Small (biological) cycle. Cycle of substances in nature

The cycle of substances in nature is a set of repeating processes of transformation or movement of substances, which has a more or less pronounced cyclic nature.

Let's start with the water cycle. This is a complex geophysical process, the main links of which are: evaporation of water, transfer of its vapor by air currents, formation of clouds and precipitation, surface and underground flow of water into the ocean.

The biological (or biotic) cycle is built into this geological water cycle. Plants absorb water from the soil and then evaporate it (see Transpiration). Part of the water absorbed by plants is used to build organic substances, which, when oxidized, again form water (see Biological oxidation). Any living organism absorbs and releases water, using the energy obtained by green plants from sunlight (see Photosynthesis). Thus, it is the energy of the Sun emitted in the form of light that “turns the wheel” of the water cycle, and not only water, but also all other substances.

Consider the nitrogen cycle. The Earth's nitrogen is found primarily in its atmosphere. Some microorganisms, both free-living (for example, cyanobacteria, Azotobacter) and symbiotic (for example, legume nodule bacteria), are capable of absorbing nitrogen from the air and fixing it in their body in the form of nitrogen-containing organic compounds, converting molecular nitrogen into ammonia, which is well absorbed by plants . From plants, nitrogen in the composition of organic compounds enters the organisms of animals and other heterotrophs.

At the final links of food chains, organic substances that enter the soil during the decomposition of corpses and with the excretions of organisms serve as food for bacteria and fungi. Certain groups of soil microorganisms (destructors) decompose organic substances into inorganic substances, which can be absorbed by green plants. So, organic compounds nitrogen is converted into ammonia in the soil, which can again be absorbed by plants. Soil chemosynthetic bacteria (see Chemosynthesis) oxidize ammonia to nitrites and nitrates, which enter the plants with water and are reduced there to ammonia. There are also microorganisms in the soil that convert ammonia into molecular nitrogen, which enters the atmosphere.

In places where there is little precipitation, nitrates formed from guano - the droppings of colonial birds feeding on fish living in the ocean - accumulate in the form of saltpeter deposits (for example, in Chile). People return it to the nitrogen cycle again, using saltpeter to fertilize fields.

Man is increasingly interfering with the cycle of substances. For example, hundreds of millions of tons of nitrogen fertilizers are synthesized, but in terms of intensity, industrial fixation of atmospheric nitrogen is inferior to biological one and is associated with environmental poisoning: excess nitrogen fertilizers are washed off by precipitation from fields into rivers. This is how they end up in water for human consumption. It turned out that nitrates are not harmless to humans - their excess contributes to the formation of malignant tumors. In addition, the synthesis of nitrogen fertilizers requires large amounts of energy. Therefore, scientists are intensively studying the mechanism of biological fixation of atmospheric nitrogen in order to develop more efficient ways to provide plants with nitrogen (see Nitrogen fixation).

The source of phosphorus in the biosphere is mainly apatite, found in many rocks. Organisms extract it from soils and aqueous solutions, including in numerous phosphorus-containing organic compounds. With the death of organisms, it returns to the soil and silt of the seas, where it can concentrate in the form of sediments (guano, deposits of fish bones, etc.). Since most soils contain insufficient amounts of phosphorus, the application of phosphorus fertilizers is extremely important for obtaining high crop yields.

You can also describe the cycle of many other elements. Each of them has its own characteristics, but it is important to emphasize that the energy for any cycle ultimately comes from the Sun.

The cycle of substances is complex, and an element “flows” from compound to compound not along one channel, but along several, which branch and merge again, and the cycles of various elements are interconnected.

The biological cycle is only a part of the geological one, but its speed is hundreds of thousands and millions of times greater, since all biological transformations are catalyzed by enzymes that are hundreds of thousands and millions of times more active than inorganic catalysts.

Another feature of the biological cycle is a very strong concentration of biologically important chemical elements, such as phosphorus, and sometimes even rare earths (for example, yttrium in horsetails).

The biological cycle is cyclical because food chains have a closed character. This ensured the long-term existence of life on Earth, since otherwise the richest reserves of any substance would quickly be exhausted.

Due to active human intervention in the processes occurring in nature, the problem of its protection has arisen (see Nature conservation).

A number of substances are lost as a result of geological and cosmic processes and leave the cycle. Thus, hydrogen, formed during the decomposition of water, evaporates from the Earth into outer space. Biogenic carbonates are deposited on the ocean floor, removing carbon from the cycle. And carbon and a number of other elements come to Earth from outer space with the solar wind and meteorites. When volcanoes erupt, they are thrown from the bowels of the earth to the surface. carbon dioxide, water and other compounds. Thus, the circulation of substances on Earth is associated with global geological, biological and astronomical processes, as well as with the conscious activity of humanity.

Mass transfer cycles of varying length in space and unequal duration in time form a dynamic system of the biosphere. V.I. Vernadsky believed that the history of most chemical elements, forming more than 99% of the mass of the biosphere, can only be understood taking into account circular migrations (cycles). At the same time, he emphasized that “these cycles are reversible only in the main part of the atoms, while some elements inevitably and constantly leave the cycle. This exit is natural, i.e. the circular process is not completely reversible.” Incomplete reversibility and imbalance of migration cycles allow certain concentrations of the migrating element, to which organisms can adapt, but at the same time, ensure the removal of excess amounts of the element from a given cycle.

That is, the integrity of the biosphere as a system is due to the continuous exchange of matter between its components, in which processes associated with the synthesis and decomposition of organic matter play a key role. They are realized both in the course of metabolism between living organisms and environment, and in the processes of mineralization of organic matter after the death of the organism as a whole or the death of its individual organs. In addition, non-biogenic in nature processes of exchange of matter between various components geographic envelope.

Abiogenic and biological cycle They are closely intertwined, forming a planetary geochemical cycle and a system of local cycles of matter. Thus, over billions of years of the biological history of our planet, a great biogeochemical cycle and differentiation of chemical elements in nature has developed, which is the basis for the normal functioning of the biosphere. That is, in the conditions of a developed biosphere, the cycle of substances is directed by the combined action of biological, geological and geochemical factors. The relationship between them may be different, but the action must be joint! It is in this sense that the terms biogeochemical circulation of substances and biogeochemical cycles are used.

The biological cycle is not a fully compensated closed cycle.

The biological, biochemical and geochemical significance of the processes carried out in the biological cycle of substances was first shown by V.V. Dokuchaev. It was further revealed in the works of V. I. Vernadsky, B. B. Polynov, D. N. Pryanishnikov, V. N. Sukachev, L. E. Rodin, N. I. Bazilevich, V. A. Kovda and other researchers .

Before we begin to study the natural biological cycles of chemical elements, it is necessary to become familiar with the most commonly used terms.

Biomass - the mass of living matter accumulated to at this moment time.

Phytomass (or plant biomass0 - the mass of living and dead organisms of plant communities that have retained their anatomical structure at a given moment in any specific area or on the planet as a whole.

Structure of phytomass - the ratio of underground and aboveground parts of plants, as well as annual and perennial, photosynthetic and non-photosynthetic parts of plants.

Rags – dead parts of plants that have retained a mechanical connection with the plant.

Decay - the amount of organic matter of plants that have died in above-ground and underground parts per unit area per unit of time.

Litter – a mass of perennial deposits of plant residues of varying degrees of mineralization.

Growth – the mass of an organism or community of organisms accumulated per unit area per unit of time.

True Gain – the ratio of the amount of growth to the amount of litter per unit time per unit area.

Primary production – the mass of living matter created by autotrophs (green plants) per unit area per unit time.

Secondary products – the mass of organic matter created by heterotrophs per unit area per unit time.

It is also necessary to distinguish between the capacity and speed of the biological cycle.

Capacity of the biological cycle – the number of chemical elements contained in the mass of a mature biocenosis (phytocenosis).

Intensity of biological cycle – the amount of chemical elements contained in the growth of biomass per unit area per unit time.

Biological turnover rate - the period of time during which an element travels from its absorption by living matter to its release from living matter.

According to L. E. Rodin and N. I. Bazilevich (1965), full cycle The biological cycle of elements on land consists of the following components:

1. Absorption of carbon by plants from the atmosphere, and nitrogen, ash elements and water from the soil, their fixation in the bodies of plant organisms, entry into the soil with dead plants or their parts, decomposition of litter and release of the elements contained in them.

2. Eating parts of plants by animals that feed on them, transforming them in the bodies of animals into new organic compounds and fixing some of them in animal organisms, their subsequent entry into the soil with the excrement of animals or with their corpses, the decomposition of both of them and the release of those contained in them elements.

3. Gas exchange between plants and the atmosphere (including soil air).

4. Lifetime secretions of some elements by above-ground plant organs and their root systems directly into the soil.

The structure of the biosphere itself general view represents two largest natural complexes of the first rank - continental and oceanic. In the modern era, the land as a whole is an eluvial system, the ocean an accumulative system. The history of the "geochemical relationship" between the ocean and land is reflected in chemical composition soils and ocean waters. The elements that are the basis of life - Si, Al, Fe, Mn, C, P, N, Ca, K - accumulate in the soil, and H, O, Na, Cl, S, Mg - form the chemical basis of the ocean.

Plants, animals and soil cover The world's landmass form a complex system. By binding and redistributing solar energy, atmospheric carbon, moisture, oxygen, hydrogen, nitrogen, phosphorus, sulfur, calcium and other biophilic elements, this system constantly forms new biomass and generates free oxygen.

There is a second system in the ocean ( aquatic plants and animals), performing the same binding functions on the planet solar energy, carbon, nitrogen, phosphorus and other biophiles through the formation of phytobiomass, releasing oxygen into the atmosphere.

You already know that there are three forms of accumulation and redistribution of cosmic energy (primarily the energy of the Sun) in the biosphere.

The essence of the first of them is this. That living organisms, and through food chains and associated animals and bacteria, build their tissues using many chemical elements and their compounds. Among the most important of them are macroelements - H, O, N, P, S, Ca, K, Mg, Si, Al, Mn, as well as microelements I, Co, Cu, Zn, Mo, etc. In this case, selective selection of light isotopes occurs carbon, hydrogen, oxygen, nitrogen and sulfur from heavier ones.

Throughout their lives and even after death, living organisms on land, aquatic and air environment, are in a state of continuous exchange with the environment. In this case, the total mass and volume of the products of intravital metabolism of organisms and the environment (metabolites) are several times higher than the biomass of living matter.

The elements of the biogeochemical cycle are the following components:

1. Continuous or regularly repeating processes of energy influx, formation and synthesis of new compounds.

2. Constant or periodic processes of transfer or redistribution of energy and processes of removal and directional movement of synthesized compounds under the influence of physical, chemical and biological agents.

3. Directed rhythmic processes of sequential transformation: decomposition, destruction of previously synthesized compounds under the influence of biogenic and abiogenic environmental influences.

4. Constant or periodic formation of the simplest mineral or organomineral components in a gaseous, liquid or solid state, which play the role of initial components for new, regular cycles of substances.

Biological are caused by the vital activity of organisms (nutrition, food connections, reproduction, growth, movement of metabolic products, death, decomposition, mineralization)

Mandatory parameters taken into account when studying biogeochemical cycles are the following main indicators:

1. Total biomass and its actual increase (phyto-, zoo-, microbial mass separately).

2. Organic litter (quantity, composition)

3. Soil organic matter (humus, undecomposed organic residues).

4. Elementary material composition soils, waters, air, precipitation, individual fractions of biomass.

5. Above-ground and underground reserves of biogenic energy.

6. Lifetime metabolites

7. Number of species of living organisms, their numbers, composition

8. Life expectancy of organisms of each species, life dynamics of populations of living organisms and soils.

9. Ecological and meteorological environment: background and assessment of human intervention.

10. Characteristics of various landscapes and their elements.

11. The amount of pollutants, their chemical, physical, biological properties.

The individual significance of a particular chemical element is assessed by the coefficient of biological absorption, which is determined by the ratio of the content of the element in plant ash (by weight) to the content of the same element in the soil (or in earth's crust).

In 1966, V. A. Kovda proposed using the ratio of the recorded phytobiomass to the annual photosynthetic increase in phytomass to characterize the average duration of the general carbon cycle. This coefficient characterizes the average duration of the overall cycle of synthesis-mineralization of biomass in a given area (or on land in general). Calculations have shown that the share of land in general this cycle fits into the period from 300-400 to 1000 years. Accordingly, at this average speed, the release of mineral compounds bound in biomass occurs, the formation and mineralization of humus in the soil.

For a general assessment of the biogeochemical significance of the mineral components of the living matter of the biosphere, V. A. Kovda proposed to compare the reserve of mineral substances of the biomass, as well as the amount of mineral substances annually involved in circulation with growth and litter, with the annual chemical runoff of rivers. It turned out that these values ​​are comparable. And this means that most of substances dissolved in river waters, passed through the biological cycle of the plant-soil system, before it joined the geochemical migration with water towards the ocean or inland depressions.

It turned out that the biogeochemical cycle indices vary greatly in different climatic conditions, under the cover of various plant communities, with different conditions natural drainage, therefore N.I. Bazilevich and L.E. Rodin proposed to calculate an additional coefficient characterizing the intensity of litter decomposition and the duration of litter preservation in the conditions of a given biogeocenosis, equal to the ratio of the mass of litter to the mass of annual litter. According to these researchers, the phytomass decomposition indices are greatest in the tundra and swamps of the north, and the lowest (about 1) in the steppes and semi-deserts.

B.B. Polynov proposed calculating the water migration index equal to the ratio of the amount of the element in the mineral residue of evaporated river or groundwater to the content of the same chemical component in rocks (or the earth’s crust). Calculation of water migration indices showed that the most mobile migrants in the biosphere are chlorine, sulfur, boron, bromine, iodine, calcium, sodium, magnesium, fluorine, strontium, zinc, uranium, and molybdenum. The least mobile are silicon, aluminum, iron, potassium, phosphorus, barium, manganese, rubidium, copper, nickel, cobalt, arsenic, lithium.

Undisturbed biogeochemical cycles are almost circular, i.e. almost reserved character. The degree of reproduction (repetition) of cycles in nature is very high (according to V.A. Kovda - 90-98%). Thus, a certain constancy of the composition, quantity and concentration of the components involved in the cycle is maintained. But the incomplete closure of biogeochemical cycles, as we will see later, has a very important geochemical significance and contributes to the evolution of the biosphere. This is why there is a biogenic accumulation of oxygen in the atmosphere, a biogenic and chemogenic accumulation of carbon compounds in the earth’s crust (oil, coal, limestones)

Let's take a closer look at the main parameters of the biogeochemical cycle on land.

The general biogeochemical cycle of elements includes biogeochemical cycles of individual chemical elements. The most important role in the functioning of the biosphere as a whole and individual geosystems of a lower classification level is played by the cycles of several chemical elements that are most necessary for living organisms due to their role in the composition of living matter and physiological processes.

BIOLOGICAL CYCLE OF SUBSTANCES The entry of substances from the soil and atmosphere into living organisms with a corresponding change in their chemical form, their return to the soil and atmosphere during the vital activity of organisms and with post-mortem residues, and their re-entry into living organisms after processes of destruction and mineralization with the help of microorganisms

Dictionary of business terms. Akademik.ru. 2001.

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Topic 3.4. BIOLOGICAL CYCLE OF ELEMENTS

3.4.1. General concept of the biological cycle of substances

Since the beginning of the study of the interaction of living organisms with the environment, it has become clear that the processes of biogenic mass transfer are cyclical in nature (see Fig. 2.3.2).

Mass transfer cycles of varying length in space and unequal duration in time form a dynamic system of the biosphere. IN AND. Vernadsky believed that the history of most chemical elements, forming more than 99% of the mass of the biosphere, can only be understood taking into account circular migrations (cycles). At the same time, he emphasized that “these cycles are reversible only in the main part of the atoms, while some elements inevitably and constantly leave the cycle. This output is natural, i.e. the circular process is not completely reversible.” Incomplete reversibility and imbalance of migration cycles allow certain concentrations of the migrating element, to which organisms can adapt, but at the same time, ensure the removal of excess amounts of the element from a given cycle.

That is, the integrity of the biosphere as a system is due to the continuous exchange of matter between its components, in which processes associated with the synthesis and decomposition of organic matter play a key role. They are realized both in the course of metabolism between living organisms and the environment, and in the processes of mineralization of organic matter after the death of the organism as a whole or the death of its individual organs. In addition, non-biogenic in nature processes of exchange of matter between various components of the geographic envelope also contribute to the cycle of matter in the biosphere.

3.4.2. Elements of biogeochemical cycle of substances.
Parameters of the biological cycle of elements on land and in the ocean

Biological cycle of substances is a set of processes of the entry of chemical organisms into living organisms, the biochemical synthesis of new complex compounds and the return of elements to the soil, atmosphere and hydrosphere (Fig.)

Abiogenic and biological cycles are closely intertwined, forming a planetary geochemical cycle and a system of local cycles of matter. Thus, over billions of years of the biological history of our planet, a great biogeochemical cycle and differentiation of chemical elements in nature has developed, which is the basis for the normal functioning of the biosphere. That is, in the conditions of a developed biosphere, the cycle of substances is directed by the combined action of biological, geological and geochemical factors. The relationship between them may be different, but the action must be joint! It is in this sense that the terms biogeochemical circulation of substances and biogeochemical cycles are used.

The biological cycle is not a fully compensated closed cycle.

The biological, biochemical and geochemical significance of the processes carried out in the biological cycle of substances was first shown by V.V. Dokuchaev. It was further revealed in the works of V.I. Vernadsky, B.B. Polynova, D.N. Pryanishnikova, V.N. Sukacheva, L.E. Rodina, N.I. Bazilevich, V.A. Kovda and other researchers.

Before we begin to study the natural biological cycles of chemical elements, it is necessary to become familiar with the most commonly used terms.

Biomass – the mass of living matter accumulated at a given point in time.

Phytomass (or plant biomass0 - the mass of living and dead organisms of plant communities that have retained their anatomical structure at a given moment in any specific area or on the planet as a whole.

Structure of phytomass - the ratio of underground and aboveground parts of plants, as well as annual and perennial, photosynthetic and non-photosynthetic parts of plants.

Rags – dead parts of plants that have retained a mechanical connection with the plant.

Decay - the amount of organic matter of plants that have died in above-ground and underground parts per unit area per unit of time.

Litter – mass of perennial deposits of plant residues varying degrees mineralization.

Growth – the mass of an organism or community of organisms accumulated per unit area per unit of time.

True Gain – the ratio of the amount of growth to the amount of litter per unit time per unit area.

Primary production – the mass of living matter created by autotrophs (green plants) per unit area per unit time.

Secondary products – the mass of organic matter created by heterotrophs per unit area per unit time.

It is also necessary to distinguish between the capacity and speed of the biological cycle.

Capacity of the biological cycle – the number of chemical elements contained in the mass of a mature biocenosis (phytocenosis).

Intensity of biological cycle – the amount of chemical elements contained in the growth of biomass per unit area per unit time.

Biological turnover rate - the period of time during which an element travels from its absorption by living matter to its release from living matter.

Field. Rodina and N.I. Bazilevich (1965), the full cycle of the biological cycle of elements on land consists of the following components:

1. Absorption of carbon by plants from the atmosphere, and nitrogen, ash elements and water from the soil, their fixation in the bodies of plant organisms, entry into the soil with dead plants or their parts, decomposition of litter and release of the elements contained in them.
2. Eating parts of plants by animals that feed on them, converting them in the bodies of animals into new organic compounds and fixing some of them in animal organisms, their subsequent entry into the soil with the excrement of animals or with their corpses, decomposition of both and the release of the elements contained in them.
3. Gas exchange between plants and the atmosphere (including soil air).
4. Lifetime secretions of certain elements by above-ground plant organs and their root systems directly into the soil.

The structure of the biosphere in its most general form represents two largest natural complexes of the first rank - continental and oceanic. In the modern era, the land as a whole is an eluvial system, the ocean an accumulative system. The history of the "geochemical relationship" between the ocean and land is reflected in the chemistry of soils and ocean waters. The elements that are the basis of life - Si, Al, Fe, Mn, C, P, N, Ca, K - accumulate in the soil, and H, O, Na, Cl, S, Mg - form the chemical basis of the ocean.

Plants, animals and soil cover of the world's land form a complex system. By binding and redistributing solar energy, atmospheric carbon, moisture, oxygen, hydrogen, nitrogen, phosphorus, sulfur, calcium and other biophilic elements, this system constantly forms new biomass and generates free oxygen.

In the ocean, there is a second system (aquatic plants and animals) that performs the same functions on the planet of binding solar energy, carbon, nitrogen, phosphorus and other biophiles through the formation of phytobiomass and the release of oxygen into the atmosphere.

You already know that there are three forms of accumulation and redistribution of cosmic energy (primarily the energy of the Sun) in the biosphere.

The essence of the first of them is this. That living organisms, and through food chains and associated animals and bacteria, build their tissues using many chemical elements and their compounds. Among the most important of them are macroelements - H, O, N, P, S, Ca, K, Mg, Si, Al, Mn, as well as microelements I, Co, Cu, Zn, Mo, etc. In this case, selective selection of light isotopes occurs carbon, hydrogen, oxygen, nitrogen and sulfur from heavier ones.

Throughout their entire life and even after death, living organisms of land, water and air are in a state of continuous exchange with the environment. In this case, the total mass and volume of the products of intravital metabolism of organisms and the environment (metabolites) are several times higher than the biomass of living matter.

The elements of the biogeochemical cycle are the following components:

1. Continuous or regularly repeating processes of energy influx, formation and synthesis of new compounds.
2. Constant or periodic processes of transfer or redistribution of energy and processes of removal and directional movement of synthesized compounds under the influence of physical, chemical and biological agents.
3. Directed rhythmic processes of sequential transformation: decomposition, destruction of previously synthesized compounds under the influence of biogenic and abiogenic environmental influences.
4. Continuous or periodic formation of the simplest mineral or organomineral components in a gaseous, liquid or solid state, which play the role of initial components for new, regular cycles of substances.

Biological are caused by the vital activity of organisms (nutrition, food connections, reproduction, growth, movement of metabolic products, death, decomposition, mineralization)

Mandatory parameters taken into account when studying biogeochemical cycles are the following main indicators:

1. Total biomass and its actual growth (phyto-, zoo-, microbial mass separately).
2. Organic litter (quantity, composition)
3. organic matter soil (humus, undecomposed organic matter).
4. Elementary material composition of soils, waters, air, sediments, individual fractions of biomass.
5. Aboveground and underground reserves of biogenic energy.
6. Lifetime metabolites
7. Number of species of living organisms, their numbers, composition
8. Life expectancy of organisms of each species, life dynamics of populations of living organisms and soils.
9. Ecological and meteorological environment: background and assessment of human intervention.
10. Characteristics of various landscapes and their elements.
11. The amount of pollutants, their chemical, physical, biological properties.

The individual significance of a particular chemical element is assessed by the coefficient of biological absorption, which is determined by the ratio of the content of the element in plant ash (by weight) to the content of the same element in the soil (or in the earth’s crust).

In 1966 V.A. Kovda proposed using the ratio of the recorded phytobiomass to the annual photosynthetic increase in phytomass to characterize the average duration of the overall carbon cycle. This coefficient characterizes the average duration of the overall cycle of synthesis-mineralization of biomass in a given area (or on land in general). Calculations have shown that the share of land in general this cycle fits into the period from 300-400 to 1000 years. Accordingly, at this average speed, the release of mineral compounds bound in biomass occurs, the formation and mineralization of humus in the soil.

For a general assessment of the biogeochemical significance of the mineral components of living matter in the biosphere, V.A. Kovda proposed to compare the reserve of mineral substances in biomass, as well as the amount of mineral substances annually involved in circulation through growth and litter, with the annual chemical runoff of rivers. It turned out that these values ​​are comparable. This means that most of the substances dissolved in river waters passed through the biological cycle of the plant-soil system, before it joined the geochemical migration with water in the direction of the ocean or inland depressions.

It turned out that the biogeochemical cycle indices vary greatly in different climatic conditions, under the cover of different plant communities, under different conditions of natural drainage, so N.I. Bazilevich and L.E. Rodin proposed to calculate an additional coefficient characterizing the intensity of litter decomposition and the duration of litter preservation under the conditions of a given biogeocenosis, equal to the ratio of the mass of litter to the mass of annual litter. According to these researchers, the phytomass decomposition indices are greatest in the tundra and swamps of the north, and the lowest (about 1) in the steppes and semi-deserts.

B.B. Polynov proposed calculating the water migration index equal to the ratio of the amount of an element in the mineral residue of evaporated river or ground water to the content of the same chemical component in rocks (or the earth’s crust). Calculation of water migration indices showed that the most mobile migrants in the biosphere are chlorine, sulfur, boron, bromine, iodine, calcium, sodium, magnesium, fluorine, strontium, zinc, uranium, and molybdenum. The least mobile are silicon, aluminum, iron, potassium, phosphorus, barium, manganese, rubidium, copper, nickel, cobalt, arsenic, lithium.

Undisturbed biogeochemical cycles are almost circular, i.e. almost reserved character. The degree of reproduction (repetition) of cycles in nature is very high (according to V.A. Kovda - 90-98%). Thus, a certain constancy of the composition, quantity and concentration of the components involved in the cycle is maintained. But the incomplete closure of biogeochemical cycles, as we will see later, has a very important geochemical significance and contributes to the evolution of the biosphere. This is why there is a biogenic accumulation of oxygen in the atmosphere, a biogenic and chemogenic accumulation of carbon compounds in the earth’s crust (oil, coal, limestones)

Let's take a closer look at the main parameters of the biogeochemical cycle on land.

The general biogeochemical cycle of elements includes biogeochemical cycles of individual chemical elements. The most important role in the functioning of the biosphere as a whole and individual geosystems of a lower classification level is played by the cycles of several chemical elements that are most necessary for living organisms due to their role in the composition of living matter and physiological processes. These most essential chemical elements include carbon, oxygen, nitrogen, sulfur, phosphorus, etc.

Biological cycle of chemical elements in common tropical communities

The bioclimatic conditions of tropical areas are very diverse. The idea of ​​the tropics as a continuous strip of jungle is completely untrue. Changing ratios of precipitation and evapotranspiration, the duration of dry and rainy seasons create a wide range of ecosystems with varying degrees of atmospheric moisture - from extremely arid or desert landscapes to constantly humid tropical forests. In the presence of a season during which evaporation exceeds precipitation, there are sparse light-colored tall-grass forests that shed their leaves during the long dry season. In drier conditions, sparse groups of trees interspersed with open spaces covered with herbaceous vegetation are typical. As aridity increases, trees are replaced by thickets of thorny bushes, and the lush cover of tall grasses is replaced by short-grass vegetation with a low degree of soil coverage.

The ratios of areas of different degrees of atmospheric moisture on the continents are not the same. Dry areas occupy the vast majority of Australia, a significant part of India, but are less common in South America. In the equatorial strip of Africa, limited to 6° N. w. and 6° S. w., areas of varying degrees of atmospheric moisture are distributed as follows:

From the above data it follows that moist forests occupy only about "/5 of the equatorial strip of Africa, and most of it is occupied by a combination of light forests and tall grass savannas. The rest of the territory is covered by more or less arid landscapes, up to almost deserted ones, where precipitation is less than 200 mm precipitation per year According to B.G. Rozanov (1977), the distribution zone of all types of tropical forests occupies 20,448 thousand km 2, or 13.33% of the world's land, the savanna zone - 14,259 thousand km 2 (9.56). %), areas of tropical deserts - 4506 thousand km 2, or 3.02%. The areas of scattered sand, lifeless rocky deserts, and salt marshes were not taken into account.

Biological cycle of elements in tropical forests. Permanently humid tropical forests are the most powerful plant formation. The abundance of heat and moisture determines the largest biomass among the biocenoses of the world's land - on average 50,000 t/km 2 of dry matter, and in some cases up to 170,000 t/km 2. The factor limiting biomass growth is the light energy required for photosynthesis. In order to maximize its use, under the cover of trees 30-40 m high, there are several more tiers of trees adapted to diffused light. A significant part of the dying and falling leaves of tall trees is intercepted by numerous epiphytes. For this reason, the chemical elements contained in the leaves are recaptured into the biological cycle without reaching the soil. In tropical rainforests, the growing season continues throughout the year. Annual production averages 2500 t/km 2 .

The biogeochemical specificity of tropical rainforests lies in the fact that almost the entire amount of chemical elements necessary to feed a huge mass of vegetation is contained in the plants themselves. The biogeochemical mass transfer cycle is highly closed. If you cut down a tropical rain forest, then along with the death of the trees, the entire system of biological circulation created over millennia will be disrupted and barren lands will remain under the cleared forest.

The biogeochemical situation in light deciduous tropical forests and savannas is close to that in deciduous forests temperate climate, but periods of suppression of biogeochemical processes are caused not by a decrease in temperature, but by the lack of rain and seasonal moisture deficiency. The biomass of dry savannas is about 200-600 t/km 2. The amount of litter (less than 150-200 t/km 2) corresponds to the conditions of tropical deserts. The biomass of deciduous tropical forests of varying degrees of moisture and tall grass park savannas occupies an intermediate position between permanently wet forests and dry savannas.

According to the available data from L.E.Rodina and N.I.Bazilevich (1965), the distribution and dynamics of masses in the vegetation of a permanently humid tropical forest are characterized by the following indicators (t/km 2):

It should be noted that the concentration of chemical elements in the wood of the trunks and branches of tropical trees is, as a rule, lower than in the leaves, which form the bulk of the litter. The nitrogen concentration in wood rarely reaches 0.5% of dry matter mass, and in leaves - about 2%. In leaves, the concentration of calcium, potassium, magnesium, sodium, silicon, and phosphorus is usually several times higher than in wood. The content of elements in the leaves of trees and in herbaceous vegetation, abundantly represented in light deciduous forests, varies slightly. The concentration of most trace elements in tree leaves and grasses is also higher than in wood, although barium and especially strontium are higher in wood.

Based on the available data, we take the average value of the sum of ash elements in the biomass of a permanently humid tropical forest to be equal to 800 t/km 2 ; the mass of these elements involved in the biological cycle is equal to 150 t/km 2 per year. For light forests, the average values ​​are 200 and 50 t/km 2 per year, respectively. Based on these figures, the approximate values ​​of the masses of trace elements annually involved in the biological cycle were determined.

Concentration of ash elements in equatorial vegetation of East Africa, % dry mass (according to V.V. Dobrovolsky 1975)

Samples: 52 - sparse grassy cover of short-grass savanna with a predominance of representatives of the genera Sporobolus, Cynodon, KyUinga, Northwestern Tanzania.

76 - Podocarpus trunk, rain forest on the southern slope of Kilimanjaro, Tanzania.

42 - forest floor of the rain forest on the southern slope of Kilimanjaro, Tanzania.

210 - papyrus stems (Cyperus papyrus), floodplain of the White Nile near the source of Lake Albert, Uganda.

Masses of trace elements involved in the biological cycle in tropical forests

The concentration levels of trace elements in the soil-forming substrate of different regions of tropical land are not the same. This is reflected in the content of elements in plants. For example, in East Africa, in cereal grasses collected in the area of ​​crystalline rocks of the Precambrian basement, the concentration of copper is 71 * 10 -4%, and in similar grasses in the area of ​​volcanic lavas - 120 * 10 -4%. The zinc concentration changes accordingly from 120 to 450 10-4%), TiOz - from 200 to 1800 10-4%.

The table compares the content of trace elements in the ash of grass and tree branches (acacia) from the savannas of East Africa. It's clear that heavy metals accumulate more strongly in grasses, and barium and strontium - in trees. It should be noted that the concentration of the latter increases with increasing aridity. In the arid regions of southern Tanzania, we found strontium concentrations in the ash of baobab branches to be about 4500 μg/g, and in one case in acacia branches 3 times higher.

The intensity of biological absorption and the concentration of trace elements in the ash of grasses and trees of the savannas of East Africa (according to V.V. Dobrovolsky, 1973)

The above-ground part of savanna grasses has a high ash content - from 6 to 10%, partly due to the admixture of small particles of mineral dust, detectable under a microscope and sometimes with the naked eye. The amount of mineral dust is 2-3% of the mass of absolutely dry matter of the aerial parts of herbs. Apparently, the admixture of mineral dust affects the increased concentration of gallium, which is poorly absorbed by plants, but is contained in highly dispersed clay material, energetically transported by the wind. But even after excluding insoluble silicate dust, the amount of ash elements in savannah cereals is 2 times greater than in the cereals of high-mountain meadows.