Biological Organization of Soil

4.1. Decomposition of Organic Matter (Destruction)

This is perhaps the most well-known function of biota. A huge amount of organic material constantly enters the soil: leaf and needle litter, dead roots, animal carcasses, excrement. If this material did not decompose, it would accumulate, and biogeochemical cycles would halt (Foth, 1990). Biota solves this problem.

The decomposition process proceeds in stages and involves different groups of organisms:

1. Physical destruction (macro- and mesofauna): Earthworms, centipedes, woodlice, insect larvae gnaw, chew, and shred plant litter. This increases the surface area accessible to microorganisms and breaks down the protective layers of tissues (Weil & Brady, 2017).

2. Biochemical destruction (microorganisms): Bacteria and fungi release enzymes (cellulases, ligninases, proteases, amylases) into the environment, which hydrolyze complex polymers (cellulose, lignin, proteins, starch) into simple monomers (glucose, amino acids, etc.). These monomers are then absorbed by microorganisms and used as a source of energy and carbon (Foth, 1990; Mukha et al., 2003).

3. Mineralization: The final stage of decomposition, where organic carbon is fully oxidized to CO₂ (so-called "soil respiration"), and the nitrogen, phosphorus, sulfur, and other elements bound in organic matter are released in mineral form (e.g., \(\mathrm{NH_4^+}\), \(\mathrm{PO_4^{3-}}\)) and become available to plants (Foth, 1990; Weil & Brady, 2017). This process is called mineralization.

It is important to understand that mineralization is not just "destruction." Thanks to it, elements removed from the biological cycle are returned to the soil solution. In essence, decomposition is the "re-melting" of dead organic material into living microbial biomass and plant-available nutrients.

4.2. Transformation of Matter and Element Cycling

Mineralization is only part of a global process — the biological cycle of matter (Mukha et al., 2003). Biota is the main driver of this cycle.

Carbon (C): Photosynthetic plants (and phototrophic microorganisms) fix atmospheric CO₂, converting it into organic matter. Then heterotrophs (animals, fungi, bacteria) return carbon to the atmosphere through respiration. Soil acts as a giant reservoir of organic carbon, and the balance between its accumulation and mineralization directly affects climate (Weil & Brady, 2017).

Nitrogen (N): Biota controls the entire nitrogen cycle. This includes:

• Fixation of atmospheric nitrogen (\(\mathrm{N_2}\)) into a plant-available form (ammonium) — a process carried out by free-living (Azotobacter) and symbiotic (rhizobia, Frankia) bacteria, as well as cyanobacteria (Foth, 1990; Weil & Brady, 2017).
• Ammonification — the conversion of organic nitrogen into ammonia (\(\mathrm{NH_3}\)).
• Nitrification — the two-stage oxidation of ammonium to nitrate (\(\mathrm{NO_3^-}\)) by nitrifying bacteria (Nitrosomonas, Nitrobacter) (Foth, 1990).
• Denitrification — the reduction of nitrates to gaseous nitrogen (\(\mathrm{N_2}\) or \(\mathrm{N_2O}\)), which closes the cycle, returning nitrogen to the atmosphere (Huang et al., 2012).

Phosphorus (P) and Sulfur (S): Although their cycles are less gaseous, they are also entirely dependent on biota. Microorganisms release organic acids and phosphatases, which convert unavailable phosphorus from minerals and organic compounds into a soluble form accessible to plants (Huang et al., 2012). Similarly, sulfur is mineralized from organic compounds (e.g., sulfate esters) by enzymes released by bacteria and fungi.

Thus, biota does not just use elements; it organizes their cyclic flow, turning soil into a self-sustaining system.

4.3. Formation and Stabilization of Soil Structure

We have touched on this before, but it is important to emphasize: soil structure is a product of biological activity. Without biota, soil would be a structureless mass.

• Aggregation: Plant roots and fungal hyphae act as a natural reinforcing framework that binds mineral particles into macroaggregates (> 250 µm) (Weil & Brady, 2017). Polysaccharides released by bacteria and roots (e.g., mucigel) act as glue, binding particles into microaggregates (< 250 µm) (Weil & Brady, 2017).
• Creation of pore space (biopores): Plant roots and burrowing animals (earthworms, ants, rodents) create channels and tunnels in the soil. These biopores are crucial pathways for water infiltration, aeration, and root growth (Foth, 1990). Earthworm burrows, for example, can penetrate several meters deep, significantly improving the water permeability of dense soils (Weil & Brady, 2017).
• Stabilization of aggregates: Metabolic products of microorganisms and decomposition products (especially humic substances) chemically bind mineral particles, making aggregates resistant to water destruction (water stability). This is a key property protecting soil from erosion (Scheffer & Schachtschabel, 2018).

As a result of these processes, that loose, crumbly, porous structure, which is the basis of fertility, is formed.

4.4. Stabilization of Organic Matter and Humus Formation

This is perhaps the most intriguing process, long remaining a mystery. Decomposition leads to mineralization. But why does organic matter (humus) accumulate in soil if microorganisms actively decompose it?

The fact is that biota not only destroys organic matter but also simultaneously stabilizes a part of it. This process includes:

1. Transformation: Microorganisms partially use the carbon from plant residues to build their own biomass. After their death, these microbial cells and their metabolites (polysaccharides, enzymes, breakdown products) become part of the soil organic matter.

2. Biochemical modification: Fungi and actinomycetes actively attack lignin — a complex polymer that is difficult to decompose. During its enzymatic modification, stable aromatic compounds are formed, which can become part of humic molecules (Scheffer & Schachtschabel, 2018).

3. Physical protection: Organic matter, especially of microbial origin, actively binds to mineral particles (clay and iron/aluminum oxides), forming stable organo-mineral complexes. These complexes physically protect organic matter from enzymatic attack, making it "preserved" for a long time (Scheffer & Schachtschabel, 2018; Huang et al., 2012).

4. Aggregate protection: Organic matter trapped inside soil aggregates becomes isolated from the microbial community. This is another mechanism of its stabilization.

Thus, humus is not a random product but the result of a purposeful biological process in which part of the organic material is converted into a stable form. This is a strategy of soil as a system: not all energy wealth should be spent immediately; part must be kept "in reserve" to ensure long-term stability and buffering capacity (Scheffer & Schachtschabel, 2018).

So, we see that biota performs four global tasks in soil: it decomposes dead organic matter, transforms and cycles chemical elements, creates and stabilizes the physical structure of soil, and forms and preserves a reserve of organic matter — humus.

This complex of interconnected processes constitutes the biological organization of soil. But now a new question arises: is biota simply an "executor" of these processes, or does it play a more complex role — the role of a regulator of the entire soil system? It is to this question that we turn in the next section.

5.1. Biotic Regulation through Feedback

Biota is not an isolated community. It is a network of direct and feedback loops that permeates the entire soil system. A classic example is the regulation of decomposition rate.

• Direct link: When a large amount of fresh organic material enters the soil (e.g., after harvest), the growth of saprotrophic microorganisms is activated (Foth, 1990). This is a direct response to changing conditions.
• Feedback: Active microbial growth leads to the uptake of available nitrogen, and its concentration in the soil solution drops. In response, microorganisms begin to more actively release enzymes for decomposing more complex nitrogen-containing compounds and suppress the activity of competitors. Thus, the growth of the microbial community itself becomes a factor altering the chemical environment.
• Closing the cycle: When the organic material is depleted, microbial activity declines. Nutrients released from dead microbial cells become available to plants again. This process is regulated by the biota itself — the intensity of decomposition depends not only on temperature or moisture but also on the density and activity of the microbial community itself.

It is important to understand that these feedbacks operate not only at the microbial level but also at the level of the entire biota. For example, earthworm activity, which improves aeration and structure, depends itself on the amount of available organic matter. More organic matter — more worms — better structure — better aeration — more active decomposition — more organic matter becomes available. Conversely, a reduction in organic matter leads to a decrease in the worm population, which worsens structure and slows processes.

5.2. Regulation through the Creation of "Hot Spots"

Soil is a heterogeneous environment. Most of the biota is concentrated not uniformly but in specific local zones, which are called "hot spots" of biological activity (Weil & Brady, 2017). These zones are centers of regulation for the surrounding soil space.

• Rhizosphere: The root zone (rhizosphere) is the most powerful regulatory center. Roots release a huge amount of organic compounds — exudates. These exudates (sugars, amino acids, organic acids) serve as food for microorganisms, and in the rhizosphere, their abundance is 10–100 times higher than in the bulk soil (Weil & Brady, 2017). This concentration of biota, in turn, creates a zone of intense decomposition, mineralization, and transformation of substances, which directly affects plant nutrition. In addition, roots and associated bacteria release regulatory molecules (signals) that can suppress pathogens or, conversely, stimulate the growth of beneficial symbionts (Weil & Brady, 2017).
• Drilosphere: The zone adjacent to earthworm burrows is also a "hot spot." The burrow walls are enriched with mucus secretions from worms and their metabolic products, creating favorable conditions for microorganisms and roots. This is a zone of enhanced matter cycling and improved aeration.
• Detritusphere: Zones of decomposition of large organic residues (e.g., straw, manure). Here, too, local conditions with high biota concentration and intensive transformation processes are created.

Thus, biota regulates soil processes not only globally but also locally, creating functional "nodes" in space that determine the direction and intensity of reactions in the surrounding soil mass.

5.3. Regulation of the Chemical Composition of the Environment

Biota can purposefully alter the pH and redox potential (Eh) of the soil solution, creating conditions favorable for certain processes.

• pH regulation: Roots and microorganisms release organic acids, which acidify the rhizosphere. This promotes the dissolution of sparingly soluble phosphates and micronutrients (Huang et al., 2012). On the other hand, during protein decomposition, ammonia (\(\mathrm{NH_3}\)) is released, which, dissolving in water, alkalizes the environment. Thus, biota actively modulates acidity, affecting the availability of elements.
• Regulation of redox potential (Eh): Under aerobic conditions, microbial respiration consumes oxygen, reducing its concentration in local zones. In the absence of oxygen (e.g., in waterlogged soils), anaerobic processes are activated, using other electron acceptors: nitrates (\(\mathrm{NO_3^-}\)), iron (\(\mathrm{Fe^{3+}}\)) and manganese (\(\mathrm{Mn^{4+}}\)) oxides, sulfates (\(\mathrm{SO_4^{2-}}\)), and even carbon dioxide (Huang et al., 2012). This sequential use of alternative acceptors is a result of the adaptation of microbial communities to changing conditions, and this regulation determines which chemical reactions will dominate in the soil at any given moment.

Thus, biota acts as a biochemical regulator, using its enzymatic systems to control the direction and rate of the most important soil reactions.

5.4. Regulation through Symbiotic and Antagonistic Interactions

Interactions between organisms within the biota are a powerful regulatory mechanism.

• Symbiosis: Symbiotic relationships permeate the entire soil system. The most famous example is mycorrhiza (symbiosis between fungi and higher plant roots) (Weil & Brady, 2017). Fungi receive carbohydrates from plants, while plants significantly increase their absorption area for water and phosphorus. Mycorrhizal networks, which can connect the roots of different plants, allow the redistribution of resources among them (e.g., carbon from one plant to another experiencing deficiency) (Weil & Brady, 2017). This is an example of regulation at the level of the entire plant community.
• Antagonism: Competition for resources and space is also a form of regulation. Many bacteria and fungi release antibiotics to suppress competitors (Weil & Brady, 2017). A well-known example is disease suppression: soil in which a diverse microbial community actively develops can suppress the development of pathogens (Foth, 1990). This phenomenon is called "disease suppressiveness," and it arises precisely from the regulatory role of biota, which creates an environment unfavorable to pathogens.

So, summarizing this section, we can say that biota is not just the "labor force" of soil, but its "brain" and "nervous system." It can perceive changes in the environment (food availability, moisture, temperature), process this information through complex biotic interactions, and produce an adequate response — whether it be activation of decomposition, alteration of soil pH, or initiation of symbiotic interaction. Soil as a system possesses homeostasis, and the main guarantor of this homeostasis is the biota.

Now that we have understood the fundamental role of biota as regulator and organizer, we must ask the final question: why does soil science study not organisms but their functions? We will answer this in the concluding section.

6.1. Different Sciences — Different Questions

Let us draw a clear boundary to avoid confusion and duplication with other disciplines that you will study within your educational project.

• Microbiology (and related disciplines): This science asks: "How is a bacterial cell structured?", "What is its genome?", "What enzymes does it synthesize?", "How does it reproduce?", "What are its cultivation conditions?", "How is this species identified?". The microbiologist studies the microorganism itself — its structure, physiology, taxonomy, and evolution. This is a discipline centered on the organism.
• Soil Science: This science asks fundamentally different questions: "What is the rate of organic matter mineralization in this soil?", "Does drought affect the intensity of the nitrogen cycle?", "How does soil structure depend on root and fungal activity?", "How does biota affect greenhouse gas emissions?". Soil science does not ask "who lives in the soil?" but "what do living organisms do to and for the soil?".

This is the key difference. Soil science views biota functionally. For the soil scientist, it is not so important which particular bacterium or fungus performs nitrification. What is important is that the process of nitrification proceeds at a certain rate, under certain conditions, and that it is critical for supplying plants with nitrogen.

6.2. Functional Redundancy and System Stability

One of the main arguments for the functional approach is the phenomenon of functional redundancy (Weil & Brady, 2017). It means that many essential soil functions can be performed not by one, but by several different species of organisms.

• Cellulose decomposition: This function is performed by many species of fungi, many species of bacteria, and even actinomycetes. If conditions change and one species disappears, its "niche" will be occupied by others, and the decomposition process will hardly suffer.
• Nitrogen fixation: It can be carried out by free-living bacteria (Azotobacter), symbiotic bacteria (rhizobia, Frankia), as well as some cyanobacteria and archaea (Weil & Brady, 2017; Huang et al., 2012). The loss of one of these groups will not halt nitrogen fixation.

It is this redundancy that provides soil with remarkable stability and buffering capacity. Soil as a system can "survive" the disappearance of individual species because their functions will be performed by other organisms. For the soil scientist, this means that it is much more important to monitor the state of functions (e.g., mineralization rate) than to try to identify all the species that perform that function.

6.3. Soil Science and the Systems Approach

Soil is an open, dynamic, hierarchically organized system. In such a system, the whole (soil functioning) is always greater than the sum of its parts (the set of species). Studying the whole through an endless enumeration of its parts is a dead end. That is why soil science uses a systems approach, where the emphasis is on interactions, fluxes of matter and energy, and feedbacks, rather than on taxonomic identification.

This does not mean that soil scientists are not interested in knowing who lives in the soil. Modern molecular methods (metagenomics, metatranscriptomics) provide us with unprecedented information about species composition. But this information is valuable to the soil scientist insofar as it helps explain why a particular function is performed in a particular way, and how the organism community responds to environmental changes (Huang et al., 2012; Weil & Brady, 2017). Species identification becomes not an end in itself but a tool for understanding the functioning of the system.

6.4. Practical Significance of the Functional Approach

Ultimately, the soil science approach is dictated by practical tasks. Agriculture, forestry, soil conservation, and environmental monitoring need not a list of species inhabiting a particular soil, but answers to specific questions:

• "Is straw decomposing fast enough in this soil?"
• "What level of biological activity ensures a stable yield?"
• "Will the application of nitrogen fertilizers accelerate denitrification and nitrogen losses to the atmosphere?"
• "Is this soil capable of self-recovery after pollution?"

Answers to these questions are provided precisely by the functional analysis of biota — measurement of enzyme activity, respiration rate, intensity of element cycling, and structural stability. We assess not how many species live in the soil, but how well the soil performs its ecosystem functions.

Summary of Section 6:

Thus, soil science, while studying biota, occupies a completely distinct position. It does not replace microbiology, zoology, or botany. It uses their data but shifts the focus of attention from the organism to the function. Soil science is interested not in taxonomic diversity per se, but in functional diversity — the set of processes that ensure the life and fertility of soil. By studying "what biota does to the soil," we gain the key to managing soil fertility, understanding its stability, and predicting its response to anthropogenic and natural changes. This is the essence of the biological organization of soil from the standpoint of systems soil science.