Sources and Composition of Soil Organic Matter

1.1 Plants—The Main Source

The primary and only initial source of carbon for soil organic matter is the products of photosynthesis by higher plants. Plants extract carbon dioxide from the atmosphere, convert it into complex organic compounds (sugars, cellulose, lignin), and return carbon to the soil in the form of plant residues. Thus, soil organic matter is transformed, "recycled" living matter (Kononova, 1966).

All plant material that enters the soil can be divided into two large groups:

1. Aboveground litter (litterfall): leaves, needles, branches, fruits, bark, stubble, and crop residues.

2. Belowground contribution: living and dying root systems, root exudates, and mucilages (rhizodeposits).

It is important to understand: in different ecosystems, the balance between these sources varies greatly. In forests, the bulk of organic matter reaches the surface as leaf litter. In herbaceous ecosystems (prairies, steppes, pastures) and agroecosystems, the main contribution comes from root systems (Kononova, 1966; Weil & Brady, 2017).

1.2 Roots and Belowground Contribution

Modern research shows that belowground phytomass, contrary to older views, is often the predominant and, crucially, more stable source of soil organic carbon (SOC) (Weil & Brady, 2017).

Why is this so?

• Quantity: In meadow and steppe ecosystems, root mass can account for 50–80% of total phytomass.
• Quality: Root tissues generally have a wider C:N ratio and contain more lignin and difficult-to-hydrolyze compounds compared to leaves. This slows their decomposition and promotes transformation not into CO₂, but into stable humic substances (Weil & Brady, 2017).
• Direct contact: Roots penetrate the mineral soil matrix, die, and decompose in situ, in direct contact with soil minerals. This creates ideal conditions for the physical and chemical stabilization of their derivatives (binding to clay particles, aggregate formation) (Foth, 1990; Weil & Brady, 2017).
• Rhizodeposition: Living roots continuously release a wide range of organic compounds into the surrounding environment (the rhizosphere)—sugars, amino acids, organic acids, vitamins, and mucilages (Eash et al., 2016; Weil & Brady, 2017). These exudates, also called root secretions, are an easily available and rapid energy source for microorganisms, stimulating their activity and triggering processes of organic matter formation and stabilization. It is generally accepted that up to 20–40% of all carbon assimilated by a plant can be released into the rhizosphere.

Thus, the role of roots is not merely the supply of "dead" biomass; it is an active, dynamic process of continuous exchange of matter and energy between the plant and the soil.

1.3 Litter and Aboveground Plant Residues

Litter is the most visible source of organic matter. It consists of leaves, needles, branches, stubble, seeds, and other aboveground plant parts that die and fall onto the soil surface (Eash et al., 2016; Kononova, 1966).

The quality (chemical composition) of litter varies widely. For instance, litter from herbaceous plants and deciduous leaves is rich in nutrients and decomposes relatively quickly, while pine needles or branches contain much lignin, resins, and waxes, making them more resistant to decomposition. Surface litter forms a specific horizon—the forest floor—which performs vital functions: protecting the soil from erosion, regulating thermal and water regimes, and acting as a "buffer" for organic matter entering the soil (Eash et al., 2016).

1.4 Microorganisms—"Secondary" Sources

Microorganisms (bacteria, fungi, actinomycetes) are not only the main decomposers and transformers of organic matter but also its direct source. Their role here is twofold.

First, microbial biomass itself is organic matter. After cell death and lysis of bacteria and fungi, their polymeric components (peptidoglycans, chitin, fungal melanins, polysaccharides) become an important part of soil humus. Numerous studies (Kononova, 1966; Weil & Brady, 2017) emphasize that the contribution of microbial residues to the formation of stable humic substances is often underestimated. Many components of microbial cells are structurally close to humic acids and possess high resistance to further decomposition.

Second, microorganisms act as a "factory" for processing and synthesizing new organic molecules. During their life processes, while decomposing complex plant polymers, microorganisms synthesize from their fragments entirely new high-molecular-weight compounds—specific humic substances (Kononova, 1966). Thus, a significant proportion of stable humus is not directly of plant origin but is a product of microbial synthesis and resynthesis.

1.5 Animals (Fauna)

Soil animals—from protozoa and nematodes to earthworms, ants, and mammals—contribute to the organic matter pool in three main ways (Eash et al., 2016; Weil & Brady, 2017):

1. Direct biomass contribution: upon dying, their bodies become part of the organic matter.

2. Mechanical transformation: "ecosystem engineers" like worms and ants shred, mix, and ingest plant residues, significantly increasing their surface area and accessibility to microbial attack. Animal feces represent a new type of organic substrate, rich in microorganisms and easily digestible compounds (Weil & Brady, 2017).

3. Creation and maintenance of structure: The activity of macrofauna (worms, ants, termites) leads to the formation of channels (biopores) and water-stable aggregates, within which organic matter becomes physically protected from rapid decomposition (Weil & Brady, 2017).

Summarizing the first block:

Soil organic matter is not a single substance. It is polygenic (has multiple origins). Plants are the main and primary source, but the major share of stable and long-lasting humus in most ecosystems is formed from the belowground contribution of roots and their metabolic products, as well as from secondary microbial synthesis.

3.1 Quantitative Superiority of Roots in Herbaceous Ecosystems

In different ecosystems, the ratio of aboveground to belowground biomass varies greatly. While in forests most phytomass is concentrated in tree trunks and canopies, in herbaceous ecosystems (steppes, prairies, pastures, meadows)—which are "storehouses" of global soil carbon—the situation is opposite.

In meadow and steppe communities, root mass is often 2–3 or more times greater than the mass of aboveground organs (Kononova, 1966; Weil & Brady, 2017). In boreal and tundra ecosystems, this ratio can be even higher. Annual root mortality is enormous. For example, under perennial grasses, root dieback can reach 3–5 tons of dry matter per hectare per year, significantly exceeding the input from aboveground litter (White, 2006). Thus, for these ecosystems, root litter is the dominant source of organic matter.

3.2 Qualitative Advantages of Root Tissues

However, it is not only a matter of quantity. The quality of root litter also plays a decisive role in its ability to form stable humus. Root tissues, especially in perennials, have several characteristic features:

1. Wider C:N ratio: Roots are generally poorer in nitrogen compared to leaves (Kononova, 1966; Weil & Brady, 2017). Nitrogen content in roots is often 0.5–1.5%, whereas in young leaves and grasses it can reach 3–5%. This means that root residues have a higher carbon-to‑nitrogen ratio. A high C:N ratio is one of the main factors slowing down the decomposition rate of organic material. Microorganisms decomposing such a substrate experience nitrogen deficiency, which inhibits their activity and, consequently, the rate of carbon mineralization to CO₂ (Eash et al., 2016; White, 2006). This gives root organic matter more time to transform into stable humic compounds.

2. Higher content of resistant compounds: Roots, especially of woody species and grasses, contain more lignin, suberin, and other hydrophobic, decomposition-resistant polymers (Weil & Brady, 2017). Lignin and suberin, unlike easily degradable cellulose and hemicellulose, are "slow" substrates. They not only decompose slowly but also serve as building blocks for the formation of stable humic substances, particularly humic acids (Kononova, 1966; White, 2006). For example, lignin can account for up to 15–20% of the dry mass of grass roots (Foth, 1990; Weil & Brady, 2017).

3.3 Spatial Factor: Contact with the Mineral Matrix

This is perhaps the most important and often underestimated aspect. Roots die and decompose directly within the soil profile, in close physical contact with mineral particles (clay, silt, sand) and soil aggregates (Weil & Brady, 2017; White, 2006). Such localization creates exceptionally favorable conditions for organic matter stabilization:

• Physical protection within aggregates: Fine root hairs and root fragments, upon dying, become enclosed within soil micro‑ and macroaggregates. The clay films and aggregates forming around them isolate organic matter from contact with microorganisms and enzymes. This phenomenon is known as physical protection and is one of the main mechanisms for long‑term stabilization of soil carbon (Foth, 1990; Weil & Brady, 2017).
• Chemical protection: Products of root decomposition (humic acids, polymers) actively interact with the surface of clay minerals and iron and aluminum oxides, forming strong organo‑mineral complexes. These complexes, in turn, are resistant to microbial degradation, further reducing the mineralization rate (Weil & Brady, 2017).
• Structure formation: Roots and their associated microflora (mycorrhizal fungi) are a powerful factor in aggregating soil particles. Fungal hyphae glue particles together, and root exudates (polysaccharides) act as adhesives. The aggregates formed are a medium where organic matter is better preserved.

3.4 Rhizodeposition—The "Hidden" Carbon Flux

In addition to dying root biomass, there is another vital but less visible source of belowground carbon—rhizodeposition (Eash et al., 2016; Weil & Brady, 2017).

During growth, living roots continuously release into the surrounding environment (the rhizosphere) a large number of diverse organic compounds. These may include:

• Simple sugars and organic acids.
• Amino acids and vitamins.
• Complex polysaccharides (mucilages, slimes).
• Dead and sloughed‑off root cap and epidermal cells.

Studies using ¹⁴C isotopic labels show that up to 20–40% of all carbon assimilated by a plant during photosynthesis can be released into the rhizosphere as exudates (Eash et al., 2016). This is an enormous figure!

Although these easily accessible compounds are quickly metabolized by microorganisms (their half‑life is hours or days), they perform critically important functions:

1. Stimulate the growth and activity of rhizosphere microflora (the rhizosphere effect), accelerating the transformation of other, more complex organic residues.

2. Contribute to the formation and stabilization of soil aggregates.

3. Act as a "seed" for the synthesis of microbial metabolites, which in turn can enter the stable organic matter pool (Weil & Brady, 2017).

Important clarification: A fraction of root exudates that is not rapidly mineralized can adsorb onto soil colloids and, thus, also transition into stable organic matter (Weil & Brady, 2017). Although this contribution is often quantitatively small, qualitatively it can be very significant for humus formation processes.

Summarizing the key points of block 3:

1. Roots are the quantitatively dominant source of organic matter in most natural herbaceous and many forest ecosystems.

2. The quality of root litter (high C:N, richness in lignin and suberin) results in its low decomposition rate and high potential for stable humus formation.

3. Direct contact with the mineral matrix provides physical and chemical protection of root residues from rapid microbial degradation.

4. Rhizodeposition is a continuous flow of readily available carbon from living roots, stimulating soil biota and promoting aggregate formation and, ultimately, stable organic matter.

Thus, in agronomic practice, when aiming to maintain or increase soil fertility, we must pay primary attention to the condition of plant root systems. It is the belowground contribution, not just the amount of crop residues, that is the key factor in the stability of soil organic matter.

In the next section, we will move to the chemical composition of plant residues—examining which specific organic compounds they consist of (cellulose, hemicellulose, lignin, etc.) and how their chemical structure influences the fate of carbon in soil.

4.1 Water‑Soluble Compounds

This is the most labile fraction. It includes simple sugars (glucose, fructose, sucrose), organic acids, amino acids, and some alcohols. These compounds are easily extracted by water and serve as "fast food" for microorganisms. They are mineralized almost completely within a few days or weeks, releasing CO₂ and providing energy for the microbial community. Their contribution to stable humus formation is minimal, but they play a vital role in initiating (priming) the decomposition of more complex polymers (Kononova, 1966).

4.2 Polysaccharides

Polysaccharides are high‑molecular‑weight polymers composed of monosaccharide units. They constitute the bulk of plant tissues.

Cellulose is the most abundant organic polymer on Earth. The cellulose molecule is a long unbranched chain of β‑D‑glucose residues linked by glycosidic bonds. Cellulose forms microfibrils and has a structural function in plant cell walls. Decomposition of cellulose requires specialized microorganisms producing the enzyme cellulase. Under aerobic conditions, it decomposes to CO₂ and water; under anaerobic conditions, to organic acids, methane, and hydrogen. Despite its complexity, cellulose is a moderately decomposable compound and, in the absence of other limiting factors, can be rather rapidly utilized by microorganisms (Kononova, 1966; Weil & Brady, 2017).

Hemicelluloses are heteropolymers containing various pentoses (xylose, arabinose) and hexoses (mannose, galactose), as well as uronic acids. Unlike cellulose, hemicelluloses have a branched structure and are more easily hydrolyzed. They decompose faster than cellulose but slower than simple sugars (Weil & Brady, 2017).

4.3 Lignin

This is unquestionably the most important and most resistant component of plant tissues regarding soil humus formation. Lignin is a complex three‑dimensional aromatic polymer built from phenylpropane units (monolignols): p‑coumaryl, coniferyl, and sinapyl alcohols. Its role in the plant is to provide mechanical strength and hydrophobicity to cell walls, especially in wood and conducting tissues.

The chemical structure of lignin makes it extremely resistant to microbial decomposition. Unlike polysaccharides, which are broken down by hydrolytic enzymes, lignin is degraded mainly by oxidative enzymes (peroxidases, laccases), produced primarily by basidiomycetes (white‑rot fungi) (Weil & Brady, 2017; White, 2006). This process is slow and energetically "unprofitable" for microorganisms. Consequently, lignin accumulates in soil, being one of the main precursors of stable humic substances, particularly humic acids (Kononova, 1966; Weil & Brady, 2017).

Important: Lignin content varies greatly among different types of plant residues. In coniferous wood, lignin can account for 25–30% of dry mass, in hardwoods up to 20–25%, in cereal straw 15–20%, and in grass leaves only 5–10%. This high lignin content in roots, as discussed above, is one reason for their high humification capacity (Weil & Brady, 2017).

4.4 Proteins and Other Nitrogen‑Containing Compounds

Proteins are the main nitrogen reserve in plants. They decompose fairly rapidly, yielding amino acids, which in turn can be mineralized to ammonia (ammonification) or used by microorganisms for synthesizing their own cell proteins (immobilization). Protein content in vegetative plant organs ranges from 5 to 20% or more (in legumes). In addition to proteins, plant tissues contain chlorophyll, nucleic acids, and free amino acids.

Nitrogen‑containing compounds are an important factor determining the decomposition rate of the entire plant residue through the carbon‑to‑nitrogen (C:N) ratio.

4.5 Lipids, Waxes, Resins, and Tannins

This is a group of compounds diverse in chemical nature but united by hydrophobic properties.

• Lipids and waxes are found mainly in the cuticle, protecting aboveground organs from desiccation, and in suberin of bark and roots. These are complex esters of fatty acids and high‑molecular‑weight alcohols. They decompose slowly and impart water‑repellent properties to residues.
• Resins and terpenes also belong to recalcitrant compounds. They are typical of conifers.
• Tannins are a group of polyphenolic compounds that can bind proteins into difficult‑to‑decompose complexes, slowing their mineralization. They play an important role in forest floor formation and likely participate in humification processes (Kononova, 1966; Weil & Brady, 2017).

4.6 Carbon‑to‑Nitrogen Ratio (C:N)

This is one of the most significant integrated indicators of plant residue quality (Eash et al., 2016; Weil & Brady, 2017; White, 2006). It reflects the balance of the substrate for the microbial community.

• Narrow C:N (below 20–25:1) is characteristic of young, nitrogen‑rich tissues (legume leaves, young grass). When decomposing such residues, microorganisms obtain sufficient nitrogen from the substrate, and after meeting their own needs, excess nitrogen is released into the soil in mineral form (mineralization). These residues decompose rapidly.
• Wide C:N (above 30–40:1) is characteristic of mature, coarse plant residues (straw, wood, perennial roots). During their decomposition, microorganisms experience acute nitrogen deficiency. They must "borrow" mineral nitrogen from the soil solution to build their own cells. This leads to immobilization of mineral nitrogen and its temporary removal from circulation, negatively affecting plant nutrition. The decomposition rate of such residues drops sharply.

That is why in agricultural practice it is important to consider the C:N ratio of added organic materials (straw, green manures, manure). Straw with a wide C:N ratio is best composted or applied together with nitrogen fertilizers to avoid nitrogen deficiency in the soil.

Summarizing block 4:

The chemical composition of plant residues is a key factor determining their resistance to decomposition and their ability to form stable humus. Among all components, lignin is the most valuable "seed" for building a long‑term reserve of soil organic carbon. However, in reality, all these compounds act in combination, and the fate of carbon in soil is determined not only by the presence of lignin but also by nitrogen content and the availability of energy (readily available carbohydrates) for microorganisms.

5.1 Microbial Biomass as a Source of Organic Matter

Soil microbial biomass is the collective mass of all living microorganisms in soil: bacteria, archaea, fungi, actinomycetes, algae, and protozoa. The mass of this biomass in the topsoil can reach 1–5 tons of dry matter per hectare (in carbon terms—0.5–2 t C/ha) (Weil & Brady, 2017; White, 2006). This is comparable to the mass of aboveground litter in many ecosystems.

The contribution of microbial biomass to the soil organic matter pool occurs through two main pathways:

1. Direct input of microbial residues (necromass): The life cycle of most soil microorganisms is very short—from a few hours to several weeks. Bacteria multiply and then die, and their cell walls and cytoplasm become part of organic matter. The annual mass of dying microbial biomass may be many times greater than its average stationary value (Eash et al., 2016). These dead cells are a high‑quality, nitrogen‑rich substrate that is quickly involved in further turnover, but some of their chemically stable components accumulate.

2. Microbial synthesis and release of exometabolites: During their life activities, microorganisms synthesize and release into the external environment a huge variety of organic compounds—exoenzymes, polysaccharides, glycoproteins, lipids, organic acids. These compounds, especially polymeric ones, often have high reactivity and actively participate in the formation of organo‑mineral complexes and aggregates. The fate of many of them is not complete mineralization but incorporation into stable humus (Weil & Brady, 2017; White, 2006).

5.2 Chemically Stable Components of Microbial Cells

Plant residues contain many readily available compounds (cellulose, hemicellulose, proteins). Microbial cells are structured differently. They contain a high percentage of polymers specifically designed to maintain structural integrity in the aggressive soil environment. These polymers are generally highly resistant to hydrolysis and oxidation. Which components of microbial cells are particularly stable and promising for humus formation?

• Fungal chitin: This is a polymer of N‑acetylglucosamine, a structural component of fungal cell walls. Chitin is a highly stable compound, resistant to decomposition. It is an important source of organic nitrogen and carbon in soil (Weil & Brady, 2017; Kononova, 1966).
• Bacterial peptidoglycan (murein): This is a giant polymer forming a rigid mesh‑like structure of bacterial cell walls. It consists of alternating residues of N‑acetylglucosamine and N‑acetylmuramic acid cross‑linked by short peptide bridges. Peptidoglycan is highly resistant and also contributes significantly to the stable pool of soil organic nitrogen (Weil & Brady, 2017).
• Microbial melanins: These are dark‑colored, high‑molecular‑weight heteropolymers of phenolic nature, synthesized by many fungi (especially from classes Ascomycota and Basidiomycota) as well as some bacteria. Melanins are strikingly similar in structure to soil humic acids. They are extremely resistant to microbial degradation and are likely direct precursors of stable humus in many soils, especially forest soils (Kononova, 1966; Weil & Brady, 2017).
• Microbial polysaccharides (exopolysaccharides): Many bacteria and fungi release slimy polysaccharides into the environment (e.g., glucans, mannans, xanthan). These compounds have a high sorption capacity for clay minerals and actively bind soil particles into aggregates. Adsorbed onto mineral surfaces, these polysaccharides are protected from rapid enzymatic hydrolysis and can persist in soil for extended periods (Weil & Brady, 2017; White, 2006).
• Glomalin: This is a glycoprotein produced by arbuscular mycorrhizal (AM) fungi. It is contained in the cell walls of hyphae and spores of AM fungi. Glomalin is exceptionally resistant to decomposition (its half‑life can be decades) and, according to estimates, is one of the main components of stable soil organic carbon in natural and agricultural ecosystems, especially those dominated by herbaceous plants (Weil & Brady, 2017).

5.3 Mechanisms of Microbial Organic Matter Stabilization

Why does microbial organic matter become stable? Unlike plant residues, which rapidly lose mass, microbial polymers enter the soil already in a "ready‑made" form for stabilization. The main mechanisms are:

• Chemical stability: Many microbial polymers (chitin, melanins, peptidoglycan) are themselves resistant to hydrolytic enzymes.
• Sorption onto minerals: Microbial exopolysaccharides and glycoproteins are actively sorbed onto the surfaces of clay minerals and iron and aluminum oxides, forming dense organic films. In this adsorbed state, they are virtually inaccessible to microbial attack (Weil & Brady, 2017).
• Physical protection within aggregates: Microbial cells and their fragments are often enclosed within soil microaggregates, where they are isolated from predators and enzymes (Foth, 1990).

5.4 Microbial Biomass as the "Engine" of Humification

We should not forget that microorganisms do not merely supply their own organic material. It is their enzymatic activity that initiates the entire process of plant residue transformation. Without microbial exoenzymes, the breakdown of lignin, cellulose, and other polymers would be impossible. The products of this breakdown (phenolic compounds, organic acids, amino acids) are the building material for the formation of specific humic acids through oxidative condensation reactions, also partly catalyzed by microbial enzymes (Kononova, 1966; Weil & Brady, 2017). Thus, microorganisms act as the organizer and main agent of the entire humus formation process.

Summarizing block 5:

1. Microbial biomass is not only a "processor" but also a crucial direct source of soil organic matter.

2. Microbial residues (necromass) and microbial metabolites contain a high proportion of chemically stable polymers (chitin, peptidoglycan, melanins, glomalin, exopolysaccharides) that directly transfer into the stable humus pool.

3. The contribution of microbial organic matter to the formation of the long‑term soil carbon reserve is often predominant, especially in stable ecosystems with slow turnover.

Thus, the modern concept of humus formation is not merely a "lignin‑protein theory" but a complex, multi‑step scheme in which microbial synthesis and subsequent stabilization of microbial polymers occupy a central place. By managing the activity of soil microbiota (creating favorable conditions, adding organic matter with an optimal C:N ratio), we also manage the process of humus accumulation.

In the next section, we will move to examining the overall structure of the soil organic matter pool—breaking down into which components (fractions) it is divided according to availability and stability.

6.1 Living Organic Matter (Biomass)

This includes living organisms inhabiting the soil: plant roots, microorganisms (bacteria, fungi, actinomycetes, algae), and soil animals (from protozoa to earthworms and rodents). Although living matter constitutes only a small fraction of total SOM (usually 1–5% of total organic carbon), its role is pivotal. It is the engine of all transformation processes. Living matter has the shortest turnover time (hours, days, years) and constantly replenishes the dead organic matter pool through its activity and mortality (Weil & Brady, 2017; White, 2006).

6.2 Fresh Plant and Animal Residues (Detritus)

This component includes undecomposed or slightly decomposed plant and animal tissues: leaf litter, stubble, dead roots, animal carcasses, and excrement. These residues still retain anatomical structure and are easily identifiable macroscopically. This pool is the primary source of organic matter for all subsequent transformations. Its chemical composition we discussed in detail in block 4. Detritus decomposes rapidly, serving as "fast food" for microorganisms. The turnover time of this pool is from several weeks to several years (Eash et al., 2016; Foth, 1990; Weil & Brady, 2017).

In soil science, this fraction is often called Particulate Organic Matter (POM) or the light fraction. It is separated from the mineral part of the soil using sieves (size > 53 μm) or flotation in heavy liquids (Eash et al., 2016; Weil & Brady, 2017). POM content is one of the key indicators of soil quality, as it is easily mobilized and serves as a rapid source of nitrogen and other nutrients.

6.3 Partially Decomposed (Transitional) Organic Matter

This is an intermediate stage. It includes already strongly comminuted, partially humified plant and microbial residues that have lost anatomical structure but have not yet turned into stable humus. This is a complex mixture of degradation products of polysaccharides, lignin, and microbial metabolites. This pool is often considered as a "slow" turnover fund. The turnover period here is no longer years but decades. This pool, in particular, includes organic matter enclosed within soil aggregates and protected from direct contact with microorganisms (physical protection) (Weil & Brady, 2017; Foth, 1990). It is from this pool that mineral nitrogen reserves are primarily replenished during mineralization.

6.4 Stabilized Organic Matter (Humus)

This is the most chemically and physically protected, long‑lived SOM fraction. It represents a complex, dark‑colored, amorphous mixture of high‑molecular‑weight organic compounds that can no longer be identified as original plant or microbial polymers. This is classical humus in the narrow sense. This pool constitutes 60–90% of all soil organic carbon (Weil & Brady, 2017; Foth, 1990).

Key characteristics of the stabilized pool:

• High resistance to decomposition: turnover time—hundreds and thousands of years. Carbon entering this pool may be "removed" from active turnover for a long time. The radiocarbon age of humic acids in some soils reaches 2000–4000 years (Weil & Brady, 2017).
• Complex chemical composition: contains humic acids, fulvic acids, and humin—specific high‑molecular‑weight acids containing aromatic rings, carboxyl and phenolic groups, and much nitrogen (Kononova, 1966). These compounds are formed through polymerization and condensation of degradation products of lignin, tannins, and microbial polymers. However, the modern concept (Weil & Brady, 2017) increasingly shifts emphasis from "chemical condensation" to microbial synthesis, emphasizing that a significant portion of stable humus is, in essence, transformed and accumulated microbial residues.
• Close association with the mineral fraction: stable humus almost always forms organo‑mineral complexes with clay minerals, iron and aluminum oxides. This sorption and aggregation are the main mechanisms of its long‑term protection from mineralization (Weil & Brady, 2017; White, 2006).

6.5 Functional Pools by Turnover Time

In soil carbon cycle models (e.g., in the RothC model), three functional pools are often distinguished (Weil & Brady, 2017; White, 2006):

• Active pool: turnover time—1–2 years. Includes living biomass, simple sugars, easily degradable proteins, and a small but most active part of POM. Provides rapid cycling of energy and mineral nitrogen.
• Slow pool: turnover time—10–50 years. Includes transformed POM, organic matter protected within aggregates, and part of microbial metabolites. It is the main reserve for maintaining fertility in the long term.
• Passive (inert) pool: turnover time—>100 years (sometimes >1000 years). Represented by stable humus firmly bound to clay minerals, as well as resistant forms of organic carbon such as charcoal (black carbon). This pool is important for long‑term carbon sequestration, but its direct role in plant nutrition is minimal.

Summarizing block 6:

Soil organic matter is a continuous continuum, extending from fresh plant residues to highly stabilized humus. Each component of this continuum performs its specific function: living matter—energy; detritus—nutrient source; transitional matter—reserve; stable humus—the foundation of the soil framework, providing cation exchange capacity, water‑holding capacity, structure, and long‑term fertility. Understanding this continuum is crucial for informed management of soil fertility: by adding specific types of organic matter, we can influence the balance between active and stable pools, regulating both current productivity and long‑term soil health.

In the concluding part of our introductory lecture, we will consider the spatial heterogeneity of organic matter—how it is distributed along the soil profile, within aggregates, and in the root zone (rhizosphere).

7.1 Vertical Distribution Along the Soil Profile

The most obvious and well‑studied pattern is the sharp decrease in organic carbon content with depth (Weil & Brady, 2017; Eash et al., 2016; Foth, 1990). In most mineral soils, the bulk of organic matter (80–90%) is concentrated in the upper 0–30 cm layer, the so‑called humus‑accumulative horizon (A‑horizon, or plow layer). With depth, organic carbon concentration declines exponentially.

Why does this happen?

1. Source: The main source of organic matter is plants. Aboveground litter falls onto the surface, and the bulk of roots is concentrated in the upper soil layers (Weil & Brady, 2017; White, 2006). Although deep roots can penetrate several meters, their biomass per unit soil volume rapidly decreases with depth.

2. Microbial activity: The upper horizons are better aerated, richer in nutrients, and have a more favorable thermal regime, providing maximum abundance and activity of microorganisms processing organic matter. With depth, microbial activity declines, slowing both decomposition and new humus formation (Eash et al., 2016).

3. Intensity of mixing: The upper layers are actively mixed by soil fauna (worms, ants, rodents) and, in agroecosystems, also by the plow. This promotes uniform distribution of organic matter in the plow layer. At depth, mixing processes weaken considerably.

However, it is important to note that in some soil types (e.g., Chernozems), organic matter is distributed relatively uniformly along the profile, forming a thick humus horizon. This is due to the high belowground contribution of herbaceous plant roots and their deep penetration (Foth, 1990; Weil & Brady, 2017). In Podzolic soils, by contrast, there is a distinct eluvial horizon (A2), depleted of organic matter, and an illuvial (B) horizon, in which humus‑iron compounds leached from above accumulate (Foth, 1990).

7.2 Horizontal Heterogeneity: "Hot Spots" and Patchiness

Even within a single horizon, organic matter is not homogeneously distributed but forms local zones of elevated concentration—"hot spots" (Weil & Brady, 2017; White, 2006).

• Rhizosphere—the main "hot spot": This is the zone of soil under the direct influence of living plant roots (usually within a radius of 1–2 mm from the root surface). Here, organic carbon concentration can be 2–10 times higher than in the bulk soil (Eash et al., 2016; Weil & Brady, 2017). This is due to root exudates, mucilages, and dead root cap cells. The rhizosphere is the epicenter of microbial activity, where intensive carbon and nitrogen exchange occurs between the plant, microorganisms, and soil.
• Microbial colonies and fecal pellets: Microorganisms are not uniformly dispersed but form colonies on plant residue surfaces, in pores, and on mineral particles. Fecal excretions of soil animals (worms, springtails, mites) also represent local accumulations of highly active organic matter enriched with microbial biomass (Eash et al., 2016; Weil & Brady, 2017).
• Root and plant tissue residues: Even within the same horizon, large fragments of roots or leaves create "islands" of fresh organic matter around which local decomposition and humification processes unfold (White, 2006).

Thus, at the microscale, soil is a mosaic of zones with high labile organic content and zones dominated by old, stabilized humus.

7.3 Microdistribution: Aggregate Level

The most important and subtle level of spatial organization is the distribution of organic matter within soil aggregates and pores (Weil & Brady, 2017; Foth, 1990).

As we noted in block 3, soil has an aggregate structure. The mechanisms of organic matter stabilization are directly related to its position within the aggregate.

• External surface of the aggregate: Here, organic matter is in direct contact with soil solution and microorganisms. It is constantly involved in active cycling and mineralization.
• Internal part (core) of the aggregate: Once inside the aggregate core, organic matter becomes physically protected from contact with microorganisms and enzymes (Foth, 1990; Weil & Brady, 2017). Diffusion of oxygen and dissolved substrates into the aggregate center is hindered, which can create local anaerobic conditions and slow mineralization. It is within aggregates that a significant portion of stable organic matter, including microbial origin (glomalin, peptidoglycan, chitin), is "preserved."
• Nano‑ and micropores: Organic matter particles entering pores smaller than 2–3 micrometers become inaccessible to bacteria due to their size. This is another mechanism of steric protection, explaining why clay soils (with more micropores) generally contain more stable organic carbon than sandy soils (Weil & Brady, 2017).

Brief summary of Lecture 1:

Thus, we have completed our first, introductory lecture on the sources and composition of soil organic matter. Let us once again briefly formulate the key conclusions:

1. The primary source of soil organic matter is plants, which create organic mass through photosynthesis (net primary production).

2. The most significant and qualitatively most important source for forming stable humus is plant roots and their exudates (rhizodeposition). The belowground contribution determines the high humus content in Chernozems and meadow soils.

3. The chemical composition of plant residues determines their fate. Lignin and other aromatic polymers are the main precursors of stable humic substances. The carbon‑to‑nitrogen (C:N) ratio regulates the decomposition rate.

4. Microorganisms are not only decomposers but also active builders of humus. Their cell walls (chitin, peptidoglycan, melanins) and metabolites (exopolysaccharides, glomalin) are direct sources of stable organic matter.

5. Soil organic matter is not a homogeneous mass but a continuum of labile (living matter, detritus, transitional forms) and stable (humic substances, organo‑mineral complexes) components. The stable pool forms the basis of long‑term fertility.

6. Spatial heterogeneity is a fundamental property of organic matter, manifested at all levels: from a sharp decrease in content with depth to microdistribution within aggregates and the rhizosphere. This mosaicity creates conditions for diversity of microbiological processes and stabilization mechanisms.

This lecture lays the foundation for understanding subsequent topics in the module: decomposition dynamics, the role of soil organisms, factors affecting humus content and quality, and practical issues of managing soil fertility through regulating organic matter.