Soil biota
Soil biota — living organisms inhabiting the soil and differing in ecological functions and taxonomic position.
Soil biota includes:
- microorganisms — bacteria, algae, fungi, actinomycetes;
- protozoa — ciliates, flagellates, rhizopods;
- worms;
- arthropods (insects);
- mollusks, etc.
In well-cultivated soils, the amount of soil biota can reach several billion per 1 g of soil, or 10 t/ha.
Importance of soil biota
Soil biota participates in:
- formation of fertility of soil;
- mineralization and humification of organic matter;
- conversion of bound forms of nutrients into mobile forms;
- nitrogen fixation;
- movement of organic and mineral substances along the soil profile;
- formation of optimal soil structure;
- processes of formation and transformation of phytoactive substances;
- maintenance of an optimal nutrient regime of soils.
In cultivated soil, soil biota, through partial binding of mineral elements and fertilizers, allows nutrients to be retained in the arable layer, favoring the creation of an optimal nutrient regime and the structuring of soil.
Soil microflora
The first living microorganisms that arose on Earth in ancient times initiated the soil-forming process. The first microbes obtained energy from the decomposition of chemical compounds and released strong acids during their vital activity, which destroyed and ground up the parent rock, creating a new type of structure. Over time, the weathered rock became enriched with organic matter.
In the arable soil layer, the mass of bacteria is from 3 to 7–8 t/ha.
Rhizosphere microorganisms process toxic substances released by plants during their vital activity. Beneficial microorganisms convert sparingly soluble compounds into forms available for plant nutrition. Nitrogen fixers, both those living on the roots of legumes and free-living ones, play a major role in plant nutrition.
According to their mode of nutrition, microorganisms are divided into autotrophic and heterotrophic. Autotrophic bacteria use photosynthesis or the chemical energy of oxidation of mineral substances—chemosynthesis—for carbon uptake. Green and purple sulfur bacteria, nitrifying bacteria, and iron bacteria have the capacity for photosynthesis. Heterotrophic bacteria take up carbon from already formed organic compounds. Most soil bacteria and actinomycetes, and almost all fungi and protozoa, are heterotrophs.
The process of oxidation of hydrogen sulfide, elemental sulfur, and sulfur-containing compounds to sulfuric acid is called sulfofication. It is carried out by sulfur bacteria and thionic bacteria. Sulfuric acid promotes the conversion of sparingly soluble mineral salts into soluble forms, or after neutralization is used as sulfates in the sulfur nutrition of plants.
Iron bacteria participate in the transformation of iron and manganese salts.
Organic nitrogen is, as a rule, unavailable to plants. In soil, mineralization of organic nitrogen (ammonification) occurs. Heterotrophic bacteria, actinomycetes, and fungi participate in this process.
Ammonia released as a result of the biochemical reactions of ammonification is partly adsorbed on clay-humus particles or neutralized by soil acidity, and partly used by soil biota. Part of the ammonia can be oxidized by autotrophs to nitrites, nitrates, and molecular nitrogen.
Autotrophs use mineral nitrogenous compounds, such as ammonium salts and nitrates. There are specific microorganisms capable of using nutrients from humus.
The optimal soil moisture for the development of microorganisms is 50–60% of maximum water-holding capacity. Anaerobic microorganisms develop at moisture from 80 to 100%.
Aerobic and anaerobic microorganisms coexist simultaneously in soil. Such coexistence is possible when aerobic bacteria that intensively absorb oxygen are present on the surface of a soil particle. At the same time, oxygen deficiency develops in the center of the particle, and conditions become anaerobic.
Various species of microorganisms are capable of destroying cellulose and pectin substances, due to which decomposition of plant residues occurs. Under the influence of ureobacteria, urea is transformed into ammonium carbonate. Ureobacteria are aerobic microorganisms developing at pH 7–8; urea serves as a nitrogen source for them, and organic acids and carbohydrates as a carbon source. Diverse soil microorganisms also break down hemicellulose, starch, and lignin.
In close proximity to the roots of higher plants, a zone favorable for the development of soil microorganisms forms—the rhizosphere. Root exudates containing various organic substances and dead plant tissues become a nutrient medium for rhizosphere microorganisms.
The number of bacteria of the genus Clostridium in 1 g of fallow soil is 69.7 thousand, whereas in the rhizosphere it is 10.7 million (V. T. Yemtsev). According to calculations, the mass of bacteria in the rhizosphere of alfalfa is 5 t/ha, whereas outside the rhizosphere it is 2.25 t/ha. The rhizosphere microflora of legumes is richer than that of cereals.
The predominant group of microflora inhabiting the rhizosphere is non-spore-forming bacteria: Azotobacter, nodule bacteria, photosynthetic bacteria, butyric acid bacteria, mycobacteria, and algae. More intensive development of algae is also observed in the rhizosphere. Ammonifiers, denitrifiers, and nitrifiers also develop in the rhizosphere.
Under certain conditions, rhizosphere microflora can play both positive and negative roles. Microorganisms, like plants, use mineral substances for nutrition. However, the scale of this competition is usually insignificant. Rhizosphere microorganisms act as biological “fixers” of nutrients, preventing leaching and removal from the root-inhabited soil layer.
Table. Content of plant-available mineral compounds of phosphorus and potassium in the root zone and in soil, mg per 100 g dry soil.
| Crop | P2O5 | K2O | ||
|---|---|---|---|---|
| outside roots | in rhizosphere | outside roots | in rhizosphere | |
| Barley | 18.6 | 22.5 | 9.3 | 12.8 |
| Winter wheat | 37.2 | 43.2 | 6.6 | 27.9 |
| Oats | 31.5 | 34.5 | 10.7 | 34.4 |
| Clover | 16.2 | 21.9 | 6.4 | 8.2 |
The situation changes substantially when substances with a wide C:N ratio are applied, for example, strawy manure or straw. In such cases, the number of microorganisms increases sharply, and consumption of nitrogen, phosphorus, and other macro- and micronutrients becomes significant. As a result, a deficiency of nutrients for plants may be created. This explains the observed decrease in crop yield in the first year after application of straw.
Biological immobilization of nutrients by microorganisms is short-lived. After death, microbial cells mineralize, and nutrients are released for subsequent use by plants, mostly in the following year.
Microorganisms release enzymes, growth stimulators, and vitamins that are absorbed by plant roots and promote their growth. In addition, they release antibiotics that suppress the development of phytopathogenic flora.
Each soil type that has arisen in the course of evolution is characterized by a certain microflora.
Table. Number of microorganisms in different soil types.
| Soils | Number of microorganisms, million per 1 g soil |
|---|---|
| Sod-podzolic, virgin | 600–1000 |
| Sod-podzolic: cultivated | 1000–2000 |
| Chernozem, virgin | 2000–2500 |
| Chernozem, cultivated | 2500–3000 |
Soil biological activity
Soil biological activity — an indicator characterizing the number of organisms living in the soil and quantitatively assessing the results of their vital activity.
Under favorable conditions, soil biota is in constant and close interaction among its components, establishing a certain equilibrium. Requirements for the living conditions of individual representatives may be simple or complex, and they may be in symbiotic or antibiotic relationships with other representatives. The latter is often achieved through the release of phytoactive substances and is used in agriculture to regulate soil biota by suppressing phytopathogenic microflora in order to create a favorable phytosanitary state of soil.
Quantitative assessment of soil biota, as a rule, is performed by counting the total number of organisms. Due to imperfect methods and an insufficient number of determinations over time, the results provide only an approximate characterization of soil biological activity. If necessary, a quantitative assessment of individual species and groups of organisms is carried out. Another method for quantitative determination of soil biological activity is the result of the vital activity of soil organisms, which is determined through the amount of absorbed oxygen and released carbon dioxide, quantitative decomposition of cellulose, soil enzymes, ammonium and nitrate nitrogen, and other compounds. In this case, the assessment characterizes the activity of individual species and groups of bacteria and only indirectly allows one to judge the activity of soil biota as a whole.
Given the diversity of soil organisms and their properties, an overall assessment of soil biological activity is difficult. Attempts have been made to use so-called biological scores, which are an averaging of individual indicators of the state of soil biological processes.
High biological activity is a factor in increasing soil fertility and its phytosanitary state, which is achieved by creating optimal conditions for the vital activity of soil biota.
List of sources
- Bazdyrev G. I., Loshakov V. G., Puponin A. I., et al. Agriculture. Textbook for universities. Moscow: Kolos Publishing House. 2000.
- Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. Agrochemistry. / Edited by B. A. Yagodin. Moscow: Kolos. 2002.
- Mineev V. G., Sychev V. G., Gamzikov G. P., et al. Agrochemistry. Textbook. / Edited by Mineev V. G. Moscow: Publishing House of the D. N. Pryanishnikov All-Russian Research Institute of Agrochemistry. 2017.
