Biogeochemical nitrogen cycle in nature. Biogeochemical cycle of nitrogen and consequences of anthropogenic impact on it

Each chemical element, making a cycle in an ecosystem, follows its own special path, but all cycles are driven by energy, and the elements participating in them alternately pass from organic to inorganic form and vice versa. Let us consider the cycles of some chemical elements, taking into account the peculiarities of their receipt from the exchange fund to the reserve and return to the exchange fund.

nitrogen cycle. Of course, this is one of the most complex and at the same time the most vulnerable cycles (Fig. 11.5).

Despite the large number of organisms involved, the cycle provides a rapid circulation of nitrogen in various ecosystems. As a rule, in quantitative terms, nitrogen follows carbon, with which it participates in the formation of protein compounds. Nitrogen, which is part of proteins and other nitrogen-containing compounds, is transferred from an organic form to an inorganic one as a result of the activity of a number of chemotrophic bacteria. Each type of bacteria does its part of the work, oxidizing ammonium to nitrite and further to nitrate. However, the nitrates available to plants "escape" from them as a result of the activity of denitrifying bacteria, which reduce nitrates to molecular nitrogen.

The nitrogen cycle is characterized by an extensive reserve fund in the atmosphere. Air by volume is almost 80% composed of molecular

Rice. 11.5.

nitrogen (N 2) and represents the largest reservoir of this element. At the same time, insufficient nitrogen content in the soil often limits the productivity of individual plant species and the entire ecosystem as a whole. All living organisms need nitrogen, which is used in various forms to form proteins and nucleic acids. But only a few microorganisms can use nitrogen gas from the atmosphere. Fortunately, nitrogen-fixing microorganisms convert molecular nitrogen into plant-available ammonium ions. In addition, the formation of nitrates by inorganic means is constantly occurring in the atmosphere, but this phenomenon plays only an auxiliary role in comparison with the activity of nitrifying organisms.

Phosphorus cycle. Phosphorus is one of the most important biogenic elements. It is part of nucleic acids, cell membranes, enzymes, bone tissue, dentin, etc. Compared to nitrogen, it occurs in relatively few chemical forms.

Phosphorus enters the exchange fund in two ways (Fig. 11.6). Firstly, due to the primary excretion of consumers and, secondly, in the process of the destruction of dead organic matter by phosphate-reducing bacteria, which convert phosphorus from the organic form into soluble phosphates: PO^ - , HPO ^ - and HjPO ^. , decomposers convert phosphorus from organic to inorganic form without oxidizing it.

Rice. 11.6. Biogeochemical cycle of phosphorus: I - exchange fund; II - reserve fund

A feature of the biogeochemical cycle of phosphorus is that, unlike nitrogen and carbon, its reserve fund is not the atmosphere, but rocks and sediments formed in past geological epochs. In this regard, the circulation of phosphorus is easily disturbed, since the bulk of the substance is concentrated in a low-active and low-mobility reserve fund buried in the earth's crust. The imperfection of the biogeochemical cycle of phosphorus is that the availability of this element is limited due to leakage into deep sediments.

Sulfur cycle. Sulfur is characterized by an extensive reserve fund in the earth's crust and a smaller one in the atmosphere and hydrosphere (Fig. 11.7). As a result of such coordination of the exchange and reserve funds, sulfur is not a limiting factor. The main source of sulfur available to organisms is various sulfates. The good water solubility of many sulfates facilitates the access of inorganic sulfur to ecosystems. Absorbing sulfates, plants restore them and produce sulfur-containing amino acids (methionine, cysteine, cystine). It is known that these amino acids play an important role in the formation of the tertiary structure of proteins by forming disulfide bridges between different parts of the polypeptide chain.

Rice. 11.7.

I - exchange fund; II, III - reserve funds

Like nitrates and phosphates, sulfates, the main form of sulfur available to plants, are reduced by autotrophs and incorporated into proteins. The organic remains of animals and plants are mineralized, and the reduced sulfur included in their composition during aerobic decomposition is oxidized enzymatically by various groups of chemotrophic microorganisms. Similar processes are carried out in reservoirs.

From the sulfoproteins contained in the soil, heterotrophic bacteria produce hydrogen sulfide. On the other hand, there are various groups of chemotrophic bacteria that can again oxidize hydrogen sulfide to sulfates, which again increases the supply of sulfur available to producers. Such bacteria do not need light. For example, chemotrophic bacteria Thiobacillus synthesize organic substances thanks to the energy obtained during the oxidation of hydrogen sulfide to sulfates in an environment where eternal darkness reigns.

The last phase of the sulfur cycle is entirely sedimentary. It consists in the precipitation of this element under anaerobic conditions in the presence of iron. Various stages of this process, especially reversible ones, further allow the use of sedimentary rock reserves. Thus, the process ends with a slow and gradual accumulation of sulfur in deep sedimentary rocks.

The carbon cycle. Carbon is the main building material of molecules of organic compounds important for life (carbohydrates, fats, proteins, nucleic acids, etc.). This bioelement is involved in a cycle with a small but mobile reserve fund in the atmosphere (Fig. 11.8), from where it is obtained by plants in the form of carbon dioxide. It is carbon dioxide, atmospheric and dissolved in water, that is the only source of inorganic carbon, from which all organic compounds that make up a living cell are produced in the process of photosynthesis. The movement of carbon along the food chains of an ecosystem is closely related to the transfer of energy - it is not for nothing that carbon dioxide and water are the end products of vital activity.

In the soil, very often the carbon cycle slows down. Organic substances are not completely mineralized, but are transformed into a complex complex of derivatives of organic acids, forming a dark-colored mass, the so-called humus. Under any conditions, the organic complex cannot be fully mineralized by aerobic means and therefore accumulates in various sedimentary rocks. Then there is stagnation or blocking of the carbon cycle - an example of this is the accumulation of coal, oil and other hydrocarbon minerals.

Rice. 11.8.

I - exchange fund; II - reserve fund

Producers cannot assimilate solid forms of carbon, therefore atmospheric air serves as its only source for plants. Now the reserves of carbon in the atmosphere in the form of CO 2 are relatively small. Thanks to the buffer system of the marine carbonate cycle, the carbon cycle becomes sustainable, but it is still vulnerable due to the small amount of the reserve fund.

Cycling involves not only biogenic elements, but also many pollutants. Some of them circulate in the environment and tend to accumulate in organisms. In such cases, the concentration of any contaminant found in the organisms increases as it moves up the food chain, as the organisms absorb the contaminants faster than they release them. Mercury, for example, can be contained in water and bottom silt in relatively harmless concentrations, while its content in the body of aquatic animals with a shell or shell can reach a lethal level for them. The action of pesticides such as DDT is based on a similar principle: their content in water can be so small that it is almost impossible to detect them, but the higher the trophic level at which the given organism is located, the greater the concentration of the pesticide in its tissues. This phenomenon is known as biological amplification or biological accumulation.

In order for life to continue to exist, chemical elements must constantly circulate from the external environment to living organisms and vice versa, passing from the cytoplasm of some organisms into a form assimilated by other organisms. The most important property of flows in ecosystems is their cyclicity. Substances in ecosystems make an almost complete cycle, getting first into organisms, then into the abiotic environment and again returning to organisms.

The critical moments of biogeochemical cycles are the capture (level of producers) and return (level of decomposers) of substances from the physical environment. These moments are associated with reduction and oxidation reactions. Restoration of chemicals is carried out ultimately due to the energy of solar radiation. At each stage of energy transfer, it is dissipated, ending at the level of decomposers, which oxidize the elements to a state in which they can already be captured by producers. In general, at the level of the exchange fund, the biogeochemical cycle can be represented by a system of steps, within each of which its own part of the oxidation process is carried out (Fig. 11.9).

Rice. 11.9.

On the way from consumers to producers, decomposers act, represented by various groups of chemotrophic bacteria, which oxidize the compounds of biogenic elements to forms available to producers. In the nitrogen and sulfur cycle, heterotrophic bacteria are included at this stage, reducing the compounds and thus making them inaccessible to plants. In the sulfur cycle, the activity of heterotrophic bacteria is balanced by the activity of several groups of aerobic and anaerobic chemotrophs at once. In the nitrogen cycle, the energy barrier is leveled due to the activity of nitrogen-fixing microorganisms.

• Denitrifying bacteria use nitrate as a source of oxygen.

The water cycle in nature (hydrological cycle) is the process of cyclic movement of water in the earth's biosphere. Consists of evaporation, condensation and precipitation.

The seas lose more water due to evaporation than they receive with precipitation, on land the situation is reversed. Water continuously circulates around the globe, while its total amount remains unchanged.

Three quarters of the earth's surface is covered with water. The water shell of the Earth is called the hydrosphere. Most of it is the salt water of the seas and oceans, and the smaller part is the fresh water of lakes, rivers, glaciers, groundwater and water vapor.

On earth, water exists in three states of aggregation: liquid, solid and gaseous. Living organisms cannot exist without water. In any organism, water is the medium in which chemical reactions take place, without which living organisms cannot live. Water is the most valuable and most necessary substance for the life of living organisms.

The constant exchange of moisture between the hydrosphere, the atmosphere and the earth's surface, consisting of the processes of evaporation, the movement of water vapor in the atmosphere, its condensation in the atmosphere, precipitation and runoff, is called the water cycle in nature.

Atmospheric precipitation partially evaporates, partially forms temporary and permanent drains and reservoirs, and partially seeps into the ground and forms groundwater.

There are several types of water cycles in nature:

A large, or world, circulation - water vapor formed above the surface of the oceans is carried by winds to the continents, falls there in the form of precipitation and returns to the ocean in the form of runoff. In this process, the quality of water changes: during evaporation, salty sea water turns into fresh water, and polluted water is purified.

A small, or oceanic, circulation - water vapor formed above the surface of the ocean condenses and falls as precipitation back into the ocean.

Intracontinental circulation - water that has evaporated above the land surface again falls on land in the form of precipitation.

In the end, the precipitation in the process of movement again reaches the oceans.

Oxygen cycle

Atmospheric oxygen is of biogenic origin, and its circulation in the biosphere is carried out by replenishing reserves in the atmosphere as a result of plant photosynthesis and absorption during the respiration of organisms and the combustion of fuel in the human economy. In addition, a certain amount of oxygen is formed in the upper atmosphere during the dissociation of water and the destruction of ozone under the action of ultraviolet radiation; part of the oxygen is spent on oxidative processes in the earth's crust, during volcanic eruptions, etc.

This cycle is very complex, since oxygen enters into various reactions and is part of a very large number of organic and inorganic compounds, and is slow. It takes about 2 thousand years to completely renew all the oxygen in the atmosphere (for comparison: about 1/3 of the atmospheric carbon dioxide is renewed annually).

Currently, an equilibrium oxygen cycle is maintained, although local disturbances occur in large densely populated cities with a large number of transport and industrial enterprises.

The carbon cycle.

This is one of the most important biospheric cycles, since carbon is the basis of organic matter. The role of carbon dioxide is especially great in the cycle. The reserves of "living" carbon in the composition of land and ocean organisms are, according to various sources, 550--750 Gt (1 Gt \u003d 1 billion tons), with 99.5% of it concentrated on land, the rest in the ocean. In addition, the ocean contains up to 700 Gt of carbon as dissolved organic matter.

The reserves of inorganic carbon are much larger. Above each square meter of land and ocean is 1 kg of atmospheric carbon and under each square meter of ocean at a depth of 4 km - 100 kg of carbon in the form of carbonates and bicarbonates. There are even more carbon reserves in sedimentary rocks - limestone contains carbonates, shales contain kerogens, etc.

Approximately 1/3 of "living" carbon (about 200 Gt) circulates, i.e., is annually absorbed by organisms in the process of photosynthesis and returned back to the atmosphere, and the contribution of the ocean and land to this process is approximately similar. Despite the fact that the biomass of the ocean is much less than the biomass of land, its biological production is created by many generations of short-lived algae (the ratio of biomass and biological production in the ocean is approximately the same as in a freshwater ecosystem.

Up to 50% (according to some sources, up to 90%) of carbon in the form of dioxide is returned to the atmosphere by soil decomposer microorganisms. Bacteria and fungi contribute equally to this process. The return of carbon dioxide during the respiration of all other organisms is thus less than during the activity of decomposers.

Some bacteria produce methane in addition to carbon dioxide. The release of methane from the soil increases with waterlogging, when anaerobic conditions are created that are favorable for the activity of methane-forming bacteria. For this reason, the emission of methane from forest soil increases sharply if the forest stand is cut down and, due to a decrease in transpiration, its waterlogging occurs. A lot of methane is emitted by rice fields and livestock.

At present, there is a violation of the carbon cycle due to the burning of a significant amount of fossil carbonaceous energy carriers, as well as the dehumification of arable soils and the drainage of swamps. In general, the content of carbon dioxide in the atmosphere increases by 0.6% annually. The content of methane increases even faster - by 1-2%. These gases are the main contributors to the increased greenhouse effect, which is 50% dependent on carbon dioxide and 33% on methane.

The nitrogen cycle is the biogeochemical cycle of nitrogen. Most of it is due to the action of living beings. Soil microorganisms play a very important role in the cycle, providing nitrogen metabolism in the soil - the cycle of nitrogen in the soil, which is present there in the form of a simple substance (gas - N2) and ions: nitrites (NO2-), nitrates (NO3-) and ammonium ( NH4+). The concentrations of these ions reflect the state of soil communities, since these indicators are affected by the state of biota (plants, microflora), the state of the atmosphere, and the leaching of various substances from the soil. They are able to reduce the concentration of nitrogen-containing substances that are detrimental to other living organisms. They can convert ammonia, toxic to living beings, into less toxic nitrates and into biologically inert atmospheric nitrogen. Thus, the soil microflora contributes to maintaining the stability of its chemical parameters.

Phosphorus cycle.

In the phosphorus cycle, unlike the carbon and nitrogen cycles, there is no gas phase. Phosphorus in nature is found in large quantities in the minerals of rocks and enters terrestrial ecosystems in the process of their destruction. Phosphorus leaching by precipitation leads to its entry into the hydrosphere and, accordingly, into aquatic ecosystems. Plants absorb phosphorus in the form of soluble phosphates from an aqueous or soil solution and include it in organic compounds - nucleic acids, energy transfer systems (ADP, ATP), and in cell membranes. Other organisms obtain phosphorus through food chains. In animal organisms, phosphorus is part of the bone tissue, dentin.

In the process of cellular respiration, organic compounds containing phosphorus are oxidized, while organic phosphates enter the environment as part of excreta. Organisms-reducers mineralize organic substances containing phosphorus into inorganic phosphates, which can again be used by plants and, thus, be again involved in the cycle.

Since there is no gas phase in the phosphorus cycle, phosphorus, like other biogenic elements of the soil, circulates in the ecosystem only if waste products are deposited in the places of absorption of this element. Disturbance of the phosphorus cycle can occur, for example, in agroecosystems, when the crop, together with the nutrients extracted from the soil, is transported over considerable distances and they are not returned to the soil at the places of consumption.

Sulfur cycle

The sulfur cycle is also closely related to living matter. Sulfur in the form of SO2, SO3, H2S and elemental sulfur is released into the atmosphere by volcanoes. On the other hand, various metal sulfides are known in nature in large quantities: iron, lead, zinc, etc. Sulfide sulfur is oxidized in the biosphere with the participation of numerous microorganisms to sulfate sulfur SO42 of soils and water bodies. Sulfates are taken up by plants. In organisms, sulfur is part of amino acids and proteins, and in plants, in addition, it is part of essential oils, etc. The processes of destruction of the remains of organisms in soils and in the silts of the seas are accompanied by very complex transformations of sulfur. When proteins are destroyed with the participation of microorganisms, hydrogen sulfide is formed. Further, hydrogen sulfide is oxidized either to elemental sulfur or to sulfates. This process involves a variety of microorganisms that create numerous intermediate sulfur compounds. Sulfur deposits of biogenic origin are known. Hydrogen sulfide can re-form "secondary" sulfides, and sulfate sulfur creates gypsum. In turn, sulfides and gypsum are again destroyed, and sulfur resumes its migration.

Biogeochemical cycles of carbon, nitrogen and oxygen(Fig. 6.9) are the most perfect. Due to their large atmospheric reserves, they are capable of rapid self-regulation. IN carbon cycle , or rather ¾ of its most mobile form ¾ CO 2, the trophic chain is clearly visible: producers¾ capture carbon from the atmosphere during photosynthesis, consumers¾ absorbing carbon together with the bodies of producers and consumers of lower orders, decomposers¾ returning carbon back into the cycle. The turnover rate of CO 2 is about 300 years (its complete replacement in the atmosphere and other elements of the cycle (Fig. 6.10).

Rice. 6.9. Scheme of the biogeochemical circulation of substances on land (according to R. Kashanov, 1984)

Rice. 6.10. Matter Circulation Rates (Cloud and Jibor, 1972)

In the World Ocean, the trophic chain: producers (phytoplankton) ¾ consumers (zooplankton, fish) ¾ decomposers (microorganisms) ¾ is complicated by the fact that some of the carbon of a dead organism, sinking to the bottom, “goes” into sedimentary rocks and is no longer involved in biological, but in the geological cycle of matter.

Forests are the main reservoir of biologically bound carbon; they contain up to 500 billion tons of this element, which is 2/3 of its reserve in the atmosphere. Human intervention in the carbon cycle leads to an increase in the content of CO 2 in the atmosphere.

Speed oxygen cycle ¾ 2000 years (Fig. 6.10), it is during this time that all the oxygen of the atmosphere passes through living matter. The main supplier of oxygen on Earth ¾ green plants. They annually produce 53 × 10 9 tons of oxygen on land, and ¾ 414 × 10 9 tons in the oceans.

The main consumer of oxygen is animals, soil organisms and plants that use it in the process of respiration. The process of oxygen circulation in the biosphere is very complex, since it is contained in so many chemical compounds.

It is estimated that 23% of oxygen is consumed annually for industrial and domestic needs, which is released during photosynthesis.

It is assumed that in the near future all the oxygen produced will be burned in furnaces, and therefore, a significant increase in photosynthesis and other radical measures are necessary.

Biogeochemical nitrogen cycle no less complex than carbon and oxygen, and covers all areas of the biosphere. Its uptake by plants is limited, since they assimilate nitrogen only in the form of its combination with hydrogen and oxygen. And this despite the fact that the reserves of nitrogen in the atmosphere are inexhaustible (78% of its volume). Reducers (destructors), and specifically soil bacteria, gradually decompose the protein substances of dead organisms and turn them into ammonium compounds, nitrates and nitrites. Part of the nitrates enters the groundwater during the cycle and pollutes them.

The danger also lies in the fact that nitrogen in the form of nitrates and nitrites is absorbed by plants and can be transferred along food (trophic) chains.

Nitrogen is returned to the atmosphere again with the gases released during decay. The role of bacteria in the nitrogen cycle is such that if only 12 of their species involved in the nitrogen cycle are destroyed, life on Earth will cease. American scientists think so.

The biogeochemical cycle in the biosphere, in addition to oxygen, carbon and nitrogen, is also carried out by many other elements that are part of organic matter - sulfur, phosphorus, iron, etc.

Biogeochemical cycles of phosphorus and sulfur, the most important biogenic elements are much less perfect, since most of them are contained in the reserve fund of the earth's crust, in the "inaccessible" fund.

Cycle of sulfur and phosphorus ¾ typical sedimentary biogeochemical cycle. Such cycles are easily broken by various kinds of influences, and part of the exchanged material leaves the cycle. It can return again to the cycle only as a result of geological processes or by extracting biophilic components by living matter.

Phosphorus contained in rocks formed in past geological epochs. It can get into the biogeochemical cycle (Fig. 6.11) if these rocks rise from the depths of the earth's crust to the land surface, to the weathering zone. By erosion processes, it is taken out to the sea in the form of the well-known mineral ¾ apatite.

Rice. 6.11. Phosphorus cycle in the biosphere (according to P. Duvigno, M. Tang, 1973; with changes)

The general cycle of phosphorus can be divided into two parts - water and terrestrial. In aquatic ecosystems, it is assimilated by phytoplankton and transmitted along the trophic chain up to third-order consumers ¾ of seabirds. Their excrement (guano) enters the sea again and enters the circulation, or accumulates on the shore and is washed into the sea.

From dying marine animals, especially fish, phosphorus again enters the sea and into the cycle, but some of the skeletons of fish reach great depths and the phosphorus contained in them again enters sedimentary rocks.

In terrestrial ecosystems, phosphorus is extracted by plants from soils and then it spreads through the food web. It returns to the soil after the death of animals and plants and with their excrement. Phosphorus is lost from soils as a result of their water erosion. The increased content of phosphorus in the waterways of its transfer causes a rapid increase in the biomass of aquatic plants, "blooming" of water bodies and their eutrophication. Most of the phosphorus is carried away to the sea and is lost there irretrievably.

The latter circumstance can lead to the depletion of reserves of phosphorus-containing ores (phosphorites, apatites, etc.). Therefore, we must strive to avoid these losses and not wait for the time when the Earth will return the “lost deposits” to the land.

Sulfur also has a main reserve fund in sediments and soil, but, unlike phosphorus, has a reserve fund in the atmosphere (Fig. 6.12). Microorganisms play the main role in the exchange fund. Some of them are ¾ reducing agents, others ¾ oxidizing agents.

Rice. 6.12. Sulfur cycle (according to Yu. Odum, 1975):

The "ring" in the center of the diagram illustrates the processes of oxidation (O) and reduction (R),
due to which sulfur is exchanged between the fund of available sulfate (SO 4)
and a fund of iron sulfides found deep in the soil and in sediments

In rocks, sulfur occurs in the form of sulfides (FeS 2 and others), in solutions ¾ in the form of an ion (SO 4) 2, in the gaseous phase in the form of hydrogen sulfide (H 2 S) or sulfur dioxide (SO 2). In some organisms, sulfur accumulates in its pure form (S 2) and, when they die, deposits of native sulfur are formed on the bottom of the seas.

In the marine environment, the sulfate ion ranks second in content after chlorine and is the main available form of sulfur, which is reduced by autotrophs and included in the composition of amino acids.

The sulfur cycle, although it is required by organisms in small quantities, is key in the general process of production and decomposition (Yu. Odum, 1986). For example, during the formation of iron sulfides, phosphorus is converted into a soluble form available to organisms.

In terrestrial ecosystems, sulfur returns to the soil when plants die, is captured by microorganisms, which reduce it to H 2 S. Other organisms and exposure to oxygen itself lead to the oxidation of these products. The resulting sulfates are dissolved and absorbed by plants from the pore solutions of the soil ¾ so the cycle continues.

However, the cycle of sulfur, as well as nitrogen, can be disturbed by human intervention (see Fig. 6.12). This is primarily due to the burning of fossil fuels, and especially coal. Sulfur dioxide (SO 2 ) disrupts the processes of photosynthesis and leads to the death of vegetation.

Biogeochemical cycles are easily broken by humans. So, extracting mineral fertilizers, it pollutes water and air. Phosphorus enters the water, causing eutrophication, nitrogenous highly toxic compounds, etc. In other words, the cycle becomes not cyclic, but acyclic. The protection of natural resources should be aimed at turning acyclic processes into cyclic ones.

Thus, the general homeostasis of the biosphere depends on the stability of the biogeochemical cycle of substances in nature. But being a planetary ecosystem, it consists of ecosystems of all levels, its integrity and stability of natural ecosystems are of paramount importance for its homeostasis.

Control questions

1. What is the place of the biosphere among the shells of the Earth and what is its fundamental difference from other shells?

2. What do the abiotic and biotic parts of the biosphere as a global ecosystem consist of?

3. What did V. I. Vernadsky understand by the living matter of the planet?

4. What biochemical principles underlie biogenic migration?

5. How is a large circulation of substances, including a large water cycle, carried out in nature?

6. What are the most important functions of living matter provided by a small circulation of substances in nature?

7. What is the role of the reserve and exchange funds in the biogeochemical cycle of substances?

8. What are the features of biogeochemical cycles of the main biogenic elements?

The history of the development of biogeochemical cycles of nitrogen on the planet is complex and contradictory. Nitrogen entered the Earth's planet as a result of condensation of interstellar cosmic protoplanetary matter, which included nitrogen and its various compounds (NO, NH 3 , HC 3 N, etc.).

Radioactive heating of the planet, the formation of a molten mantle was accompanied by the release of gaseous nitrogen compounds and its accumulation in the primary atmosphere, in which N 2 dominates (n · 10 15 t) even now. Cooling lava, gas fumaroles of volcanoes continue to supply nitrogen, its oxides, ammonium chloride and carbon dioxide to the biosphere.

Electrochemical discharges, photochemical reactions, ultrahigh temperatures and pressure contributed to the emergence of non-cellular molecular forms of organic nitrogenous compounds on the planet.

The appearance of free-living nitrogen-fixing bacteria and heterotrophic bacteria probably marked the beginning of the biogenic enrichment of the primary biosphere with nitrogen compounds, the formation of amino acids, proteins, mineral nitrogen compounds (ammonium, nitrate salts). It is possible that biogenic nitrogen fixation preceded the onset of photosynthesis, proceeded in anoxic anaerobic conditions of the distant past, and was carried out by microorganisms of the Clostridium type. Bacteria of this genus are still the most important agents of nitrogen fixation under anaerobic conditions.

Biological nitrogen fixation by microorganisms is much more widespread in nature than it seemed 20-30 years ago. In addition to bacteria of the Rhizobium group, which fix nitrogen in nodule formations on the roots of leguminous plants, nonsymbiotic (associative) nitrogen fixation by numerous heterotrophic bacteria and fungi is widely developed (Umarov, 1983). This type of nitrogen fixation is carried out by hundreds of species of various microorganisms living in the rhizosphere of plants, in the soil and on the surface of stems and leaves (phyllosphere).

On average, associative (non-symbiotic) nitrogen fixation in ecosystems is 40-50 kg/ha per year; but in the world literature there are indications that non-symbiotic nitrogen fixation in the tropics reaches 200-600 kg/ha per year (Umarov, 1983). At the same time, most (> 90%) of the nitrogen mass is fixed in the rhizosphere using the energy of root secretions and dying small roots. Therefore, in the presence of vegetation cover, soils always fix several times more nitrogen than soils of pure fallows.

As established by the studies of Umarov (1983), associative nitrogen fixation is characteristic of most species of herbaceous and many woody plants, including their cultivated forms. Meadow, chernozem and chestnut soils (90-330 kg/ha), as well as mountain forest soils of the Caucasus (up to 180 kg/ha) have a high potential for nitrogen fixation in the rhizosphere. Only during the growing season in the fields, this type of fixation can give soils 30-40 kg/ha of additional nitrogen. This is not surprising, since nitrogen-fixing microorganisms can make up from 20 to 80% of their total population.

There is a clear positive relationship between the processes of nitrogen fixation by microorganisms and plant photosynthesis in ecosystems. The higher the productivity of plant photosynthesis, the more nitrogen is fixed in the soil. This is the most important mechanism of nitrogen biogeochemistry in the biosphere and in agriculture.

The role of blue-green algae in the biogeochemistry of nitrogen is great, numerous species of which also have the ability to fix nitrogen simultaneously with the process of photosynthesis. Blue-green algae (Cyanophyta) enrich soils with nitrogen, especially irrigated rice fields, river, lake and swamp waters and sediments. But they also live on the surface of bare rocks or desert soils.

The development of vegetation cover and plant-associated microorganisms significantly increased the involvement of atmospheric nitrogen in the composition of biomass. The complication of life forms on the planet caused the lengthening of food chains, the accumulation of living and dead organic matter on land and in the ocean. This created the possibility of long-term existence of organic nitrogen compounds in the biosphere and lithosphere. The role of herbaceous plants is especially great in this. The ground and underground parts of herbaceous vegetation annually consume from 20-25 to 600-700 kg / ha of nitrogen (usually the roots contain 2-6 times more nitrogen than the ground part). In this case, the total biomass, as a rule, contains 10-50 times more carbon than nitrogen. All this confirms the enormous overall role of carbon and nitrogen in the creation of phytomass (Titlyanova, 1979). But nitrogen compounds are easily leached from plant tissues by rain moisture. Entering the soil, they are re-consumed by plants.

How complex and little studied are the biogenic cycles of nitrogen, evidenced by the established facts of the transfer of nitrogen compounds from plant to plant (of the same and different species) through root excretions into the soil, and possibly by direct contact of the roots. This amazing mechanism shows how “thrifty” plants are in nitrogen nutrition. This phenomenon probably also exists in the biogeochemistry of other elements.

As is known, the protein content of wheat grain and the content of nitrogen in them increases with a decrease in precipitation in the steppes of the Russian Plain. This has already been established for the content of total nitrogen in the biomass of herbaceous plants. In steppe conditions, the nitrogen content in the dry biomass of grasses reaches 2-2.6%; with increasing humidity, it decreases to 1-1.5%.

All these facts testify to the enormous role of vegetation (especially grasses) and microorganisms in the biogeochemistry of nitrogen on land. The development of the vegetation cover, the emergence of the soil-forming process (300-400 million years ago), the formation of the humus shell and fine soil, its removal and accumulation in the form of sedimentary rocks expanded the process of transferring atmospheric nitrogen into the biosphere, raising its content in the latter to the level n 10 15 t.

At the same time, it must be emphasized that the return of nitrogen to the atmosphere through denitrification is as universal a process as fixation and nitrification. This process ensures the global nitrogen cycle on the planet.

Redox conditions within soils are highly heterogeneous. Even in aerated soils, there are areas of oxygen deficiency where denitrification can occur. The abundance of fresh mobile organic matter and the supersaturation of soils with moisture always sharply intensify the processes of denitrification after rains, during swamping, and during irrigation. Denitrification is even more pronounced in water landscapes (swamps, lakes, estuaries, etc.).

This directed planetary biogeochemical process has a polycyclic character. The predominant part of the nitrogen fixed in nature, through microcyclic repeated transformations, nitrification and denitrification, is ultimately returned in the form of molecular gaseous nitrogen (N 2) to the atmosphere. But as the biosphere developed, the duration of existence and the size of the mass of organic and mineral biogenic nitrogen compounds on the planet increased. The amount of buried organic sediments increased. The duration of individual microcycles of the general terrestrial biogeochemical nitrogen cycle varies in the present era from short (days, weeks, months) in microbial tissues to significant (years) in herbaceous vegetation ecosystems and to long (decades, centuries, millennia) in woody ecosystems and in soil humus. Complete terrestrial cycles of nitrogen found in the sediments of rivers, lakes, seas, in combustible fossils of the earth's crust, cover a time of the order of tens of millennia, hundreds of thousands and millions of years.

The natural biogeochemical cycles of nitrogen (as well as carbon) in the biosphere were "almost closed", but had the character of a directed expanded reproduction of reserves in the biosphere. The biosphere not only did not give up the completely captured masses of nitrogen and carbon, but progressively increased their total reserves in a fixed form (in humus, peat, in the mass of fossil coals, oil, shale, bitumen, etc.).

The anthropogenic epoch has made noticeable changes in the established natural cycles of nitrogen. The main thing that has happened and is happening is (apart from agriculture) the emergence in the biosphere of a new anthropogenic industrial mechanism for fixing masses of nitrogen in the form of tens of millions of tons of nitrogen fertilizers, as well as the release of nitrogen oxides into the environment from large masses of fossil fuels burned (heating plants, transport, aviation). , missiles). Technogenic sources of nitrogen compounds in the biosphere are growing rapidly, doubling every 6-7 years. Already in the 70-80s of the XX century. 50-60 million tons/year of nitrogen fertilizers are produced annually in the world (in terms of nitrogen). At the beginning of the XXI century. this value can reach 100-150 million tons/year. Probably, by this time, the technogenic influx of nitrogen into the biosphere can be equal to or exceed all biogenic forms of its input.

In the anthropogenic era, especially in the modern period, the process of enriching the environment with nitrogen compounds has noticeably increased. As we noted earlier, the process of technogenic nitrogenization of the environment is taking place, accompanied by a complex set of positive (increase in yields, an increase in the proportion of proteins in the diet) and negative (cancer, methemoglobinemia, increased soil acidity and precipitation) consequences. The destruction of forests, steppes (and mycorrhiza), the replacement of legumes by cereals, the destruction of the humus horizons of soils rich in microflora, and the reduction of the soil surface also caused additional changes in the biogeochemistry of nitrogen in the biosphere. All these changes, often of an opposite nature, have not been studied and quantified. Apparently, nevertheless, there is a tendency to reduce the role of biogenic nitrogen fixation in its general circulation on the planet.

It was against this background of disturbances in the normal nitrogen cycle in nature that mineral fertilizers of soils made the above-mentioned changes in the income items of the nitrogen balance and in the geography of its distribution, and also raised the overall level of the concentration of nitrates and ammonium salts in soils and waters. But an even more serious factor in the disruption of the balance, level of concentration and forms of nitrogen compounds in the atmosphere and especially in the hydrosphere and soils turned out to be the modern fuel and energy and transport economy.

According to tentative data, the emission of ammonia and various nitrogen oxides during the combustion of coal, oil, fuel oil, gasoline, peat, shale, etc. together is about 200-350 million tons annually in the form of gases and aerosols. Oxidation of ammonia and nitrogen oxides leads to the formation of mainly nitric acid and partly ammonium salts, which precipitate on land and the ocean surface. Even if these figures are exaggerated even twice, we still have to admit that the emission of nitrogen compounds into the atmosphere has already become a noticeable component in the income items of the nitrogen cycle on our planet.

In the light of these facts, it is necessary to better understand the future needs of agriculture in nitrogen fertilizers, the ways of global, air and water migration of nitrogen compounds on the planet and to find out the areas where the accumulation of nitrate and ammonium compounds occurs predominantly. This is all the more necessary since emissions of nitrogen oxides into the atmosphere will continue and even increase. The facts of precipitation of acidified atmospheric waters in Canada, Scandinavia, and the USA have already been established, which is accompanied by a decrease in the pH of soils and local waters (usually under the influence of combined precipitations with dilute sulfuric acid solutions). Acidification of the environment will increase the weathering of minerals, the removal of calcium, magnesium and other plant nutrients from soils, which will increase the need for liming fields.

One more factor of disturbance of the normal level of concentration and nitrogen cycle in nature should be pointed out. These are wastes from industrial livestock and poultry farming, as well as waste and sewage from modern large cities. Waste and effluents of this origin are very large. There are more than 3 billion head of livestock in the world, producing huge amounts of waste. Modern poultry farms, industrial livestock enterprises, cities create numerous centers of abnormally high content of nitrogen and phosphorus in the form of organic and mineral compounds, which locally saturate soils, streams, rivers, lakes, estuaries and estuaries. Sometimes in such soils, the content of N-NO 3 reaches 400 parts per million (ppm), and N-NH 4 - up to 2200 ppm. According to scientists, urban runoff, animal waste and soil erosion play an equally important, and sometimes even greater role in soil and water pollution with nitrogen compounds to toxic levels (Cooke and Williams, 1970).

An increase in the concentration of nitrogen compounds in natural waters is an alarming fact. In the river waters of forest areas of temperate climate, the content of nitrates reaches 0.3-0.5 mg/l, and in arid climate - 1.2-1.7 mg/l. In the drainage waters of irrigation systems, the concentration of NO 3 is usually about 5-6 mg/l, but sometimes 10-15 mg/l. In soil solutions of saline irrigated soils NO 3 concentrations up to 100-300 mg/l were observed. In groundwater, sometimes there is a concentration of nitrates of the order of 10-15 and even 50-100 mg/l. For 25 years (1945-1970) of regular observations in the state of Illinois, the content of nitrate nitrogen in surface runoff waters, according to the average and maximum data, increased two-three and even four times.

Not only surface waters are enriched with excess concentrations of nitrates, but also underground waters - the main source of drinking water supply for the population. Nitrates penetrate into groundwater to depths of 10-15 m and even more, causing an increase in their concentration to 10-15 mg/l N, which is already clearly dangerous for people (in terms of NO 3, this is 45-60 mg/l).

The total balance of nitrogen for the territory of the USA was calculated (Accumulation of Nitrate, 1972). The total inputs of nitrogen into the US soils are expressed as 21.0 million tons of N per year (including with atmospheric precipitation 5.6 million tons, with mineral fertilizers 7.5 million tons and nutrient fixation 4.8 million tons) . Of this amount, about 17 million tons are used for food production and textile raw materials, and 4 million tons are not used.

All types of denitrification (including more than 10 million tons in the aquatic environment) account for about 18.5 million tons, and about 1.5 million tons annually remain in soils and waters. The data on denitrification are clearly exaggerated here. The residual nitrogen in waters and soils is at least two to three times higher. As a result of considering the elements of the modern biogeochemical cycle of nitrogen on land, the following are outlined: main forms receipt of its compounds:

• biogenic fixation of nitrogen in soils by microorganisms of symbiotic and non-symbiotic types;
• entry into solutions with metabolites of food chains, with dead organic matter, with mineralization products of soil organic matter;
• intake of nitrogen oxides from combustion products of fossil fuels;
• introduction of nitrogen compounds into soils in the form of organic and mineral fertilizers;
• transfer and accumulation of nitrates during the evaporation of groundwater.

Expenditure items The balance of nitrogen on land is composed of the following main forms:

• absorption of mineral nitrogen compounds by higher and lower plants and their entry into the food chains of ecosystems;
• the transition of nitrogen compounds into organic forms with the formation of humus;
• denitrification and eventually return to the atmosphere of most of the nitrogen in the gaseous molecular form of N 2 and partly in the form of oxides and ammonia;
• flushing, removal and alienation of nitrogen compounds from biological cycles to geological ones; burial for a geologically long time in sedimentary rocks, fossil fuels or salt deposits.

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Topic 3.5. BIOGEOCHEMICAL CYCLES OF THE MOST IMPORTANT CHEMICAL ELEMENTS:
CARBON, OXYGEN, NITROGEN, SULFUR, PHOSPHORUS, POTASSIUM, CALCIUM,
SILICA, ALUMINUM, IRON, MANGANESE AND HEAVY METALS

Let's at least in general terms get acquainted with the biogeochemical cycles of such important elements for the biosphere as carbon, oxygen, nitrogen, sulfur, phosphorus, potassium, calcium, as well as elements that are very common in nature, such as silicon, aluminum and iron.

Biogeochemical cycle of carbon.

The carbon content in the Earth's atmosphere is 0.046% in the form of carbon dioxide and 0.00012% in the form of methane. Its average content in the earth's crust is 0.35%, and in living matter it is about 18% (Vinogradov, 1964). The entire process of the emergence and development of the biosphere is closely related to carbon, since it is carbon that is the basis of protein life on our planet, i.e. Carbon is the most important chemical component of living matter. It is this chemical element, due to its ability to form strong bonds between its atoms, that is the basis of all organic compounds.

The index of biogenic enrichment of soils in relation to the earth's crust, and of plants in relation to soils, is 100 and 1000 for carbon, respectively (Kovda, 1985).

The main reservoir of carbon in the biosphere, from which this element is borrowed by living organisms for the synthesis of organic matter, is the atmosphere. Carbon is contained in it, mainly in the form of CO 2 dioxide. A small proportion of atmospheric carbon is included in the composition of other gases - CO and various hydrocarbons, mainly methane CH 4 . But they are unstable in an oxygen atmosphere, and enter into chemical interactions with the formation, ultimately, of the same CO 2 .

From the atmosphere, carbon is assimilated by autotrophic producer organisms (plants, bacteria, cyanobionts) in the process of photosynthesis, as a result of which, on the basis of interaction with water, organic compounds are formed - carbohydrates. Further, as a result of metabolic processes, with the participation of substances coming with aqueous solutions, more complex organic substances are also synthesized in organisms. They are not only used to form plant tissues, but also serve as a source of nutrition for organisms that occupy the next links of the trophic pyramid - consumers. Thus, along the trophic chains, carbon passes into the organisms of various animals.

The return of carbon to the environment occurs in two ways. First, in the process of breathing. The essence of the processes of respiration is the use by organisms of oxidative chemical reactions that provide energy for physiological processes. The oxidation of organic compounds, for which atmospheric or water-dissolved oxygen is used, results in the decomposition of complex organic compounds with the formation of CO 2 and H 2 O. As a result, carbon in the composition of CO 2 returns to the atmosphere, and one branch of the cycle closes.

The second way to return carbon is the decomposition of organic matter. Under the conditions of the biosphere, this process mainly takes place in an oxygen environment, and the same CO 2 and H 2 O are the end products of decomposition. But most of the carbon dioxide does not enter directly into the atmosphere. Carbon released during the decomposition of organic matter remains mainly in dissolved form in soil, ground and surface waters. Either in the form of dissolved carbon dioxide, or as part of dissolved carbonate compounds - in the form of HCO 3 - or CO 3 2- ions. It can, after a more or less prolonged migration, partially return to the atmosphere, but a larger or smaller proportion of it always precipitates in the form of carbonate salts and binds in the composition of the lithosphere.

Part of atmospheric carbon directly enters the hydrosphere from the atmosphere, dissolving in water. Mostly, carbon dioxide is absorbed from the atmosphere, dissolving in the waters of the World Ocean. Part of the carbon, in one form or another, dissolved in the waters of the land, also enters here. CO 2 dissolved in sea water is used by marine organisms to create a carbonate skeleton (shells, coral structures, echinoderm shells, etc.). It is part of the layers of carbonate rocks of biogenic origin, and for a more or less long time "falls out" of the biospheric circulation.

In anoxic environments, the decomposition of organic matter also proceeds with the formation of carbon dioxide as the final product. Here, oxidation proceeds due to oxygen borrowed from mineral substances by chemosynthetic bacteria. But the process under these conditions is slower, and the decomposition of organic matter is usually incomplete. As a result, a significant part of carbon remains in the composition of organic matter that has not completely decomposed and accumulates in the thickness of the earth's crust in bituminous silts, peat bogs, and coals.

Carbon stores are living biomass, humus, limestones and caustobioliths. Natural sources of carbon dioxide, in addition to volcanic exhalations, are the processes of decomposition of organic matter, the respiration of animals and plants, the oxidation of organic substances in soil and other natural environments. Technogenic carbon dioxide is 20 x 10 9 tons, which is still much less than its natural release into the atmosphere. Over the billions of years since the appearance of life on Earth, all the carbon of the atmosphere and hydrosphere has repeatedly passed through living organisms. For only 304 years, living organisms absorb as much carbon as it is contained in the atmosphere. Consequently, the carbon composition of the atmosphere can be completely renewed in just 4 years, and it can be conditionally considered that atmospheric carbon completes its cycle over this period. The cycle of carbon, which is part of the soil humus, is estimated at 300-400 years.

The role of carbon in the biosphere is clearly illustrated by the scheme of its circulation (Fig. 3.5.1).

Rice. 3.5.1. Diagram of the biogeochemical carbon cycle

This scheme clearly shows that plants, using the mechanism of photosynthesis, perform the function of oxygen producers and are the main consumers of carbon dioxide.

However, the carbon cycle is not closed. Which is very important, including for us. This element is often removed from the geochemical circulation for a long time in the form of carbonate rocks, peat, sapropels, coals, and humus. Thus, part of the carbon all the time falls out of the biological cycle, binding in the lithosphere as part of various rocks. Why, then, is there no shortage of carbon in the atmosphere? The reason is that its loss is compensated by the constant supply of CO 2 to the atmosphere as a result of volcanic activity. That is, deep carbon dioxide and carbon monoxide are constantly entering the atmosphere. This allows us to maintain the balance of carbon in the biosphere of our planet.

Human economic activity intensifies the biological cycle of carbon and can contribute to an increase in primary, and, consequently, secondary productivity. But further intensification of technogenic processes may be accompanied by an increase in the concentration of carbon dioxide in the atmosphere. Increasing the concentration of carbon dioxide to 0.07% sharply worsens the breathing conditions of humans and animals. Calculations show that, provided that the current level of extraction and use of fossil fuels is maintained, it will take a little more than 200 years to achieve such a concentration of carbon dioxide in the Earth's atmosphere. In some large cities, this threat is quite real now.

Biogeochemical cycle of oxygen

As you remember, oxygen is the most common element not only in the earth's crust (its clarke is 47), but also in the hydrosphere (85.7%), as well as living matter (70%). This element also plays a significant role in the composition of the atmosphere (more than 20%). Due to its exceptionally high chemical activity, oxygen plays a particularly important role in the biosphere. It determines the redox and alkaline-acid conditions of solutions and melts. It is characterized by both ionic and non-ionic forms of migration in solutions.

The evolution of geochemical processes on Earth is accompanied by a steady increase in the oxygen content. At present, the amount of oxygen in the atmosphere is 1.2x10 15 tons. The scale of oxygen production by green plants is such that this amount could be doubled in 4000 years. But this does not happen, since approximately the same amount of organic matter that is formed as a result of photosynthesis decomposes during the year. In this case, almost all of the released oxygen is absorbed. But due to the openness of the biogeochemical cycle due to the fact that part of the organic matter is preserved and free oxygen gradually accumulates in the atmosphere.

The main "factory" for the production of oxygen on our planet is green plants, although various chemical reactions also occur in the earth's crust, as a result of which free oxygen is released.

Another migration cycle of free oxygen is associated with mass transfer in the system of natural waters - the troposphere. In the water of the ocean there is from 3x10 9 to 10x10 9 m 3 of dissolved oxygen. The cold water of high latitudes absorbs oxygen, and, entering the tropics with ocean currents, releases it into the atmosphere. The absorption and release of oxygen also occurs when the seasons change, accompanied by a change in water temperature.

Oxygen is consumed in a huge number of oxidative reactions, most of which are of a biochemical nature. In these reactions, the energy absorbed during photosynthesis is released. In soils, silts, and aquifers, microorganisms develop that use oxygen to oxidize organic compounds. The reserves of oxygen on our planet are enormous. It is part of the crystal lattices of minerals and is released from them by living matter.

Thus, the general scheme of the oxygen cycle in the biosphere consists of two branches:

• the formation of free oxygen during photosynthesis;
• uptake of oxygen in oxidative reactions

According to the calculations of J. Walker (1980), the release of oxygen by the vegetation of the world's land is 150x10 15 tons per year; release by photosynthetic organisms of the ocean - 120x10 15 tons per year; absorption in the processes of aerobic respiration - 2 10 x10 15 tons per year; biological nitrification and other processes of decomposition of organic matter - 70x10 15 tons per year.

In the biogeochemical cycle, oxygen flows between individual components of the biosphere can be distinguished (Fig. 3.5.2).

Rice. 3.5.2. Scheme of the biogeochemical cycle of oxygen

Under modern conditions, the oxygen fluxes established in the biosphere are disturbed by technogenic migrations. Many chemical compounds discharged by industrial enterprises into natural waters bind oxygen dissolved in water. An increasing amount of carbon dioxide and various aerosols are emitted into the atmosphere. Soil pollution and, especially, deforestation, as well as desertification of land over vast areas, reduce the production of oxygen by land plants. A huge amount of atmospheric oxygen is consumed when fuel is burned. In some industrialized countries, more oxygen is burned than is produced by photosynthesis.

Biogeochemical cycle of hydrogen

In the earth's crust, free hydrogen is unstable. It quickly combines with oxygen to form water, and also participates in other reactions. In addition, due to its negligible atomic mass, it is able to escape into space (dissipate). A significant amount of hydrogen comes to the Earth's surface during volcanic eruptions. Gaseous hydrogen is constantly formed as a result of some chemical reactions, as well as in the process of vital activity of bacteria that decompose organic matter under anaerobic conditions.

Organisms fix hydrogen in the planet's biosphere, binding it not only in organic matter, but also participating in the fixation of hydrogen by the mineral matter of the soil. This becomes possible as a result of the dissociation of acidic metabolic products with the release of the H+ ion. The latter, as a rule, forms a hydronium ion (Н3О+) with a water molecule via a hydrogen bond. When the hydroxonium ion is absorbed by some silicates, they are transformed into clay minerals. Thus, as V.V. Dobrovolsky, the intensity of the production of acidic metabolic products is an important factor in the hypergene transformation of crystalline rocks and the formation of a weathering crust.

Of the cyclic processes on the Earth's surface, in which hydrogen is involved, one of the most powerful is the water cycle: more than 520,000 cubic meters of moisture passes through the atmosphere every year. To create the phytomass of the World land that existed before human intervention, according to V.V. Dobrovolsky (1998) about 1.8x1012 tons of water were split and, accordingly, 0.3x1012 tons of hydrogen were bound.

During the water cycle in the biosphere, the isotopes of hydrogen and oxygen are separated. Water vapor during evaporation is enriched in light isotopes, therefore atmospheric precipitation, surface and ground waters are also enriched in light isotopes compared to ocean waters, which have a stable isotopic composition.

Biogeochemical nitrogen cycle

Nitrogen and its compounds play the same important and indispensable role in the life of the biosphere as carbon. The biophilicity of nitrogen is comparable to the biophilicity of carbon. The index of biogenic enrichment of soils in relation to the earth's crust, and of plants in relation to soils, for nitrogen is 1000 and 10000, respectively (Kovda, 1985).

The main reservoir of nitrogen in the biosphere is also the air envelope. About 80% of all nitrogen reserves are concentrated in the planet's atmosphere, which is associated with the direction of biogeochemical fluxes of nitrogen compounds formed during denitrification. The main form in which nitrogen is contained in the atmosphere is molecular - N 2. As an insignificant impurity, the atmosphere contains various nitrogen oxide compounds NO x, as well as ammonia NH 3. The latter is the most unstable under the conditions of the earth's atmosphere and is easily oxidized. At the same time, the magnitude of the redox potential in the atmosphere is insufficient for the stable existence of oxide forms of nitrogen, therefore its free molecular form is the main one.

Primary nitrogen in the atmosphere probably appeared as a result of degassing processes in the upper mantle and from volcanic ejecta. Photochemical reactions in the high layers of the atmosphere lead to the formation of nitrogen compounds and their noticeable flow to land and the ocean with precipitation (3-8 kg/ha of ammonium nitrogen per year and 1.5-6 kg/ha of nitrate). This nitrogen is also included in the general biogeochemical flow of dissolved compounds migrating with water masses, participates in soil-forming processes and in the formation of plant biomass.

Unlike carbon, atmospheric nitrogen cannot be directly used by higher plants. Therefore, fixative organisms play a key role in the biological cycle of nitrogen. These are microorganisms of several different groups that have the ability, through direct fixation, to directly extract nitrogen from the atmosphere and, ultimately, fix it in the soil. These include:

• some free-living soil bacteria;
• symbiotic nodule bacteria (existing in symbiosis with legumes);
• cyanobionts, which are also symbionts of fungi, mosses, ferns, and sometimes higher plants.

As a result of the activity of nitrogen fixing organisms, it binds in soils in the nitrite form (compounds based on NH 3).

Nitrite nitrogen compounds are able to migrate in aqueous solutions. At the same time, they are oxidized and converted into nitrate salts of nitric acid HNO 3. In this form, nitrogen compounds can be effectively assimilated by higher plants and used for the synthesis of protein molecules based on C-N peptide bonds. Further, along the trophic chains, nitrogen enters the organisms of animals. It returns to the environment (in aqueous solutions and soil) in the processes of excretory activity of animals or the decomposition of organic matter.

The return of free nitrogen to the atmosphere, as well as its extraction, is carried out as a result of microbiological processes. This link in the cycle functions due to the activity of soil denitrifying bacteria, which again convert nitrogen into a molecular form.

In the lithosphere, as part of sedimentary deposits, a very small part of nitrogen is bound. The reason for this is that mineral nitrogen compounds, unlike carbonates, are very soluble. The loss of a certain proportion of nitrogen from the biological cycle is also compensated by volcanic processes. Thanks to volcanic activity, various gaseous nitrogen compounds enter the atmosphere, which, under the conditions of the geographic envelope of the Earth, inevitably passes into a free molecular form.

Thus, the following can be considered the main specific features of the nitrogen cycle in the biosphere:

• predominant concentration in the atmosphere, which plays the exclusive role of a reservoir from which living organisms draw the reserves of nitrogen they need;
• the leading role in the nitrogen cycle of soils and, in particular, soil microorganisms, whose activity ensures the transition of nitrogen in the biosphere from one form to another (Fig. 3.5.3).

Rice. 3.5.3. Scheme of the biogeochemical cycle of nitrogen

Therefore, the biosphere contains a huge amount of nitrogen in a bound form: in the organic matter of the soil cover (1.5x10 11 tons), in plant biomass (1.1x10 9 tons), in animal biomass (6.1x10 7 tons). Nitrogen is also found in large quantities in some biogenic fossils (saltpeter).

At the same time, a paradox is observed - with a huge nitrogen content in the atmosphere, due to the extremely high solubility of nitric acid salts and ammonium salts, there is little nitrogen in the soil and is almost always insufficient for plant nutrition. Therefore, the need for cultivated plants in nitrogen fertilizers is always high. Therefore, according to various estimates, from 30 to 35 million tons of nitrogen are annually introduced into the soil in the form of mineral fertilizers. Thus, the input from nitrogen fertilizers is 30% of the total nitrogen inputs to land and ocean. This often leads to significant environmental pollution and severe diseases in humans and animals. The losses of nitrate forms of nitrogen are especially great, since it is not sorbed by the soil, is easily washed out by natural waters, is restored to gaseous forms, and up to 20-40% of it is lost for plant nutrition. A significant violation of the nitrogen cycle is the ever-increasing amount of animal waste, industrial waste and sewage from large cities, the release of ammonium and nitrogen oxides into the atmosphere during the combustion of coal, oil, fuel oil, etc. The penetration of nitrogen oxides into the stratosphere (exhaust from supersonic aircraft, rockets, nuclear explosions) is dangerous, as this can cause the destruction of the ozone layer. All this naturally affects the biogeochemical cycle of nitrogen.

Biogeochemical sulfur cycle

Sulfur is also one of the elements that play an extremely important role in the circulation of substances in the biosphere. It is one of the chemical elements most necessary for living organisms. In particular, it is a component of amino acids. It predetermines important biochemical processes of a living cell, is an indispensable component of plant nutrition and microflora. Sulfur compounds are involved in the formation of the chemical composition of soils; they are present in significant amounts in groundwater, which plays a decisive role in the processes of soil salinization.

The sulfur content in the earth's crust is 4.7x10-2%, in the soil - 8.5x10-2%, in the ocean - 8.8x10-2% (Vinogradov, 1962). However, in saline soils, the sulfur content can reach values ​​measured in whole percentages. Thus, the main reservoir from which it is drawn by living organisms is the lithosphere. This is due to the fact that the stable existence of sulfur compounds in the modern atmosphere of the Earth, containing free oxygen and H 2 O vapor, is impossible. Hydrogen sulfide (H 2 S) is oxidized in an oxygen environment, and oxygen sulfur compounds, reacting with H 2 O, form sulfuric acid H 2 SO 4, which falls on the Earth's surface as part of acid rain. Therefore, sulfur oxides SO x, although they can be absorbed by plants directly from the atmosphere, this process does not play a significant role in the sulfur cycle.

Sulfur has several isotopes, of which S 32 (> 95%) and S 34 (4.18%) are the most common in natural compounds. As a result of biological and biogeochemical processes, there is a change in the ratio of these isotopes towards an increase in the content of the lighter isotope in the upper humus soil horizons.

The isotopic composition of sulfur in groundwater, soil-groundwater and water-soluble sulfates from horizon C of sulfate-soda solonchaks is similar.

In the composition of the earth's crust, sulfur compounds exist mainly in two mineral forms: sulfide (salts of hydrosulfide acid) and sulfate (salts of sulfuric acid). Native sulfur is rarely found, which is unstable and tends, depending on the values ​​of the redox potential of the medium, to form either oxygen or hydrogen compounds.

The primary, deep in origin, mineral form of sulfur in the earth's crust is sulfide. Sulfide compounds are practically insoluble in the biosphere, and therefore sulfide sulfur is not absorbed by plants. But, at the same time, sulfides in an oxygen environment are unstable. Therefore, sulfides on the earth's surface, as a rule, are oxidized, and as a result, sulfur is included in the composition of sulfate compounds. Sulfate salts have a fairly good solubility, and sulfur in the geographic envelope actively migrates in aqueous solutions as part of the SO 4 2- sulfate ion.

It is in this sulfate form that sulfur, as part of aqueous solutions, is effectively absorbed by plants, and then by animal organisms. Assimilation is facilitated by the fact that sulfate sulfur compounds are able to accumulate in soils, participating in the processes of exchange sorption and, at the same time, being part of the soil absorbing complex (SAC).

The decomposition of organic matter in an oxygen environment leads to the return of sulfur to the soil and natural waters. Sulfate sulfur migrates in aqueous solutions and can be reused by plants. If the decomposition takes place in an oxygen-free environment, the leading role is played by the activity of sulfur bacteria, which reduce SO 4 2- to H 2 S. Hydrogen sulfide is released into the atmosphere, where it is oxidized and returned to other components of the biosphere in sulfate form. Part of the sulfur in a reducing environment can bind in sulfide compounds, which, when the oxygen supply is resumed, are oxidized again and go into the sulfate form.

The biogeochemical cycle of sulfur consists of 4 stages (Fig. 3.5.4):

1. the assimilation of sulfur compounds by living organisms (plants and bacteria) and the inclusion of sulfur in the composition of proteins and amino acids.
2. The transformation of organic sulfur by living organisms (animals and bacteria) into the final product - hydrogen sulfide.
3. Oxidation of mineral sulfur by living organisms (sulfur bacteria, thionic bacteria) in the process of sulfate reduction. At this stage, hydrogen sulfide, elemental sulfur, its thio and tetra compounds are oxidized.
4. Recovery of mineral sulfur by living organisms (bacteria) in the process of desulfification to hydrogen sulfide. Thus, the most important link in the entire biogeochemical cycle of sulfur in the biosphere is the biogenic formation of hydrogen sulfide.

Rice. 3.5.4. Scheme of the biogeochemical cycle of sulfur

The removal of sulfur from the biospheric cycle occurs as a result of the accumulation of sulfate deposits (mainly gypsum), the layers and lenses of which become components of the lithosphere. Losses are compensated, firstly, in the processes of volcanism (the entry of H 2 S and SO x into the atmosphere, and from there, with precipitation, to the Earth's surface). And secondly, as a result of the activity of thermal waters, with which sulfide compounds enter the upper horizons of the earth's crust and the bottom of the World Ocean.

Thus, the characteristic features of the sulfur cycle include the secondary role of atmospheric migration processes, as well as the variety of forms of occurrence due to its transition from sulfide to sulfate forms and vice versa, depending on changes in redox conditions.

Industrial processes release large amounts of sulfur into the atmosphere. In some cases, a significant concentration of sulfur compounds in the air causes environmental disturbances, including acid rain. The presence of sulfur dioxide in the air negatively affects both higher plants and lichens, and epiphytic lichens can serve as indicators of elevated sulfur levels in the air. Lichens absorb moisture from the atmosphere with their entire thallus, so the sulfur concentration in them quickly reaches the maximum permissible level, which leads to the death of organisms.

The entry of sulfur into the general cycle according to J.P. Friend (1976) is as follows:

During degassing of the earth's crust - 12x10 12 g / year; during the weathering of sedimentary rocks - 42x10 12 g / year; anthropogenic inputs in the form of sulfur dioxide - 65x1012 g/year, which in total is 119x10 12 g/year. Significant amounts of sulfur are annually conserved in the form of sulfides and sulfates - 100x10 12 g / year and, thus, are temporarily removed from the general biogeochemical circulation.

Thus, the anthropogenic entry of sulfur into the biosphere significantly changes the circulation of this element, and the entry of sulfur into the biosphere exceeds its consumption, as a result of which, its gradual accumulation should occur.

Biogeochemical cycle of phosphorus.

The cycle of phosphorus in nature is very different from the biogeochemical cycles of carbon, oxygen, nitrogen and sulfur, since the gaseous form of phosphorus compounds (for example, PH 3) practically does not participate in the biogeochemical cycle of phosphorus. That is, phosphorus is generally not capable of accumulating in the atmosphere. Therefore, the role of the "reservoir" of phosphorus, from which this element is extracted and used in the biological cycle, as well as for sulfur, is played by the lithosphere.

Phosphorus in the lithosphere is contained in the form of phosphate compounds (salts of phosphoric acid). The main share among them falls on calcium phosphate - apatite. This is a polygenic mineral, which is formed in various natural processes, both deep and supergene (including biogenic). Phosphate compounds are able to dissolve in water, and phosphorus as part of the PO 4 3- ion can migrate in aqueous solutions. Of these, phosphorus is absorbed by plants.

The index of biogenic enrichment of soils in relation to the earth's crust, and of plants in relation to soils, for phosphorus, as well as for nitrogen, is 1000 and 10000, respectively (Kovda, 1985). For plants, phosphorus of nonspecific organic compounds and humus is the most accessible, and it is he who plays the main role in the small (local) biological cycle of phosphorus.

Animals are even greater concentrators of phosphorus than plants. Many of them accumulate phosphorus in the tissues of the brain, skeleton, shells .. There are several ways to assimilate phosphorus by consumer organisms. First, direct assimilation from plants in the process of nutrition. Secondly, aquatic filter-feeding organisms extract phosphorus from organic suspensions. Thirdly, organic phosphorus compounds are assimilated by sludge-eaters during their processing of biogenic sludge.

The return of phosphorus to the environment occurs during the decomposition of organic matter. But this return is far from complete. In general, phosphorus compounds are characterized by a tendency to be carried out in the form of aqueous solutions and suspensions to the final water bodies of runoff, to the greatest extent - to the World Ocean, where it accumulates as part of sedimentary deposits of various genesis. This part of phosphorus can return to the exogenous circulation again only as a result of tectonic processes stretching over hundreds of millions of years. Under natural conditions, maintaining the balance is ensured by the relatively weak mobility of phosphorus compounds, as a result of which the phosphorus extracted by plants from the soil is mostly returned to it as a result of the decomposition of organic matter. In soils and rocks, phosphorus is quite easily fixed. Phosphorus fixers are hydroxides of iron, manganese, aluminum, clay minerals (especially minerals of the kaolinite group). However, fixed phosphorus can be desorbed by 40-50% and used by plants. This process depends on pH and Eh environmental conditions. Increased acidity, the formation of carbonic acid, contribute to the desorption of phosphorus, enhancing the migration of phosphorus compounds.

Phosphorus compounds with divalent iron are formed in the reducing environment, which also contributes to the removal of phosphorus from the soil.

Migration of phosphorus is also possible due to water and wind erosion. Therefore, the biogeochemical cycle of phosphorus is much less closed and less reversible than the cycles of carbon and nitrogen, and environmental pollution with phosphorus is especially dangerous (Fig. 3.5.5).

Rice. 3.5.5. Scheme of the biogeochemical cycle of phosphorus

The main features of the phosphorus cycle are thus:

• no atmospheric transport;
• the presence of a single source - the lithosphere;
• tendency to accumulation in the final reservoirs of runoff.

With intensive agricultural exploitation of land, the loss of phosphorus in the landscape becomes almost irreversible. Compensation is possible only through the use of phosphate fertilizers. It is known that phosphate fertilizers are an important and necessary link in obtaining high yields of agricultural crops. However, all known reserves of phosphate deposits are limited and, according to scientists' predictions, they can be depleted in the next 75-100 years. At the same time, harmful phosphate compounds have recently become one of the most important factors in the pollution of river and lake waters.

Thus, in recent times, the general picture of the distribution of phosphorus migration by him in the biosphere has been sharply disturbed by man. Here are the components of this phenomenon: firstly, the mobilization of phosphorus from agricultural ores and slags, the production and use of phosphate fertilizers, and secondly, the production of phosphorus-containing preparations and their use in everyday life; thirdly, the production of phosphorus-containing food and feed resources, their export and consumption in areas of population concentration; fourthly, the development of fisheries, the extraction of marine mollusks and algae, which entails the redistribution of phosphorus from the ocean to land. As a result, the process of land phosphatization is observed, but this process manifests itself extremely unevenly. The content of phosphorus in the environment of large cities is increasing. On the contrary, countries that actively export organic products and do not use phosphate fertilizers are losing phosphorus reserves in their soils.

Biogeochemical cycles of potassium and sodium

The clarke of potassium in the earth's crust is 2.89, and that of sodium is 2.46, i.e. their relative contents are very close.

Potassium consists of a mixture of 3 isotopes: 39 K - 93.08%; 40 K -0.0119%; 41 K - 6.91%. The 40K isotope is unstable and transforms into neighboring isobars of calcium and argon.

The conversion of potassium into argon was the basis for the development of the potassium-argon method of nuclear geochronology.

The cosmic abundance of potassium, as an odd element, is small compared to even calcium and oxygen. In terms of size, the potassium ion is the largest among the other leading cations of the lithosphere. Therefore, the bulk clarke of potassium ranks second after oxygen in the earth's crust.

Potassium is a reactive metal that does not occur in its native state. In all chemical compounds on Earth, it acts as a monovalent metal. Potassium metal “burns out” in air, quickly oxidizing to K 2 O. The number of mineral species is 115 (three times less than that of calcium and half that of sodium). The most important minerals: halides - sylvin, carnallite, nitrates - K- saltpeter, silicates - K-feldspars (orthoclase, microcline), phlogopite, muscovite, biotite, glauconite, leucite. By chemical properties, potassium is close to sodium, which determines their joint migration. But their behavior in the zone of hypergenesis and the biosphere as a whole is sharply different. Most of the potassium during the hypergene transformation of silicates remains in the composition of secondary clay minerals, so potassium is much more firmly retained within the World land than sodium and, as we will see below, calcium. Nevertheless, a partial release of potassium ions in the processes of hypergenesis occurs and it is actively involved in the biogeochemical cycle.

This is due to the fact that potassium plays a very important role in the life of living organisms. In a humid climate, during the weathering of potassium-containing minerals, potassium is easily leached and carried by aqueous solutions. However, the removal of potassium in the weathering crust is less intense than that of calcium and sodium. This is due to the fact that a large potassium ion is sorbed to a greater extent by finely dispersed minerals. It has long been known that potassium ions are more easily sorbed by some colloids (for example, iron and aluminum hydroxides) than sodium ions. Cation exchange reactions with clay minerals also promote potassium fixation. In soils, there is also an exchange between potassium and hydroxonium ions, which have comparable ionic sizes. In this way, potassium can be fixed in hydromicas, kaolinite, montmorillonite. Potassium is absorbed to a greater extent than sodium by terrestrial vegetation.

Therefore, a significant part of the potassium is stored in soils, while most of the sodium is carried into the ocean. In the composition of the runoff from the continents, sodium is almost 2.5 times more than potassium.

Potassium is an essential element of living organisms. They contain 0.1 to 0.01% potassium. In the ashes of cultivated plants up to 25-60% K 2 O. Some organisms are able to concentrate potassium in significant quantities. So, in some algae, the potassium content reaches 3% of live weight. Land plants absorb potassium from the soil. With a lack of potassium, the leaves turn pale and die, the seeds lose their germination. Potassium easily penetrates into the cells of organisms and increases their permeability to various substances. It has a significant effect on metabolism and is necessary for plants for photosynthesis. In addition, potassium improves the flow of water into plant cells and reduces the evaporation process, thereby increasing the resistance of plants to drought. With a lack or excess of potassium, the intensity of photosynthesis decreases, and the intensity of respiration increases. The lack of potassium in soils leads to a significant decrease in plant productivity.

That is why the clarke of potassium in living matter is as high as that of nitrogen. Especially a lot of potassium is accumulated by some seaweeds (up to 5%).

About 1.8 x 109 tons of potassium are involved in the biological cycle on land every year (Dobrovolsky, 1998). The mass of potassium released from the system of biological circulation on land is partially retained in dead organic matter and sorbed by the mineral matter of the soil (clay minerals), and partially involved in water migration.

The amount of potassium currently bound in the dead organic matter of the pedosphere is, according to various authors, from 3x109 to 6x109 tons. More than 61x106 tons of potassium in the dissolved state (in the form of free ions) and 283x106 tons of potassium in suspension (clay particles, organic matter, etc.) enter the ocean annually with the continental water runoff. Potassium also actively migrates in the ocean surface-atmosphere system as part of aerosols: the average concentration of this element in atmospheric precipitation over the ocean is 15%. The concentration of potassium in precipitation over the continents is noticeably higher, 0.7% on average. A significant amount of potassium is transported with dust from land to the ocean. According to V.V. Dobrovolsky, this value is at least 43x10 6 tons per year.

In the hypergene zone, large concentrations of potassium are rare and are represented by evaporites - sylvite and carnallite. Even less common are potassium nitrates in the form of potassium nitrate of organogenic origin (formed in arid climates).

The clarke of sodium in living matter is very low - 0.008 (more than two orders of magnitude lower than that of potassium), which indicates a low consumption of sodium by living matter. However, sodium is needed in small amounts by all living organisms.

In a humid climate, sodium easily leaves the biological cycle and is carried out of the landscape with liquid runoff. As a result, a general depletion of the latter in sodium is observed. The sodium content in plant organisms is usually very low. Animal organisms need increased amounts of this element, since it is part of the blood. Affects the activity of the cardiovascular system and kidneys. Therefore, animals sometimes need to be fed with table salt.

In dry climates, sodium is concentrated in ground and lake waters and accumulates in saline soils (evaporative barrier action). Accordingly, the vegetation of halophyte communities also contains increased amounts of sodium.

However, the role of the biological cycle of sodium, unlike potassium, is relatively small. But its water migration is very significant. According to the characteristics of migration in the biosphere, sodium is very similar to chlorine. It forms easily soluble salts, therefore it accumulates in the oceans and participates in atmospheric migration.

The main source of mobile sodium in the biosphere is weathering igneous rocks (the main source of chlorine is volcanism).

Technogenesis has made significant adjustments to the biogeochemical pathways of sodium migration. Of primary importance is the extraction of halite (common salt), soda and mirabilite. Irrigation of lands in arid regions also has a significant effect on the nature of the biogeochemical cycles of sodium.

Biogeochemical cycles of calcium and magnesium.

Calcium atoms contain the magic number of protons: 20 in the nucleus and this determines the strength of its nuclear system. Among the light elements, calcium is represented by the maximum number of stable isotopes - 6, which have a distribution: 40 Ca - 96.97% (double magic Z \u003d N \u003d 20) 42 Ca - 0.64, 43 Ca - 0.145, 44 Ca - 2.06, 46 Ca-0.0033, 48 Ca -0.185%. In terms of distribution in the solar system, it takes 15th place, but among metals it is in 5th place.

In nature, it behaves like a reactive metal. Easily oxidized to form CaO. In geochemical processes, it acts as a doubly charged Ca + 2 cation

Its ionic radius is very close to that of sodium. The number of mineral species is 390, so it belongs to the main mineral-forming elements. In terms of the number of minerals formed, it ranks 4th after oxygen, hydrogen and silicon. For example: carbonates - calcite, aragonite, dolomite; sulfates - anhydrite, gypsum; halides - fluorite; phosphates: apatite; silicates - garnets, pyroxenes, amphiboles, epidote, plagioclases, zeolites.

Plagioclases are the most common minerals in the earth's crust. The calcium clarke in the lithosphere is 2.96. Calcium silicates are weakly stable in the hypergenesis zone and are the first to be destroyed during the weathering of rocks.

Calcium has a relatively high migration capacity, largely determined by the climate. In chemical weathering processes, calcium is leached from minerals by natural waters. In relation to weathering, calcium minerals form the following sequence: plagioclase - calcium augite - calcium amphibole. In the plagioclase group, calcium-rich varieties are weathered faster than sodium ones. At the same time, natural solutions that actively remove calcium contain significant amounts of bicarbonate ion. On the other hand, in the soils of the humid zones, a significant deficiency of calcium is observed. There is very little of it in the weathering crusts. This is explained by the high migration mobility of this element.

In the ion sink from the continents, calcium occupies the first place among the cations. It is transported by rivers mainly in the form of suspensions of carbonates, sulfates and bicarbonate in a dissolved state. The geochemical history of calcium in the ocean is related to the carbonate equilibrium system, water temperature, and the activities of living organisms.

Calcium is one of the most important elements of living organisms - from the simplest to higher mammals. Cold waters of high latitudes and sea depths are undersaturated with CaCO 3 due to low temperatures and pH, therefore carbonic acid contained in water dissolves CaCO 3 of bottom sediments. That is why marine organisms at high latitudes avoid building their skeletons from CaCO 3 . In the equatorial latitudes, a region of CaCO 3 supersaturation has been established. There is a massive growth of coral reefs here, many organisms living here have massive carbonate skeletons and shells.

The migration of calcium in the ocean with the participation of living organisms is the most important link in its circulation. According to A.P. Vinogradov rivers annually bring into the ocean and 1 * 10 15 tons of CaCO 3. Where does he go? Approximately the same amount of it is annually buried in the bottom sediments of the ocean. Ocean living organisms concentrate calcium in the form of aragonite and calcite. Aragonite, however, is unstable and eventually transforms into calcite. In the ocean, we are faced with unique phenomena of the rapid growth of large crystals in individual organisms. In some shells of bivalve mollusks, calcite crystals more than 7 cm long are found; sea urchins with long calcite needles live in tropical seas. In many echinoderms, adaptation of the living body of organisms to the form of crystals is observed. In this case, we meet a special kind of symbiosis between organisms and crystals.

In an arid climate, calcium easily precipitates from solutions in the form of carbonates, forming strata of chemogenic carbonate rocks and illuvial-carbonate horizons in soils.

A small part of the calcium ions of sea water is deposited in closed reservoirs under evaporite conditions by chemical means.

Calcium plays an important role in soil formation processes. It is a part of the soil-absorbing complex, participates in the exchange reactions of the soil solution, causing the buffer capacity of soils in the acid range of the environment. Calcium humates play an important role in the formation of soil structure. In addition, calcium is actively involved in the precipitation of sesquioxides and manganese, often forming concretions together with these elements and silica.

In soils of the acid series, characterized by a significant manifestation of the leaching process, the phenomenon of biogenic accumulation of calcium in the litter and accumulative surface horizons of soils is observed. It belongs to the group of biophilic elements. Therefore, calcium is actively involved in the biological cycle. The scale of calcium involvement varies significantly in different natural zones.

In agricultural landscapes, a significant part of calcium is alienated along with the harvest.

But the violation of the biogeochemical cycle of calcium at present occurs not only and not so much due to the alienation of part of it with agricultural products, but also due to the use of carbonate rocks in construction, agriculture (liming the soil), and the metallurgical industry.

The magnesium clack is inferior to the calcium clack at 1.87, but the distribution of magnesium is very heterogeneous. In size, the magnesium ion is close to the ions of divalent iron and nickel and, together with them, is included in the composition of olivines and pyroxenes, concentrating in basic and especially ultrabasic igneous rocks.

At the same time, magnesium accumulates in the ocean and salt lakes and migratory capacity approaches such elements as sodium and potassium. This is due to the good solubility of magnesium chlorides and sulfates. Unlike other alkaline earth and alkali metals, magnesium, due to the small size of the ions, easily enters the crystal lattice of clay minerals, forming secondary magnesian aluminosilicates.

Magnesium is a biophilic element. It is part of chlorophyll, which, with a lack of this element, is destroyed. The plant reacts to a lack of magnesium in the soil by the outflow of chlorophyll from old leaves to young ones. Movement is along the veins of the leaf. Therefore, they remain green for a long time, while the interveinal areas of the leaf turn yellow. Animal diseases are also known. Associated with magnesium deficiency. However, the biophilicity of magnesium is less than that of calcium and potassium.

In humid landscapes, magnesium, like calcium, is leached from soils, although its mobility is lower. Than calcium. This is due to the action of several geochemical barriers. First, magnesium is actively absorbed by living matter; secondly, it, like potassium, enters the crystal lattices of secondary silicates and, finally, is sorbed by clay colloids and humus. Nevertheless, a significant part of magnesium is removed with liquid runoff, and in the composition of ground and river waters, magnesium is in second place after calcium.

Under arid conditions, the distribution of magnesium is affected by the high solubility of its chlorides and sulfates. As a result, the accumulation of these salts on the evaporation barriers and the formation of solonchaks are observed.

Magnesium enters the ocean from weathering rocks and the scale of this supply is significant (especially in the past). According to V.M. Goldschmidt, over the course of geological history, 12.6 g of magnesium entered the ocean from the continents for every kilogram of ocean water. However. The magnesium content in the water of modern oceans is only 1.3 g. This is due to the repeated participation of each magnesium atom in a large geological cycle, the deposition of dolomites and other sedimentary rocks containing magnesium.

The migration of magnesium has changed significantly over the course of geological history. If Precambrian limestones contain up to 12.6% magnesium, then modern ones contain only 1%. The formation of dolomites in the open seas ceased at the end of the Paleozoic. Currently, dolomites are deposited only in some lagoons.

The technophilicity of magnesium is still much lower than that of calcium and sodium. Until the beginning of the 20th century, only dolomite and magnesite were used. It is only recently that alloys containing magnesium have been widely used. In landscapes depleted in magnesium, its insignificant accumulation is observed due to the introduction of magnesium-containing fertilizers and soil liming with the use of dolomite.

Thus, in general, the biogeochemical cycles of all alkali and alkaline earth metals are characterized by the openness of global annual cycles. As a result, an intensive accumulation of these elements in the sediments of the World Ocean is observed: up to 99% of calcium, 98% of potassium and over 60% of sodium are concentrated according to V.V. Dobrovolsky in sedimentary rocks.

Biogeochemical cycle of silicon.

Silicon is the second most abundant chemical element (after oxygen) in the earth's crust. Its clarks in the earth's crust are 29.5, in the soil - 33, in the ocean - 5x10-5. However, despite the enormous abundance of silicon and its compounds in nature (quartz and silicates make up 87% of the lithosphere), the biogeochemical cycles of silicon (especially on land) have not yet been studied enough.

No wonder V.I. Vernadsky believed that no organism in the biosphere can exist without silicon, which is necessary for the formation of cells and tissues of plants and animals, their skeletons. Living matter extracts silicon from natural waters and soils for nutrition and the functioning of biochemical processes, then releasing it with excrement and upon death. As a result of the death of billions of organisms, huge masses of silica are deposited at the bottom of water bodies. This is how the biogeochemical cycle of silicon is formed. IN AND. Vernadsky emphasized that the history of silica cannot be understood without studying the results of the vital activity of organisms.

M.Strakhov proved the possibility of exclusively biogenic extraction of SiO2 from surface waters. However, the supply of dissolved silica into the ocean from land is insufficient for the normal development of phytoplankton. That is why organisms with a siliceous skeleton are poorly developed in the temperate and tropical latitudes in the ocean. With the current saturation of water with silica, for the normal development of diatom phytoplankton, each silicon atom must be used many times during the year (tens and even hundreds of times). Of the entire mass of silica produced in the surface photosynthetic layer, no more than 0.1 part reaches the bottom sediments, and often it is only 0.05-0.01 part. The rest of the silica again becomes water-soluble. Later, it is captured from the water by new generations of diatoms, siliceous sponges and radiolarians. Nevertheless, the 0.1-0.01 part of the remnants of diatom plankton skeletons reaching the bottom leads to significant accumulations of sedimentary siliceous rocks. This branch of the silicon cycle is relatively static and irreversible, and part of the silica is removed from the biogeochemical cycle in this way.

For us, another, more dynamic branch of the cycle, which is actually cyclical, is more important. This is the silicon that passes from phytoplankton organisms into the environment and back many times a year. In these transitions, the most important function of the water biogeochemical cycle of silicon is manifested - the function of mass and energy transfer of matter from the surface to deeper zones of the World Ocean.

The second feature of the biogeochemical cycle of silicon in the World Ocean is its inextricable link with carbon.

The continental branch of the silicon cycle is complex. The water migration of silica is closely related to landscape and geochemical conditions: the composition of vegetation, and the lithology of the underlying sediments. The mobility of silica increases sharply with an increase in the pH of the medium, especially in the alkaline range. At pH=10-11 silica concentration can reach 200 mg/l. Strongly increases the solubility of amorphous silica and the rise in temperature. Sulfates, bicarbonates and carbonates of magnesium and calcium sharply reduce the solubility of silica and cause its precipitation. In a strongly acidic environment pH=1-2, the solubility of silica also greatly increases. Some plants are silicon concentrators.

A powerful mechanism that drives this cycle is the vegetation cover of the land, in which various processes of formation of silicon-containing organogenic minerals (bioliths) take place. In this case, bioliths are understood as minerals that are formed inside the body in the course of its vital activity. Their role in the silicon cycle is extremely important, but not well understood. Basically, silica encrusts cell walls. Most silica bioliths contain cereals, sedges, horsetails, ferns, mosses, palm trees, pine needles, spruces, leaves and bark of elm, aspen, and oak. According to Parfenov and Yarilov, the silica content in feather grass ash can reach 80%. In bamboo trunks, formations composed of opal are sometimes found, reaching a length of 4 cm and weighing up to 16 g! The genesis of soil silicic acid under certain conditions is directly related to the accumulation of this element by living organisms. The most striking example is the formation of solods, the silicic acid of which was accumulated due to the activity of diatoms. During the vital activity of blue-green algae, iron, manganese and silica are “captured” with the formation of bioliths. The ratio of the processes of accumulation and removal of silica under the conditions of the temperate zone is shifted towards accumulation. Land vegetation, especially coniferous forests, acts as a powerful mechanism that pumps masses of silica from rocks, soils and natural waters and returns them back to the landscape in the form of bioliths. In the future, the opal of bioliths passes into chalcedony and even into secondary quartz. A significant part of the silicic acid of bioliths is included in active migration in soil and ground waters in the form of colloidal and true solutions.

As a result of the impact of silica aerosols on living organisms (animals and humans), a serious disease develops - silicosis.

Biogeochemical cycles of aluminum, iron and manganese

As you already know, aluminum is one of the three most common elements in the earth's crust. His Clark is 8.05. Iron is the second most abundant metal after aluminum and the fourth among all elements of the earth's crust. His clark is 4.65. The content of manganese in the earth's crust is much lower than -0.1%. These two elements occupy adjacent places in D.I. Mendeleev and have a similar structure of electron shells. However, manganese migrates more actively, because the pH value at which its hydroxide precipitates is higher than for iron. Iron and manganese are actively involved in the biological cycle, as they are part of many enzymes. Iron is involved in the formation of chlorophyll and is part of hemoglobin. Manganese takes part in redox reactions - respiration, photosynthesis and nitrogen assimilation. The participation of aluminum in the biological cycle is limited. Although it is the most common metal in the earth's crust, its biophilicity is very low, the clarke of living matter is only 5x10-3.

The biogeochemical cycles of iron and manganese depend to a decisive extent on the conditions of moisture, the reaction of the environment, the degree of soil aeration, and the conditions of decomposition of organic matter. The migration of aluminum is less dependent on redox conditions, since it has a constant valency. At the same time, the amphoteric nature of this element determines the strong dependence of its migration on the acid-base conditions of the environment: in a strongly acidic medium, it behaves as a cation, and in a strongly alkaline medium, as an anion. In neutral and slightly alkaline waters of steppes and deserts, it almost does not migrate, the highest mobility of this metal is in strongly acidic waters of areas of active volcanism and zones of oxidation of sulfide deposits. Under the protection of organic colloids, aluminum actively migrates in swamp waters. However, the migration rate of aluminum is generally much lower than that of iron and manganese, and its minerals are more stable. The low mobility of aluminum determines the residual (due to the removal of more mobile elements) accumulation of its hydroxides in the weathering crust of the humid tropics and the formation of bauxites.

It is known that compounds of aluminum, iron, and manganese in soils with leaching regime migrate in the vertical direction and form illuvial horizons enriched in sesquioxides and manganese. Many researchers have proved that the migration of sesquioxides under the conditions of the leaching type of water regime occurs in the form of highly dispersed sols stabilized by acid humus. In this case, an important role is played by the creation of an anaerobic environment, which causes the formation of compounds of ferrous iron and manganese. Of decisive importance are aggressive fulvic acids, which destroy soil minerals and form easily mobile complex compounds with aluminum, iron and manganese.

Iron and manganese compounds actively migrate with lateral soil runoff, forming accumulations of nodules in swamps. Meadow and gley soils, shallow lakes and lagoons. This indicates the ability of these compounds to migrate over very long distances. The precipitation of iron in accumulative landscapes occurs in the form of iron carbonates, oxides of varying degrees of hydration, as well as phosphates and humates. In steppes and deserts, under alkaline conditions, these elements migrate weakly.

Migration of iron and manganese is also possible in the composition of living matter. After the death of organisms and their mineralization in the soil, some of these elements are fixed in the soil, while the other part enters natural waters. Returning to the soil, they begin a new biogeochemical cycle.

As a result of weathering processes, iron is carried out into the oceans in large quantities. The removal of iron by rivers into the ocean occurs in various forms - in the form of coarse suspensions of fragments of minerals and rocks containing iron in the crystal lattice (silicates, including clay minerals), in the form of colloids containing iron in the absorbed state, in the form of hydrates, humates and organic compounds of ferrous iron.

Iron deficiency in plants leads to a disease known as chlorosis. However, the direct accumulation of iron in significant amounts is characteristic of only a few organisms. Iron bacteria are unique in this respect, oxidizing ferrous iron, resulting in the formation of limonite. Diatoms are able to absorb iron from insoluble colloids. Iron is also consumed by zooplankton with red blood (small crustaceans). With the death of these organisms and the dissolution of detrital parts, a certain amount of iron also passes into solution in the form of hydrates and other forms. As a special case of iron concentration by organisms, the presence of magnetite and goethite in the teeth of some modern gastropods can be noted.

The biogeochemical cycle of iron and manganese is significantly disturbed by technogenic processes, and, despite the significantly higher content of iron in the earth's crust, the technophilicity of these elements is approximately equal. In the noosphere, aluminum plays an extremely important role, but its technophilicity is almost 100 times lower than that of iron.

Biogeochemical cycles of heavy metals.

Heavy metals are usually called chemical elements having an atomic mass of more than 50 units. Despite the relatively low abundance of these elements in nature, they have a great influence on biogeochemical processes in the biosphere. Since many of them have a pronounced toxic effect on living organisms.

Numerous studies have established that the following 9 elements are the most toxic: Cr, As, Ni, Sb, Pb, Vo, Cd, Hg, Ta. Polish scientists have ranked heavy metals according to their pollution potential into 4 groups. The group of elements with a very high pollution potential includes cadmium, mercury, lead, copper, thallium, tin, chromium, antimony, silver, and gold.

Bismuth and uranium belong to the group of elements with a high pollution potential. Molybdenum, barium, manganese, titanium, iron, selenium, tellurium. The group of elements with an average pollution potential includes fluorine, beryllium, vanadium, rubidium, nickel, cobalt, arsenic, germanium, indium, cesium, tungsten. Elements with a low pollution potential - strontium, zirconium, lanthanum, niobium.

As you can see, 4 metals from the first group (with a very high pollution potential) are lead, mercury, cadmium and chromium.

To a certain extent, every major city is the cause of biogeochemical anomalies, including those dangerous for humans.

It is well known that lead and zinc accumulate in high traffic areas, along highways and in industrial centers. Soils in rural areas contain 10-20 times less lead. Than the soil of cities. Lead has the ability to accumulate in soil organic matter.

The availability of heavy metals to plants depends on the plant species, soil and climatic conditions. In each plant species, the concentrations of heavy metals can vary in different parts and organs, and also depend on the age of the plants.

Soil factors that significantly affect the availability of heavy metals to plants include: granulometric composition, reaction of the soil environment, organic matter content, cation exchange capacity and drainage. In heavier soils, there is less danger of possible adsorption of excess (toxic) amounts of heavy metals by plants. With an increase in the pH of the soil solution, the probability of the formation of insoluble hydroxides and carbonates increases. It was believed that in order to minimize the availability of toxic metal in the soil, it is necessary to maintain a pH of at least 6.5. Metals can form complex compounds with soil organic matter, and therefore, in soils with a high humus content, they are less available for uptake by plants. The exchange capacity of cations depends mainly on the content and mineralogical composition of the clay part of soils and the content of organic matter in them. The higher the exchange capacity of cations, the greater the holding capacity of soils in relation to heavy metals.

Excess water in the soil contributes to the appearance of metals with low valence in a more soluble form.

Priority pollutants of the biosphere are mercury, lead, cadmium, zinc, and copper. An increase in their concentration in water, soil, air and biota is a direct indicator of danger to animals and humans.