cover crops, and complex crop rotations, we can not only increase yields and soil resilience to stress but also make a real contribution to mitigating global warming. Soil is not just a medium for plant growth. It is a huge, underestimated, and still poorly used tool for regulating the planet’s climate.

So, we have seen that soil organic carbon is not only the basis of fertility of a particular field but also a critical regulator of the global atmosphere. Not only our harvest but also the climate in which we will live depends on the state of soil carbon.

In the final, seventh section of our lecture, we will summarise and answer the key practical question: Why is SOC the main integral indicator of soil health?

Excellent, colleagues. We are concluding our lecture. We have examined what soil organic carbon (SOC) is, what pools it consists of, how it is stabilised, what its dynamics are, what sequestration is, and what its global role is. Now we come to the culmination—answering the question that has direct practical significance for every agronomist, soil scientist, and farmer: Why is SOC considered the main integral indicator of soil health?

7.1. SOC as an indicator of physical soil health

Physical soil health is determined by its structure, density, permeability, and water‑holding capacity. SOC plays a fundamental role here:

1. Aggregation and structure: Organic matter, especially the active pool (microbial exopolysaccharides, glomalin) and humus, is the main “glue” binding mineral particles into water‑stable aggregates (Foth, 1990; Weil, 2017). Good structure provides an optimal ratio of pores of different sizes: large pores for aeration and drainage, small pores for water retention.

2. Resistance to erosion: Aggregated soil better withstands the destructive impact of raindrops and wind. Therefore, high SOC content is a direct indicator of soil resistance to degradation (Eash et al., 2016; Weil, 2017).

3. Water regime: SOC acts like a “sponge”. Humus can hold 4–5 times its weight in water (Foth, 1990; Weil, 2017). High SOC content increases field capacity (FC), improves infiltration, and reduces surface runoff. This is critically important under drought conditions (Huang, 2012).

4. Reduction of compaction: Soils rich in organic matter are less prone to compaction under heavy machinery, as organic material cushions the pressure (Weil, 2017).

Conclusion: From the level of SOC we can reliably infer how well structured the soil is, how resistant it is to water and wind erosion, and how well it manages water.

7.2. SOC as an indicator of chemical soil health

Chemical soil health is determined by its ability to retain and supply nutrients, and to maintain an optimal pH.

1. Cation exchange capacity (CEC): Humus is the main source of negative charges in most soils (especially sandy and loamy sands). The contribution of SOC to CEC can reach 25–90% in surface horizons (Foth, 1990; Weil, 2017). The higher the SOC content, the more cations (Ca²⁺, Mg²⁺, K⁺) the soil can retain, preventing leaching and making them available to plants.

2. Buffering capacity: Humus contains weakly acidic functional groups (carboxyl, phenolic) that act as a buffer, counteracting sharp pH changes (Huang, 2012; Weil, 2017). Soils with high SOC better resist both acidification and alkalisation.

3. Complexation and detoxification: Organic molecules can form stable chelate complexes with metal ions, including toxic ones (Al³⁺, Pb²⁺, Cd²⁺) and micronutrients (Fe³⁺, Cu²⁺, Zn²⁺). This reduces phytotoxicity and simultaneously increases micronutrient availability to plants (Foth, 1990; Scheffer et al., 2018).

4. Reserve of nutrients: SOC is the main storehouse of nitrogen (N), phosphorus (P), and sulphur (S) in organic form. Their mineralisation provides long‑term plant nutrition (Foth, 1990; Weil, 2017).

Conclusion: From the level of SOC we can judge the soil’s ability to retain and supply nutrients, its resistance to chemical stresses, and its reserve of “slow‑release” forms of nutrients.

7.3. SOC as an indicator of biological soil health

Biological health is the diversity, activity, and abundance of soil biota (microorganisms, fungi, invertebrates).

1. Energy (food) source: SOC, especially the active pool, is the main source of carbon and energy for heterotrophic microorganisms (Eash et al., 2016; White, 2006). Without organic carbon, the soil food web simply cannot exist. The higher and more diverse the organic input, the richer and more active the microbial community.

2. Habitat: The aggregated structure, created with the participation of SOC, forms a complex system of pores and channels that is “home” to a huge diversity of organisms (from bacteria to earthworms) (Weil, 2017).

3. Enzyme activity: Microorganisms secrete enzymes that decompose complex organic polymers (cellulose, lignin) into simple compounds. This activity depends directly on the presence and quality of substrate (Foth, 1990; Huang, 2012).

Conclusion: From the level of SOC (especially the active pool) we can judge the biological richness of the soil, its ability for self‑purification, nutrient cycling, and maintenance of biological diversity.

7.4. SOC — a “win‑win” indicator

The most important property of SOC as a health indicator is its universality and sensitivity:

1. Sensitivity to management: SOC, especially its active fraction, responds to changes in land use (ploughing, organic amendments, No‑Till) within a few years (Eash et al., 2016; Weil, 2017). This makes it an excellent tool for monitoring the effectiveness of agronomic practices.

2. Integrativeness: It does not merely show the presence of carbon. It reflects the complex interaction of all processes in the soil—from microbial activity to the physical state of the mineral matrix. Loss of SOC is almost always a signal that other health indicators (structure, biota, fertility) are also degrading (Scheffer et al., 2018).

3. Measurability: Although precise fractionation of pools requires sophisticated methods, total SOC content is measured by standard and accessible methods (wet or dry combustion), allowing its use in routine analysis (Weil, 2017).

7.5. Practical significance: from theory to practice

Understanding SOC as the main indicator of soil health changes our approach to agronomy:

• We move from the paradigm of “applying fertilisers to obtain yield” to the paradigm of “feeding the soil to build carbon capital”.
• When deciding on a tillage system or crop rotation, we first ask: “How will this affect the SOC balance in the long term?”
• Monitoring SOC becomes as mandatory as agrochemical analysis for macronutrients. This allows us to see long‑term trends and adjust management strategies to prevent degradation.