Biogeochemical cycles (biogeochemical cycles) are cyclic metabolic processes between various components of the biosphere, caused by the vital activity of organisms. Features of the circulation of water and some substances in the biosphere Biogeochemical c

The circulation of chemical elements (substances) in the biosphere is called biogeochemical cycles.The exchange of chemical elements between living organisms and the inorganic environment is called biogeochemical cycle, or biogeochemical cycle. Living organisms play a decisive role in these processes. The elements necessary for life are conventionally called biogenic (life-giving) elements, or nutrients. There are two groups of nutrients:

• Macrotrophic substances include elements that form the chemical basis of the tissues of living organisms. This carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur.
• Microtrophic substances include elements and their compounds that are also necessary for the existence of living systems, but in extremely small quantities. Such substances are often called microelements. This iron, manganese, copper, zinc, boron, sodium, molybdenum, chlorine, vanadium and cobalt. Lack of microelements can have a strong impact on living organisms (in particular, limit the growth of plants), as well as a lack of nutrients.

Biogeochemical carbon cycle

Due to their participation in the cycle, nutrients can be used repeatedly. The reserves of nutrients are variable: some of them are bound and are part of the living biomass, which reduces the amount remaining in the ecosystem environment. And if plants and other organisms did not eventually decompose, the supply of nutrients would be exhausted and life on Earth would cease. From this we can conclude that the activity of heterotrophic organisms, primarily decomposers, is a decisive factor in maintaining the cycle of nutrients and preserving life.

Global carbon cycle. Numbers - millions of billions of grams (10 15 g); for funds - on average, for flows - per year

Consideration of the characteristics of BHC cycles of several important elements should, naturally, begin with carbon. Carbon is the basis of organic compounds, and therefore the carbon cycle is of particular importance to living organisms. The most important feature of this cycle is the presence of reserves of CO 2, carbon dioxide, in the atmosphere, from which living organisms can draw it. The movement of carbon through living organisms is closely related to the movement of other nutrients. For example, the ratio of carbon to nitrogen fluxes through living matter is approximately 6:1 (six carbon atoms to one nitrogen atom), and the ratio of carbon to phosphorus fluxes is approximately 100:1. Naturally, this reflects the ratio of the elements themselves in the composition of living matter.

Industrial emissions of carbon monoxide (CO) into the atmosphere equal or even exceed natural emissions.

The carbon cycle is particularly important because of its influence on climate. Carbon dioxide and methane are the most important greenhouse gases. CH 4 stabilizes the ozone layer and is an important greenhouse gas. Methane is released by swamps and shallow waters, as well as by the intestinal endosymbionts of ruminants. Methods are now being developed to combat them using antibiotics. The result of such a struggle is an increase in live weight gain and a decrease in the greenhouse effect in the atmosphere (i.e., to some extent, inhibition of global warming).

Biogeochemical nitrogen cycle

The circulation of nutrients is usually accompanied by their chemical transformations. Nitrate nitrogen, for example, can be converted into protein nitrogen, then converted into urea, converted into ammonia and again synthesized into the nitrate form under the influence of microorganisms. The biochemical nitrogen cycle involves various mechanisms, both biological and chemical. The diagram of nitrogen circulation in the biosphere is shown in the figure.

Biogeochemical cycle of phosphorus

One of the simplest cycles is the phosphorus cycle. The main reserves of phosphorus contain various rocks, which gradually (as a result of destruction and erosion) release their phosphates to terrestrial ecosystems. Phosphates are consumed by plants and used for the synthesis of organic substances. When animal carcasses are decomposed by microorganisms, phosphates are returned to the soil and then used again by plants. In addition, part of the phosphates is carried out with the water flow into the sea. This ensures the development of phytoplankton and all food chains involving phosphorus. Some of the phosphorus contained in seawater can return to land in the form of guano - the excrement of seabirds. Where they form large colonies, guano is mined as a very valuable fertilizer.

Some organisms may play an extremely important role in the phosphorus cycle. Mollusks, for example, by filtering water and extracting small organisms and their remains, capture and retain large amounts of phosphorus. Despite the fact that the role of mollusks in the food chains of coastal marine communities is small (they do not form dense aggregations with high biomass, their nutritional value is low), these organisms are of paramount importance as a factor in preserving the fertility of the sea zone where they live. Populations of mollusks are similar to natural batteries, only instead of electricity, they accumulate and retain phosphorus, which is necessary to support life in coastal zones of the seas. In other words, the population of these organisms is more important for the ecosystem as a “mediator” in the exchange of matter between living and inanimate nature (community and biotope).
This example is a good illustration of the fact that the value of a species in nature does not always depend on indicators such as its abundance or raw material qualities. This value can only manifest itself indirectly and is not always revealed by superficial examination.

Nitrogen makes up almost 78% of the atmosphere's mass. The main part of it forms N2 molecules from two atoms. Most organisms are not able to use this nitrogen due to the strong bond of the atoms. They require nitrogen in chemical forms such as ammonia, ammonium ions, nitrate and nitrite ions, which participate in chemical reactions with oxygen. Therefore, fixed nitrogen is important for this biogeochemical cycle.

The natural biogeochemical cycle of nitrogen is shown in Fig. 16. The total nitrogen flow into the biosphere is about 14·10 10 t/year. The main supplier of fixed nitrogen is nitrogen-fixing bacteria. The most famous of them are found in the nodules of legume plants. The traditional method of increasing fertility is based on their activities. Legumes are grown in the field, then they are plowed, and the nitrogen accumulated in the nodules passes through the soil. The following year, the field is sown with other crops that use this nitrogen. Some nitrogen binds during thunderstorms. An electrical discharge heats the air to a temperature at which various nitrogen oxides are formed. As in the case of carbon, a certain amount of nitrogen compounds comes from the depths of the Earth.

The reverse process - the reduction of nitrate ions is carried out by a chain of bacteria:

· ammonifying bacteria decompose nitrogenous organic compounds, forming ammonia (NH 3) or ammonium ions (NH 4 +);

· Nitrifying bacteria oxidize ammonia into nitrous acid – NO 2 –. (nitrites);

· nitrate bacteria convert nitrous acid into nitric acid – NO 3 – (nitrates) and the cycle begins again.

Rice. 15. Biogeochemical nitrogen cycle

Anthropogenic nitrogen flow into the biosphere approximately equal to natural. The greatest contribution comes from the use of nitrogen fertilizers (8·10 10 t/g). The consequence may be an increase in the content of nitrites, nitrates and nitrosamines in products with a wide range of toxic effects.

The source of nitrogen oxides (2·10 10 t/g) are many metallurgical processes, transport and fuel combustion in the production of heat and electricity. Nitrogen oxides are involved in the formation of acid rain and photochemical smog.

Ecosystems absorb a certain amount of nitrogen. Its excess is washed out and accumulates in water bodies. The process of increasing biogenic elements (not just nitrogen compounds) in water is called eutrophication. Its main causes are the discharge of industrial and municipal wastewater into water bodies, the chemicalization of agriculture and the concentration of livestock farming. Currently, this phenomenon has affected 90% of all lakes in the world. The process sometimes causes irreversible damage to aquatic ecosystems and deteriorates water quality (see section 6.2.3.). The main measures to reduce eutrophication are wastewater treatment and control over the use of fertilizers.

Nitrogen gas(N2) in the atmosphere is extremely inert, in other words, a very large amount of energy is needed for the bonds in the nitrogen (N2) molecule to break and other compounds, such as oxides, to form. However, nitrogen is an essential component of biological molecules, such as proteins, nucleic acids, etc. Only some bacteria are capable of converting atmospheric nitrogen into a form accessible to organisms (nitrites and nitrates). This process is called nitrogen fixation and is the main route of nitrogen entry into the biotic component of the ecosystem.

Nitrogen fixation

Nitrogen fixation- an energy-intensive process, since it requires the destruction of a very strong bond between two nitrogen atoms in its molecule. Bacteria use the enzyme nitrogenase and the energy contained in ATP for this. Non-enzymatic nitrogen fixation requires much more energy, obtained in industry from the combustion of fossil fuels, and in the atmosphere as a result of ionizing factors, such as lightning and cosmic radiation.

Nitrogen so important for soil fertility, and the agricultural need for it is so great that enormous quantities of ammonia are produced annually in chemical plants, which is used in nitrogen fertilizers such as ammonium nitrate (NH4NO3) or urea.

Now scale of industrial nitrogen fixation comparable to natural ones, but we still have little idea of ​​the possible consequences of the gradual accumulation of nitrogen compounds available to organisms in the biosphere. There are no compensatory mechanisms that return the nitrogen we fix to the atmospheric pool.

Nitrogen cycle. Nitrogen makes up 79% of the volume of the atmosphere - the main reservoir of this element.

Relatively small amount of fixed nitrogen(5-10%) is given by ionization in the atmosphere. The resulting nitrogen oxides, interacting with rainwater, produce the corresponding acids, which, once in the soil, ultimately turn into nitrates.

Probably, main natural source of fixed nitrogen- representatives of the legume family, for example clover, soybeans, alfalfa, peas. The roots of legumes have characteristic thickenings called nodules, in which nitrogen-fixing bacteria of the genus Rhizobium live intracellularly. This symbiosis is mutualistic, since the plant receives fixed nitrogen in the form of ammonia from the bacteria, and in return supplies them with energy and some organic substances, such as carbohydrates. Per unit area, nodule bacteria can produce 100 times more fixed nitrogen than free-living bacteria. It is not surprising that leguminous plants are often sown to enrich the soil with this element, at the same time obtaining a harvest of high-quality forage grasses.

All nitrogen fixers bind nitrogen in the form of ammonia, but it is immediately used for the synthesis of organic compounds, primarily proteins.

Decomposition and denitrification

Most plants as a source of nitrogen use nitrations. Animals, in turn, directly or indirectly obtain assimilable nitrogen from plants. In Fig. Figure 10.11 shows how nitrates are formed after the decomposition of dead tissue proteins by saprotrophic bacteria and fungi. This process involves oxidative reactions involving oxygen and aerobic bacteria. Proteins are first broken down into amino acids, and then the amino acids produce ammonia. The same product is formed during the decomposition of animal excreta and feces. Chemosynthetic bacteria Nitrosomonas and Nitrobacter carry out the so-called nitrification - they gradually oxidize ammonia to nitrates.

Denitrification

In a sense, a process reverse nitrification, is denitrification, also carried out by bacteria, which as a result reduces soil fertility. Denitrification occurs under anaerobic conditions when nitrate is used in respiration instead of oxygen as an oxidizer of organic compounds (electron acceptor). The nitrates themselves are reduced, usually to nitrogen. Therefore, denitrifying bacteria are facultative aerobes.

Fixation of atmospheric nitrogen in nature occurs in two main directions - abiogenic and biogenic. The first pathway involves mainly reactions of nitrogen with oxygen. Since nitrogen is chemically very inert, large amounts of energy (high temperatures) are required for oxidation. These conditions are achieved during lightning strikes when the temperature reaches 25,000 °C or more. In this case, the formation of various nitrogen oxides occurs. There is also the possibility that abiotic fixation occurs as a result of photocatalytic reactions on the surface of semiconductors or broadband dielectrics (desert sand).

However, the main part of molecular nitrogen (about 1.4 × 10 8 t/year) is fixed biotically. For a long time it was believed that only a small number of species of microorganisms (albeit widespread on the Earth’s surface) could bind molecular nitrogen: bacteria Azotobacter And Clostridium, nodule bacteria of leguminous plants Rhizobium, cyanobacteria Anabaena , Nostoc etc. It is now known that many other organisms in water and soil have this ability, for example, actinomycetes in the tubers of alder and other trees (160 species in total). All of them convert molecular nitrogen into ammonium compounds (NH 4 +). This process requires significant energy expenditure (to fix 1 g of atmospheric nitrogen, bacteria in legume nodules consume about 167.5 kJ, that is, they oxidize approximately 10 g of glucose). Thus, the mutual benefit from the symbiosis of plants and nitrogen-fixing bacteria is visible - the former provide the latter with a “place to live” and supply the “fuel” obtained as a result of photosynthesis - glucose, the latter provide the nitrogen necessary for plants in a form that they can absorb.

Nitrogen in the form of ammonia and ammonium compounds, resulting from biogenic nitrogen fixation processes, is quickly oxidized to nitrates and nitrites (this process is called nitrification). The latter, not connected by plant tissues (and further along the food chain by herbivores and predators), do not remain in the soil for long. Most nitrates and nitrites are highly soluble, so they are washed away by water and eventually end up in the world's oceans (this flow is estimated at 2.5-8x10 7 t/year).

Nitrogen included in the tissues of plants and animals, after their death, undergoes ammonification (decomposition of nitrogen-containing complex compounds with the release of ammonia and ammonium ions) and denitrification, that is, the release of atomic nitrogen, as well as its oxides. These processes occur entirely due to the activity of microorganisms under aerobic and anaerobic conditions.

In the absence of human activity, the processes of nitrogen fixation and nitrification are almost completely balanced by the opposite reactions of denitrification. Part of the nitrogen enters the atmosphere from the mantle with volcanic eruptions, part is firmly fixed in soils and clay minerals, in addition, nitrogen is constantly leaking from the upper layers of the atmosphere into interplanetary space.

WALKING IN NATURE

Nitrogen, after hydrogen, helium and oxygen, is the fourth most abundant element in the solar system. Nitrogen has been found in the spectra of stars, including in the photosphere of the Sun, in meteorites, comets, the solar wind and in interstellar gas clouds. Molecular nitrogen is observed in the atmospheres of Venus and Mars, and ammonia is characteristic of Jupiter and Saturn. In all space objects, nitrogen is found only in a reduced state.

Nitrogen ranks 20th in abundance in the earth's crust. The vast majority of it is concentrated in the following main reservoirs: the atmosphere (3.86-1015 t), the lithosphere (1.7-1015 t), the hydrosphere (2.2-1013 t) and the biosphere (~ 10"" t). In the atmosphere, free nitrogen in the form of molecular Na is 78.09% by volume (or 75.6% by mass), not counting its minor impurities in the form of ammonia and oxides.

In the lithosphere, the average nitrogen content is 6-10~3 wt.%. The bulk of nitrogen in silicates is in a chemically bound state in the form of NHJ, isomorphically replacing the potassium ion in the silicate lattice. In addition, nitrogen minerals are also found in nature: ammonia (NH4C1), released from volcanoes in fairly large quantities, buddingtovate (NH4AlSi308 - 0.5 H2O) - the only ammonium aluminosilicate found with peolitic water. In the very near-surface regions of the lithosphere, a number of minerals have been discovered, consisting mainly of nitrate salts. Among them is the well-known saltpeter (NaN03), large accumulations of which are characteristic of a dry desert climate (Chile, Central Asia). For a long time, saltpeter was the main source of fixed nitrogen. (Now the industrial synthesis of ammonia from air nitrogen and hydrogen is of primary importance.) Nitrides have also been found in nature: sylvestrine (Fe6N2) in the lavas of Vesuvius and osbornite (TiN), sinoite (Si2N20), Carlsbergite (CrN) in meteorites.

Compared to silicate minerals, fossil organic matter is significantly enriched in nitrogen. Oil contains from 0.01 to 2% nitrogen, and coal - from 0.2 to 3%. As a rule, diamonds have a high nitrogen content (up to 0.2%).

In the hydrosphere, the average nitrogen content is 1.6-10-3 wt.%. The bulk of this nitrogen is molecular nitrogen dissolved in water; chemically bound nitrogen, which is approximately 25 times less, is represented by nitrate and organic forms. Water contains ammonia and nitrite nitrogen in smaller quantities. The concentration of fixed nitrogen in the ocean is approximately 10* times less than in soils suitable for agricultural production. This calls into question optimistic statements about the limitless reserves of the World Ocean.

Although the name nitrogen means “non-life-sustaining,” it is actually an essential element for life. In plant organisms it is contained on average 3%, in living organisms up to 10% of dry weight. Nitrogen accumulates in soils (on average 0.2 wt.%). The average nitrogen content in animal and human protein is 16%. Humans and animals cannot synthesize 8 essential amino acids (valine, isoleucine, leucine, phenylalanine, tryptophan, methionine, threonine, lysine), and therefore their main source of these amino acids is plant and microorganism proteins.

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

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

Biogeochemical carbon cycle.

The carbon content of 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 – about 18% (Vinogradov, 1964). The entire process of the emergence and development of the biosphere is closely related to carbon, because 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.

Carbon is absorbed from the atmosphere by autotrophic producing organisms (plants, bacteria, cyanobionts) during the process of photosynthesis, as a result of which, based on interaction with water, organic compounds - carbohydrates - are formed. Further, as a result of metabolic processes involving substances supplied with aqueous solutions, more complex organic substances are synthesized in organisms. They are not only used for the formation of plant tissues, but also serve as a source of nutrition for organisms occupying the next links of the trophic pyramid - consumers. Thus, through trophic chains, carbon passes into the organisms of various animals.

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

The second way to return carbon is the decomposition of organic matter. Under the conditions of the biosphere, this process mainly occurs in an oxygen environment, and the end products of decomposition are the same CO 2 and H 2 O. But most of the carbon dioxide does not enter directly into the atmosphere. The carbon released by the decomposition of organic matter largely remains in dissolved form in soil, groundwater and surface water. 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. After a more or less long migration, it may partially return to the atmosphere, but a larger or smaller proportion of it is always precipitated in the form of carbonate salts and is bound in the lithosphere.

Part of the atmospheric carbon directly enters the hydrosphere from the atmosphere, dissolving in water. Mainly, 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 comes 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 carbonate rock layers of biogenic origin, and for a more or less long time “falls out” of the biosphere cycle.

In oxygen-free environments, the decomposition of organic matter also occurs with the formation of carbon dioxide as the final product. Here, oxidation occurs due to oxygen borrowed from mineral substances by chemosynthetic bacteria. But the process under these conditions proceeds more slowly, and the decomposition of organic matter is usually incomplete. As a result, a significant part of the carbon remains in the composition of incompletely decomposed organic matter 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 caustobiolites. Natural sources of carbon dioxide, in addition to volcanic exhalations, are the processes of decomposition of organic matter, the respiration of animals and plants, and the oxidation of organic substances in the soil and other natural environments. Technogenic carbon dioxide amounts to 20x10 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 in the atmosphere and hydrosphere has repeatedly passed through living organisms. In just 304 years, living organisms absorb as much carbon as is contained in the atmosphere. Consequently, in just 4 years the carbon composition of the atmosphere can be completely renewed, and we can conditionally assume that atmospheric carbon completes its cycle during this period. The cycle of carbon contained in soil humus is estimated at 300-400 years.

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

Rice. 3.5.1. Diagram of the biogeochemical carbon cycle

This diagram clearly shows that plants, using the mechanism of photosynthesis, act as oxygen producers and are the main consumers of carbon dioxide.

However, the biological carbon cycle is not closed. Which is very important, including for us. This element is often removed from the geochemical cycle for a long period of time in the form of carbonate rocks, peats, sapropels, coals, and humus. Thus, part of the carbon constantly falls out of the biological cycle, becoming bound in the lithosphere as part of various rocks. Why, then, is there no carbon deficit in the atmosphere? The reason is that its loss is compensated by the constant flow of CO 2 into 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 carbon balance in the biosphere of our planet.

Human economic activity intensifies the biological carbon cycle and can help increase 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. An increase in carbon dioxide concentration 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 already quite real.

Biogeochemical oxygen cycle

As you remember, oxygen is the most common element not only of the earth’s crust (its clarke is 47), but also of 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 nonionic forms of migration in solutions.

The evolution of geochemical processes on Earth is accompanied by a steady increase in oxygen content. Currently, the amount of oxygen in the atmosphere is 1.2 x 10 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 exchange in the natural water – troposphere system. Ocean water contains from 3x10 9 to 10x10 9 m 3 of dissolved oxygen. Cold water at high latitudes absorbs oxygen, and when it travels with ocean currents to the tropics, it releases it into the atmosphere. The absorption and release of oxygen also occurs when the seasons of the year change, accompanied by changes in water temperature.

Oxygen is consumed in a huge number of oxidative reactions, most of which are biochemical in nature. These reactions release energy absorbed during photosynthesis. In soils, silts, and aquifers, microorganisms develop that use oxygen to oxidize organic compounds. The oxygen reserves on our planet are enormous. It is part of the crystalline 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:

• formation of free oxygen during photosynthesis;
• absorption of oxygen in oxidative reactions

According to calculations by J. Walker (1980), the release of oxygen by the vegetation of the world's land is 150x10 15 tons per year; excretion by photosynthetic organisms of the ocean - 120x10 15 tons per year; absorption in aerobic respiration processes – 2 10 x 10 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 oxygen cycle

Under modern conditions, the oxygen flows established in the biosphere are disrupted by technogenic migrations. Many chemical compounds discharged by industrial enterprises into natural waters bind oxygen dissolved in water. Increasing amounts of carbon dioxide and various aerosols are being released 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 burning fuel. In some industrialized countries, more oxygen is burned than is produced through photosynthesis.

Biogeochemical hydrogen cycle

Free hydrogen is unstable in the earth's crust. It quickly combines with oxygen to form water and also participates in other reactions. In addition, due to its negligible atomic mass, it is capable of evaporating into space (dissipating). A significant amount of hydrogen reaches the Earth's surface during volcanic eruptions. Hydrogen gas is constantly formed as a result of certain chemical reactions, as well as during the life of bacteria that decompose organic matter under anaerobic conditions.

Organisms fix hydrogen in the biosphere of the planet, binding it not only in organic matter, but also participating in the fixation of hydrogen in 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 (H3O+) with a water molecule through a hydrogen bond. When hydronium ions are absorbed by some silicates, they are transformed into clay minerals. Thus, as emphasized by V.V. Dobrovolsky, the intensity of production of acidic metabolic products is an important factor in the hypergene transformation of crystalline rocks and the formation of weathering crust.

Of the cyclic processes on the Earth's surface in which hydrogen participates, one of the most powerful is the water cycle: more than 520 thousand cubic meters of moisture pass through the atmosphere every year. To create the phytomass of the world's land mass, which existed before human intervention, according to V.V. Dobrovolsky (1998) approximately 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. During evaporation, water vapor 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 irreplaceable 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, is 1000 and 10000 for nitrogen, 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 flows of nitrogen compounds formed during denitrification. The main form in which nitrogen is contained in the atmosphere is molecular - N 2. As minor impurities, the atmosphere contains various nitrogen oxide compounds NOx, as well as ammonia NH3. The latter is the most unstable under the conditions of the earth's atmosphere and is easily oxidized. At the same time, the value of the redox potential in the atmosphere is not sufficient for the stable existence of oxide forms of nitrogen, which is why 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 exudates. Photochemical reactions in high layers of the atmosphere lead to the formation of nitrogen compounds and their noticeable entry onto land and into the ocean with precipitation (3-8 kg/ha of ammonium nitrogen per year and 1.5-6 kg/ha of nitrate nitrogen). This nitrogen is also included in the general biogeochemical flow of dissolved compounds migrating with water masses and 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 nitrogen cycle. 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;
• symbiont 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 is bound in soils in nitrite form (compounds based on NH 3).

Nitrite nitrogen compounds are capable of migrating in aqueous solutions. At the same time, they are oxidized and converted into nitrates - salts of nitric acid HNO 3. In this form, nitrogen compounds can be effectively absorbed by higher plants and used for the synthesis of protein molecules based on peptide C-N bonds. Further, through 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 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 thanks to the activity of soil denitrifying bacteria, which again convert nitrogen into 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 amount 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 Earth's geographic envelope, inevitably turns into a free molecular form.

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

• predominant concentration in the atmosphere, which plays the exclusive role of a reservoir from which living organisms draw the reserves of nitrogen they need;
• a 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 nitrogen cycle

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

At the same time, there is a paradox - 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 almost always not enough to feed plants. Therefore, the need of cultivated plants for nitrogen fertilizers is always high. Therefore, according to various estimates, from 30 to 35 million tons of nitrogen are added to the soil annually in the form of mineral fertilizers. Thus, inputs from nitrogen fertilizers account for 30% of the total nitrogen inputs to land and oceans. This often leads to significant environmental pollution and serious illnesses for humans and animals. The losses of nitrate forms of nitrogen are especially large, since it is not sorbed by the soil, is easily washed out by natural waters, is reduced to gaseous forms, and up to 20-40% of it is lost for plant nutrition. A significant disruption of the nitrogen cycle is the ever-increasing amount of animal waste, industrial waste and wastewater from large cities, the release of ammonium and nitrogen oxides into the atmosphere when burning coal, oil, fuel oil, etc. The penetration of nitrogen oxides into the stratosphere (exhausts from supersonic aircraft, rockets, nuclear explosions) is dangerous, as this can cause the destruction of the ozone layer. All this naturally affects the biogeochemical nitrogen cycle.

Biogeochemical sulfur cycle

Sulfur is also one of the elements that plays an extremely important role in the cycle 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 and is an indispensable component of plant nutrition and microflora. Sulfur compounds participate in the formation of the chemical composition of soils and are present in significant quantities 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 conditions of the modern Earth’s atmosphere, containing free oxygen and H 2 O vapor, is impossible. Hydrogen sulfide (H 2 S) oxidizes in an oxygen environment, and oxygen compounds of sulfur, reacting with H 2 O, form sulfuric acid H 2 SO 4, which falls on the surface of the Earth as part of acid rain. Therefore, sulfur oxides SOx, 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 the most common in natural compounds are S 32 (>95%) and S 34 (4.18%). As a result of biological and biogeochemical processes, a change occurs in the ratio of these isotopes towards an increase in the content of the lighter isotope in the upper humus horizons of soils.

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

In 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 rare, which is unstable and tends, depending on the redox potential of the environment, to form either oxygen or hydrogen compounds.

The primary, deep-seated mineral form of sulfur in the earth's crust is sulfide. Sulfide compounds are practically insoluble under biosphere conditions, and therefore sulfide sulfur is not absorbed by plants. But, at the same time, sulfides are unstable in an oxygen environment. 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 fairly good solubility, and sulfur in the geographical shell actively migrates in aqueous solutions as part of the sulfate ion SO 4 2-.

It is in this sulfate form that sulfur, in 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 exchange sorption processes and being part of the soil absorption 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 used again by plants. If decomposition occurs 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. In a reducing environment, part of the sulfur can be bound in sulfide compounds, which, when oxygen access is restored, are oxidized again and turn into the sulfate form.

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

1. assimilation of sulfur compounds by living organisms (plants and bacteria) and the inclusion of sulfur in proteins and amino acids.
2. Conversion 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, the oxidation of hydrogen sulfide, elemental sulfur, and its thio- and tetra-compounds occurs.
4. Reduction 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 biosphere 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 surface of the Earth). 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 forms 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 increased sulfur content in the air. Lichens absorb moisture from the atmosphere with their entire thallus, so the concentration of sulfur 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 weathering of sedimentary rocks – 42x10 12 g/year; anthropogenic inputs in the form of sulfur dioxide are 65x1012 g/year, which totals 119x1012 g/year. Significant amounts of sulfur are annually preserved in the form of sulfides and sulfates - 100x10 12 g/year and, thus, are temporarily removed from the general biogeochemical cycle.

Thus, the anthropogenic entry of sulfur into the biosphere significantly changes the cycle 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 phosphorus cycle in nature is very different from the biogeochemical cycles of carbon, oxygen, nitrogen and sulfur, since the gas form of phosphorus compounds (for example PH 3) practically does not participate in the biogeochemical cycle of phosphorus. That is, phosphorus is not capable of accumulation in the atmosphere at all. Therefore, the role of a “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 is calcium phosphate - apatite. This is a polygenic mineral formed in various natural processes - both deep and supergene (including biogenic). Phosphate compounds can dissolve in water, and phosphorus in 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, is for phosphorus, as well as for nitrogen, 1000 and 10,000, respectively (Kovda, 1985). For plants, the most accessible phosphorus is from nonspecific organic compounds and humus, and it is this phosphorus that plays the main role in the small (local) biological phosphorus cycle.

Animals are even greater concentrators of phosphorus than plants. Many of them accumulate phosphorus in the tissues of the brain, skeleton, and shells. There are several ways for phosphorus to be absorbed by consumer organisms. Firstly, direct absorption from plants during nutrition. Secondly, aquatic filter-feeding organisms extract phosphorus from organic suspensions. Thirdly, organic phosphorus compounds are absorbed by sludge-eating organisms when they process biogenic sludge.

Phosphorus is returned to the environment 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 into the final reservoirs of flow, to the greatest extent into the World Ocean, where it accumulates in sedimentary deposits of various origins. This part of phosphorus can only return to the exogenous cycle as a result of tectonic processes stretching over hundreds of millions of years. Under natural conditions, maintaining balance is ensured by the relatively weak mobility of phosphorus compounds, as a result of which phosphorus extracted by plants from the soil is mostly returned to it as a result of the decomposition of organic matter. Phosphorus is quite easily fixed in soils and rocks. Phosphorus fixers are hydroxides of iron, manganese, aluminum, clay minerals (especially minerals of the kaolinite group). However, fixed phosphorus can be 40-50% desorbed and used by plants. This process depends on pH and Eh environmental conditions. Increased acidity and the formation of carbonic acid contribute to the desorption of phosphorus and increased migration of phosphorus compounds.

In a reducing environment, compounds of phosphorus with divalent iron are formed, 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 pollution of the environment 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:

• lack of atmospheric transport;
• the presence of a single source - the lithosphere;
• tendency for runoff to accumulate in terminal reservoirs.

With intensive agricultural exploitation of land, phosphorus losses in the landscape become almost irreversible. Compensation is possible only through the use of phosphorus fertilizers. It is known that phosphorus 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 may 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 in the biosphere has been sharply disrupted by humans. Here are the components of this phenomenon: firstly, the mobilization of phosphorus from agricultural ores and slags, the production and use of phosphorus fertilizers, 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 fishing, the extraction of sea shellfish and algae, which entails the redistribution of phosphorus from the ocean to land. As a result, a process of phosphatization of land is observed, but this process manifests itself extremely unevenly. The phosphorus content in the environment of large cities is increasing. On the contrary, countries that actively export organic products and do not use phosphorus fertilizers lose phosphorus reserves in their soils.

Biogeochemical cycles of potassium and sodium

The Clarke of potassium in the earth's crust is 2.89, and 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 turns 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 other leading cations in the lithosphere. Therefore, the volume clarke of potassium ranks second after oxygen in the earth's crust.

Potassium is a chemically active metal and is not found in its native state. In all chemical compounds on Earth it acts as a monovalent metal. Metallic potassium “burns” in air, quickly oxidizing to K 2 O. The number of mineral species is 115 (three times less than calcium and half as much as sodium). The most important minerals: halogens - sylvite, carnallite, nitrates - K- saltpeter, silicates – K-feldspars (orthoclase, microcline), phlogopite, muscovite, biotite, glauconite, leucite. The chemical properties of potassium are close to sodium, which determines their joint migration. But their behavior in the hypergenesis zone and the biosphere as a whole is sharply different. Most of the potassium during the supergene transformation of silicates remains in the composition of secondary clay minerals, therefore potassium is much more firmly retained within the world's landmass than sodium and, as we will see later, calcium. And yet, partial release of potassium ions occurs in the processes of hypergenesis 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 humid climates, when potassium-containing minerals are weathered, potassium is easily leached and transported by aqueous solutions. However, the removal of potassium in the weathering crust occurs less intensely than calcium and sodium. This is due to the fact that large potassium ions are more sorbed 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 hydronium ions, which have comparable ionic sizes. In this way, potassium can be fixed in hydromicas, kaolinite, and montmorillonite. Potassium, to a greater extent than sodium, is absorbed by terrestrial vegetation.

Therefore, much of the potassium is retained in soils, while most of the sodium is carried into the ocean. The runoff from continents contains almost 2.5 times more sodium than potassium.

Potassium is an essential element of living organisms. They contain from 0.1 to 0.01% potassium. The ash of cultivated plants contains up to 25-60% K 2 O. Some organisms are capable of concentrating potassium in significant quantities. Thus, 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, and the seeds lose their viability. Potassium easily penetrates 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 plant resistance to drought. With a deficiency or excess of potassium, the rate of photosynthesis decreases and the rate of respiration increases. A lack of potassium in soils leads to a significant reduction in plant productivity.

That is why the clarke of potassium in living matter is as high as that of nitrogen. Some seaweeds accumulate especially much potassium (up to 5%).

About 1.8x109 tons of potassium are involved annually in the biological cycle on land (Dobrovolsky, 1998). The mass of potassium released from the biological cycle system on land is partially retained in dead organic matter and sorbed by soil mineral matter (clay minerals), and is 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. Every year, more than 61x106 tons of potassium in a dissolved state (in the form of free ions) and 283x106 tons of potassium in suspension (clay particles, organic matter, etc.) enter the ocean with continental water runoff. Potassium also actively migrates in the ocean surface-atmosphere system in the composition of aerosols: the average concentration of this element in precipitation over the ocean is 15%. The concentration of potassium in precipitation over the continents is noticeably higher, on average 0.7%. Significant amounts of potassium are transferred with dust from land to the ocean. According to V.V. Dobrovolsky, this value is no less than 43x10 6 tons per year.

In the supergene 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 low sodium consumption by living matter. However, sodium is needed in small quantities by all living organisms.

In humid climates, sodium easily leaves the biological cycle and is carried with liquid runoff outside the landscape. As a result, there is a general depletion of the latter in sodium. The sodium content of 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 concentrates in ground and lake waters and accumulates in saline soils (the effect of an evaporation barrier). Accordingly, the vegetation of halophytic communities contains increased amounts of sodium.

However, the role of the biological cycle of sodium, in contrast to potassium, is relatively small. But its water migration is very significant. In terms of migration patterns in the biosphere, sodium is very similar to chlorine. It forms easily soluble salts, therefore it accumulates in the World Ocean 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. The extraction of halite (table salt), soda and mirabilite is of primary importance. The nature of sodium biogeochemical cycles is also significantly influenced by land irrigation in dry areas.

Biogeochemical cycles of calcium and magnesium.

Calcium atoms contain a 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, with the following distribution: 40 Ca - 96.97% (twice magic Z=N=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 ranks 15th, but among metals it is in 5th place.

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

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 Clarke of calcium in the lithosphere is 2.96. Calcium silicates are weakly stable in the hypergenesis zone and are the first to be destroyed during weathering of rocks.

Calcium has a relatively high migration ability, largely determined by climate characteristics. During 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 weather faster than sodium varieties. At the same time, natural solutions that vigorously remove calcium contain significant amounts of bicarbonate ion. But in the soils of humid zones there is a significant calcium deficiency. There is very little of it in weathering crusts. This is explained by the high migration mobility of this element.

In the ion sink from the continents, calcium ranks first among cations. It is carried out by rivers mainly in the form of suspended 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 protozoa to higher mammals. Cold waters of high latitudes and deep seas are undersaturated with CaCO 3 due to low temperatures and pH, so the carbonic acid contained in the water dissolves CaCO 3 of bottom sediments. This is why marine organisms in high latitudes avoid building their skeletons from CaCO 3 . In equatorial latitudes, a region of CaCO 3 supersaturation has been established. There is massive growth of coral reefs here, and many of the 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 cycle. According to A.P. Vinogradov rivers annually bring 1*10 15 tons of CaCO 3 into the ocean. Where did he go? About the same amount is buried annually in ocean bottom sediments. Ocean living organisms concentrate calcium in the form of aragonite and calcite. Aragonite, however, is unstable and eventually turns into calcite. In the ocean we encounter unique phenomena of 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 spines 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 encounter a special type 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 portion of seawater calcium ions is chemically precipitated in closed reservoirs under evaporitic conditions.

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

In acidic soils, characterized by a significant manifestation of the leaching process, the phenomenon of biogenic accumulation of calcium in the litter and accumulative surface soil horizons is observed. It is part of the group of biophilic elements. Therefore, calcium actively participates in the biological cycle. The extent of calcium involvement varies significantly in different natural areas.

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

But the disruption of the biogeochemical cycle of calcium currently 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 of soils), and the metallurgical industry.

The magnesium clarke is inferior to the calcium clarke and is 1.87, but the distribution of magnesium is very heterogeneous. The size of the magnesium ion is close to the ions of ferrous iron and nickel and, together with them, is part 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 has a migration capacity approaching that of elements such 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 magnesium aluminosilicates.

Magnesium is a biophilic element. It is part of chlorophyll, which is destroyed if there is a lack of this element. The plant responds to a lack of magnesium in the soil by the outflow of chlorophyll from old leaves to young ones. Movement occurs along the leaf veins. 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, magnesium is less biophilic than 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 carried out 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, there is an accumulation of these salts on evaporation barriers and the formation of salt marshes.

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

The migration of magnesium has varied significantly throughout 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 significantly lower than that of calcium and sodium. Until the beginning of the twentieth century, only dolomite and magnesite were used. Only recently have alloys containing magnesium become widely used. In landscapes depleted of magnesium, a slight accumulation of magnesium is observed due to the application of magnesium-containing fertilizers and liming of soils using 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, intensive accumulation of these elements is observed in the sediments of the World Ocean: 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 (after oxygen) chemical element in the earth's crust. Its clarke in the earth's crust is 29.5, in 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 sufficiently studied.

No wonder V.I. Vernadsky believed that no organism in the biosphere could exist without silicon, which is necessary for the formation of cells and tissues of plants and animals, and their skeletons. Living matter extracts silicon from natural waters and soils for nutrition and the functioning of biochemical processes, then releasing it in excrement and when it dies. 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 to the ocean from land is insufficient for the normal development of phytoplankton. That is why in temperate and tropical latitudes in the ocean, organisms with a siliceous skeleton are poorly developed. Given the current saturation of water with silica, for the normal development of phytoplankton and diatoms, each silicon atom must be used many times during the year (tens and even hundreds of times). Of the total mass of silica produced in the surface photosynthetic layer, no more than 0.1 part reaches bottom sediments, and often this is only 0.05-0.01 part. The rest of the silica again becomes water-soluble. Subsequently, it is captured from the water by new generations of diatoms, siliceous sponges and radiolarians. However, 0.1-0.01 of the remains 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.

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

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

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

A powerful mechanism driving this cycle is the vegetation cover of the land, in which various processes of formation of silicon-containing organogenic minerals (bioliths) occur. In this case, bioliths are understood as minerals formed inside the body during its life. Their role in the silicon cycle is extremely large, but has not been sufficiently studied. Basically, silica encrusts cell membranes. The most bioliths of silica are found in cereals, sedges, horsetails, ferns, mosses, palms, pine needles, spruce needles, leaves and bark of elm, aspen, and oak. In the ash of feather grass, the silica content, according to Parfenov and Yarilov, 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 some conditions is directly related to the accumulation of this element by living organisms. The most striking example is the formation of malts, the silicic acid of which accumulated due to the activity of diatoms. During the life 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 conditions of the temperate zone is shifted towards accumulation. Land cover, 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. Subsequently, the opal of the bioliths turns into chalcedony and even into secondary quartz. A significant part of the silicic acid of bioliths is included in active migration in soil and groundwater in the form of colloidal and true solutions.

As a result of exposure to 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 clarke is 8.05. Iron ranks second in abundance after aluminum among metals and fourth among all elements of the earth's crust. His clarke is 4.65. The manganese content in the earth's crust is significantly lower than -0.1%. These two elements occupy adjacent places in D.I.’s periodic table of elements. Mendeleev and have a similar structure of electronic 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 absorption. 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 critically on moisture conditions, environmental reactions, 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 valence. At the same time, the amphoteric nature of this element causes a strong dependence of its migration on the acid-base conditions of the environment: in a strongly acidic environment it behaves as a cation, and in a strongly alkaline environment it behaves as an anion. In neutral and slightly alkaline waters of steppes and deserts, it almost does not migrate; the mobility of this metal is highest in highly 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 weak 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 bauxite.

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 with sesquioxides and manganese. Many researchers have proven that the migration of sesquioxides under leaching type water conditions occurs in the form of highly dispersed sols stabilized by acidic humus. In this case, an important role is played by the creation of an anaerobic environment, which causes the formation of compounds of divalent 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 intrasoil 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. 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 in an alkaline environment, these elements migrate weakly.

Migration of iron and manganese is also possible in 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 into the oceans in huge 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 an absorbed state, in the form of hydrates, humates and organic compounds of ferrous iron.

Lack of iron leads to a disease in plants known as chlorosis. However, direct accumulation of iron in significant quantities is characteristic of only a few organisms. In this regard, iron bacteria are unique because they oxidize divalent iron, resulting in the formation of limonite. Diatoms are able to absorb iron from insoluble colloids. Iron is also consumed by red-blooded zooplankton (small crustaceans). When these organisms die and detrital parts dissolve, a certain amount of iron also goes into solution in the form of hydrates and other forms. As a special case of iron concentration by organisms, one can note the presence of magnetite and goethite in the teeth of some modern gastropods.

The biogeochemical cycle of iron and manganese is significantly disrupted 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 with an atomic mass of more than 50 units. Despite the relatively low occurrence 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 found that the following 9 elements are the most toxic: Cr, As, Ni, Sb, Pb, Vo, Cd, Hg, Ta. Polish scientists ranked heavy metals according to their pollution potential into 4 groups. The group of elements with a very high contamination potential includes cadmium, mercury, lead, copper, thallium, tin, chromium, antimony, silver, and gold.

The group of elements with a high contamination potential includes bismuth and uranium. Molybdenum, barium, manganese, titanium, iron, selenium, tellurium. The group of elements with average contamination potential includes fluorine, beryllium, vanadium, rubidium, nickel, cobalt, arsenic, germanium, indium, cesium, and tungsten. Elements with low contamination potential are 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 large city is the cause of the occurrence of biogeochemical anomalies, including those dangerous to humans.

It is well known that the accumulation of lead and zinc occurs in areas of heavy vehicle traffic, along highways and in industrial centers. Soils in rural areas contain 10-20 times less lead. Than the soils 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 influence the availability of heavy metals for plants include: particle size distribution, 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. As the pH of the soil solution increases, the likelihood of the formation of insoluble hydroxides and carbonates increases. It is believed that to minimize the availability of toxic metals 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 are therefore less available for plant uptake in soils with high humus content. The exchange capacity of cations depends mainly on the content and mineralogical composition of the clay part of the soil and the content of organic matter in them. The higher the exchange capacity of cations, the greater the retention capacity of soils with respect to heavy metals.

Excess water in the soil promotes the appearance of low-valence metals in a more soluble form.

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

The term "biogeochemistry" was proposed by the Russian scientist V.I. Vernadsky and means the field of science about the exchange of substances between living and nonliving matter in the biosphere ("bio" refers to living organisms, and "geo" refers to rocks, air and water). Geochemistry studies the chemical composition of the Earth and the migration of elements between different parts of the biosphere: lithosphere, hydrosphere and atmosphere.

For the normal existence of most ecosystems and the organisms that inhabit them, the cycles of elements such as hydrogen, carbon, nitrogen, sulfur and phosphorus, which are part of any living matter, are of maximum importance.

In the cycles of any chemical elements and substances, two parts or two “funds” are distinguished:

1) reserve fund- a large mass of substances slowly moving in the biogeochemical cycle;

2) exchange (moving) fund- a smaller, but more active mass of substance, which is characterized by rapid exchange between living organisms and their immediate environment.

In general, biogeochemical cycles are usually divided into two main types:

1) the circulation of gaseous substances with a reserve fund in the atmosphere or hydrosphere (ocean); 2) sedimentary cycle with a reserve fund and the earth's crust. Reserve funds in the atmosphere and hydrosphere are easily accessible, so such cycles are relatively stable. Sedimentary biogeochemical cycles tend to be less stable.

The amazing constancy of the percentage content of various chemical elements in the components of the ecosystem is historically due to the existence of continuous and balanced cycles of substances, which creates the opportunity for self-regulation (homostasis) of the system and maintaining its stability.

The processes of new formation of organic matter during photosynthesis and the processes of its destruction (decay) determine the speed and balance of the cycles of elements in the biosphere and occur only due to solar energy coming from outside. Consequently, the speed and direction of the cyclic movement of elements in an ecosystem are determined by the energy flows passing through the biological community.

A generalized diagram of biogeochemical cycles, combined with a simplified diagram of energy flow (Fig. 10.1), shows how the unidirectional flow of energy drives the cycle of matter. Noteworthy is the fact that the chemical elements involved in the cycle process repeatedly go through the same path, and energy flows only in one direction.

In Fig. In Fig. 10.1, the reserve fund is designated as a fund of nutrients, and the exchange fund is represented by a dark ring, going from autotrophs to heterotrophs and from them again to autotrophs. Sometimes the reserve fund is referred to as unavailable, while the active exchange fund is referred to as available. For example, agronomists typically measure soil fertility by assessing the concentration in the soil of those forms of nutrients that are directly available to plants.

The exchange fund is formed by substances that return to the cycle in two main ways - either as a result of intravital release of metabolic products into the external environment by animals and plants, or during the destruction (mineralization) of dead organic matter (detritus) by microorganisms.

The human influence on biogeochemical cycles lies in the fact that with anthropogenic intervention these processes may cease to be closed and in some places of the boiosphere there may be a deficiency, and in others an excess of some substances. Ultimately, measures to protect natural resources should be aimed at preventing cyclical disruptions, i.e. balance of cycles of the most important elements in the biosphere. Knowledge of the features of biogechemical cycles is a necessary condition for the rational use of natural resources and the conservation of natural ecosystems.

Rice. 10.1. Diagram of the biogeochemical cycle against the background of a simplified diagram of energy flow:

Р G - gross primary production; P N - net primary production (can be consumed by heterofs in the system itself or exported); P - secondary production, R 1 - respiration of autotrophs (plants); R 2 - respiration of heterotrophs (animals and bacteria)

Any ecosystem can be represented as a series of blocks through which various substances pass. As a rule, three active blocks participate in the cycles of mineral substances in an ecosystem: living organisms, dead organic detritus, and available inorganic substances in the habitat.

Let's consider the biogeochemical cycles of nitrogen, phosphorus and sulfur. The biogeochemical cycle of nitrogen (a biogenic element found in proteins and nucleic acids) can serve as an example of a very complex, well-balanced cycle of a gaseous substance. The biogeochemical phosphorus cycle is a sedimentary cycle with less perfect regulation of the phosphorus cycle.

The biogeochemical sulfur cycle exemplifies the functional connection between the atmosphere, water and the earth's crust, as sulfur actively circulates in and between each of these “reservoirs”. Microorganisms play a key role in the nitrogen and sulfur cycles.

Nitrogen cycle, which includes both gas and mineral phases, despite the large number of organisms participating in it, ensures rapid circulation of nitrogen in various ecosystems (Fig. 10.2).

Rice. 10.2. Diagram of the nitrogen cycle (gray rectangle – nitrogen reserve fund)

The main source and reservoir of nitrogen is the atmosphere, the mass of which consists of 79% of this element. The participation of living organisms in the nitrogen cycle is subject to a strict hierarchy: only certain types of microorganisms (bacteria) carry out biochemical processes of transformation of nitrogen compounds at certain key stages of this cycle.

Most organisms living in the biosphere cannot directly use gaseous molecular nitrogen (N 2). Plants absorb nitrogen only as part of nitrate ions (NO 3 -) or ammonium ions (NH 4 +). Nitrates are formed mainly as a result of the activity of microorganisms - nitrogen fixers, which include symbiotic bacteria of the genus Rhizobium, living in nodules on the roots of leguminous plants, bacteria of the genus Azotobacter, living in the soil; and cyanobacteria (blue-green). All nitrogen-fixing microorganisms are capable of fixing atmospheric nitrogen thanks to a very complex metabolism, including molybdenum and hemoglobin as catalysts. Symbiotic nitrogen-fixing microorganisms penetrate the tissues of the root system of legume plants. Plants provide symbiotic bacteria with habitat and food (sugars), and they supply the plant with organic nitrogen, which they synthesize from nitrogen gas. Free-living non-symbiotic microorganisms - nitrogen fixers ( Azotobacter in cyanobacteria) also absorb nitrogen gas and convert it into organic form. In this case, nitrogen is included in the synthesized protein molecules. After the death of nitrogen-fixing bacteria and the mineralization of organic matter, nitrogen in nitrate form (NO 3 -) enriches the soil.

Animals can absorb nitrogen only in organic matter of plant or animal origin. Through typical food chains (plants - herbivores - predators), organic nitrogen is transferred from microorganisms - nitrogen fixers to plants and all other organisms of the ecosystem. Processes occur in soils ammonification(formation of ammonium ions) and nitrification(formation of nitrate ions), consisting of a series of sequential reactions during which, with the participation of different groups of microorganisms, dead organic matter is destroyed.

Molecular nitrogen returns to the atmosphere and the biogeochemical nitrogen cycle closes during the life of bacteria - denitrifiers sort of Pseudomonas, reducing nitrates to free nitrogen and oxygen under anoxic (anaerobic) conditions.

Nitrates are constantly formed from molecular nitrogen in small quantities without the participation of nitrogen-fixing microorganisms during electrical lightning discharges in the atmosphere. These nitrates then fall with rain to the soil surface. Another source of atmospheric nitrogen entering the biogeochemical cycle is volcanoes, which compensate for the loss of nitrogen excluded from the cycle when it is deposited on the bottom of the oceans.

In order to compare the scale of various processes of atmospheric nitrogen entering the biogeochemical cycle, it is necessary to keep in mind the following: the average annual supply of nitrate nitrogen of abiotic origin (lightning discharges) from the atmosphere into the soil does not exceed 10 kg/ha, free nitrogen-fixing microorganisms contribute up to 25 kg/ha. ha, while symbiotic nitrogen-fixing bacteria Rhizobium on average they produce up to 200 kg/ha.

The majority of the nitrogen contained in organic matter is converted by denitrifying bacteria into nitrogen gas (N2) and returned to the atmosphere. Only about 10% of mineral nitrogen is absorbed from the soil by higher plants and becomes available to multicellular organisms.

Phosphorus cycle. Phosphorus is part of energy-rich organic substances - adenose nitriphosphate (ATP) and adenosine diphosphate (ADP), which are carriers and energy accumulators in plant and animal cells. The main source of phosphorus for plants are phosphate ions (PO 4 -). Plants absorb phosphate ions from the environment (soil or water) and, through the process of biosynthesis, incorporate phosphorus into the organic substances that form plant biomass. Animals, by eating plants, obtain phosphorus in organic form. Thus, by converting phosphorus from mineral to organic form, plants make it available to animals. The phosphorus cycle in the biosphere is associated with metabolic processes in plants and animals. This important biogenic element, the content of which in terrestrial parts of plants and algae varies from 0.01 to 0.1%, and in animals from 0.1% to several percent, during the circulation process gradually passes from organic compounds into phosphates, which can again be used by plants (Fig. 10.3 ).

Rice. 10.3. Diagram of the phosphorus cycle (gray rectangle - phosphorus reserve fund)

If we compare the phosphorus content in living and nonliving matter of the biosphere, it turns out that the disproportion is very large. Therefore, phosphorus is one of the most deficient biogenic macroelements that determine the development of life.

The natural biogeochemical cycle of phosphorus in the biosphere is not balanced. The main reserves of phosphorus are contained in rocks (apatites, phosphorites), from which, through the process of leaching, water-soluble phosphates (PO 4 3-) enter terrestrial and aquatic ecosystems. Once in terrestrial ecosystems, phosphorus is absorbed by plants from an aqueous solution in the form of inorganic phosphate ion (PO 4 3-) and is included in various organophosphorus compounds. Through food chains, phosphorus-containing organic matter moves from plants to other organisms in the ecosystem. Chemically bound phosphorus enters the soil with the remains of plants and animals, where it is exposed to microorganisms and converted into mineral phosphorus compounds available to plants during photosynthesis. The removal of phosphates from terrestrial ecosystems into continental water bodies enriches the latter with phosphorus. River runoff annually carries about 2 million tons of phosphorus into the World Ocean.

In marine ecosystems, mineral phosphorus becomes part of phytoplankton, which serves as food for other marine organisms, and accumulates in the tissues of marine animals, such as fish. Some of the organic phosphorus compounds migrate through food chains within shallow depths, while the other part sinks to greater depths in the process of deposition of dead organic matter. Dead remains of organisms lead to the accumulation of phosphorus at different depths. It follows that phosphorus, entering water bodies in one way or another, saturates and often oversaturates their ecosystems. The return movement of phosphorus from the World Ocean to land and terrestrial water bodies is limited (catch of fish and other organisms by humans) and does not compensate for the removal of phosphorus from land. And only in significant time intervals, when in the process of tectonic movement of the earth’s crust the bottom of the oceans becomes dry land, does this biogeochemical cycle close.

Sulfur cycle. There are numerous gaseous sulfur compounds, such as hydrogen sulfide (H 2 S) and sulfur dioxide (SO 3).

However, the predominant part of the cycle of this element is sedimentary in nature and occurs in soil and water.

A detailed diagram of the sulfur cycle is shown in Fig. 10.4.

Figure 10.4. Diagram of the sulfur cycle

The main source of sulfur available to living organisms is sulfates (SO 4 2-). Many sulfates are soluble in water, and this determines the availability of inorganic sulfur for plants, since many elements (including sulfur) can only enter living organisms in dissolved form. Plants, absorbing sulfates, reduce them and produce essential sulfur-containing amino acids (methionine, cysteine, cystine), which play an important role in creating the tertiary (spatial) structure of proteins. Animals and microorganisms, consuming plant biomass for food, assimilate sulfur-containing organic compounds.

When dead organic matter (fallen leaves, dead organisms, excrement products) decomposes by heterotrophic bacteria, sulfur again transforms into inorganic form (mainly in the form of hydrogen sulfide H 2 S). Some bacteria can produce hydrogen sulfide from sulfates in anaerobic(oxygen-free) conditions. Another small group of bacteria can reduce hydrogen sulfide to elemental sulfur (S).

On the other hand, there are bacteria that again oxidize hydrogen sulfide to sulfates, due to which the supply of sulfur in a form accessible to plants again increases. Such bacteria are called chemosynthetic, since they synthesize organic substances using the energy of oxidation of simple chemicals (in this case, hydrogen sulfide). In this circumstance they differ from photosynthetic organisms, which create organic substances using light energy.

The last phase of the sulfur cycle is completely sedimentary (taking place in sedimentary rocks). It is characterized by the precipitation of this element under anaerobic conditions in the presence of iron. The process thus ends with a slow and gradual accumulation of sulfur in deep-lying sedimentary rocks.

In general, the ecosystem requires significantly less sulfur compared to nitrogen and phosphorus. Therefore, sulfur is less often a limiting factor for the development of plants and animals. At the same time, the sulfur cycle is a key one in the overall process of creating the decomposition of organic matter of biomass in the biosphere. For example, when iron sulfides form in sediments, phosphorus changes from an insoluble form to a soluble form and becomes available to photosynthetic organisms. This serves as a clear confirmation that one cycle is connected to and regulated by another.

Carbon cycle. Carbon, as the most important structural element, is part of any organic matter, therefore its cycle largely determines the intensity of the formation and destruction of organic matter in various parts of the biosphere. In nature, carbon exists in the two most common mineral forms - in the form of carbonates (limestones) and in the form of a mobile form of carbon dioxide (carbon dioxide, CO 2). In the biochemical carbon cycle, the atmospheric pool of carbon dioxide is relatively small (711 billion tons) compared to carbon reserves in the oceans (39,000 billion tons), fossil fuels (12,000 billion tons) and terrestrial ecosystems (3,100 billion tons).

Approximately 93% of carbon dioxide is found in the ocean, which can hold much more of the chemical than other reservoirs. Most of the carbon dioxide coming from the atmosphere into the surface layers of sea water interacts with water to form carbonic acid and its dissociation products. Thus, in the ocean there is always carbonate system- the sum of all inorganic dissolved carbon compounds (carbon dioxide CO 2, carbonic acid H 2 CO 3 and its dissociation products).

All these compounds are interconnected and can transform into each other through chemical reactions when environmental conditions change. For example, if the acidity of water increases (at low pH values), carbonic acid molecules break down into water H 2 O and carbon dioxide CO 2, while the latter can be removed from the ocean into the atmosphere. Alkaline conditions, on the contrary, contribute to the formation of carbonate ions (CO3 2-), sparingly soluble calcium carbonates (CaCO 3) and magnesium (MgCO 3), which sink to the bottom in the form of sediment and for some time remove carbon from the cycle in the ocean.

As can be seen from Fig. 10.5, carbon contained in the atmosphere or hydrosphere (in the form of carbon dioxide CO 2) during photosynthesis is included in the organic matter of plants and further through the food chain enters animal organisms and microorganisms. The reverse process of transition of carbon from organic to mineral form occurs during the respiration of all animal and plant organisms (oxidation of organic matter to carbon dioxide (CO 2) and water (H 2 O)). The process of releasing carbon dioxide from organic matter does not occur immediately, but gradually, in parts at each trophic level. In the soil, the biogeochemical carbon cycle very often slows down, since organic substances are not completely mineralized, but are transformed into organic complexes - humus.

A feature of the functioning of terrestrial ecosystems is the significant and relatively long-term accumulation of the organic form of carbon in the biomass of plants and animals, as well as in humus. Thus, the biomass of terrestrial ecosystems can also be considered as a significant carbon reserve in the biosphere.

Rice. 10.5. Carbon cycle diagram (gray rectangles are carbon reserves)

The oceanic branch of the biogeochemical carbon cycle has its own characteristics, which, given the significant amount of carbon contained in water, determine the important role of the World Ocean in the cycle of this element. In the aquatic environment, unlike terrestrial ecosystems, the main photosynthetic organisms are single-celled microscopic algae floating in the water column (phytoplankton).

The life activity of phytoplankton organisms is quite active and is accompanied by both the accumulation of organic carbon in the form of biomass and the release of dissolved organic carbon. Animals and bacteria consume these organic forms of carbon.

A feature of the functioning of the aquatic ecosystem is the rapid transition of organic forms of carbon along the food chain from one organism to another. Unlike terrestrial ecosystems, the ocean does not form significant reserves of organic carbon in the biomass of living organisms. Most of the organic carbon in the hydrosphere is reconsumed and eventually oxidized to its mineral form, carbon dioxide (CO2). The other part of the dead organic matter (detritus), under the influence of gravity, settles into the deep layers of the water column and is deposited at the bottom, where it can remain for a long time in the form of organic sediments.

A small part of organic matter and the carbon contained in it, according to the terminology of V.I. Vernadsky, escapes the cycle and “goes into geology” - into sediments in the form of peat, coal, oil and limestone in aquatic ecosystems.

The current balance of carbon dioxide in the atmosphere is presented in table. 10.1.

Table 10.1

Annual balance of CO 2 in the atmosphere

Source: Tarko A.M. Stability of biosphere processes and Le Chatelier's principles // Reports of the Russian Academy of Sciences. 1995. T. 343. No. 3. P. 123.

Thus, out of about 6.41 billion tons of carbon dioxide emitted annually by industry, 3.3 billion tons, i.e. more than 50% remains in the atmosphere. Over the past 150 years, this has already led to an increase in carbon dioxide in the atmosphere by more than 25% and stimulated the greenhouse effect. In turn, changes in the Earth's climate regime can and are already leading to global climate change.

In general, about 0.2% of the mobile carbon stock is in constant circulation in the biosphere. Biomass carbon is renewed in 12 years, in the atmosphere - in 8 years, which confirms the highest balance of the biogeochemical carbon cycle.

Test questions and assignments.

1. What are biogeochemical cycles and how are they related to ecosystems?

2. Describe the reserve and exchange fund in the cycle of chemical elements.

3. Indicate the blocks of ecosystems through which the biogeochemical cycles of elements pass.

4. In the cycle of which nutrients do microorganisms play a key role?

5. For which elements is the atmosphere a reserve fund?

ECOLOGY OF POPULATIONS

Each biological species that exists in nature is a complex complex of intraspecific groups of organisms with similar structural features, physiology and lifestyle. Populations are such intraspecific groups of organisms.

Population - a group of organisms of the same species, capable of maintaining their numbers for a long time, occupying a certain space and functioning as part of the biotic community of the ecosystem

A biotic community is a collection of populations of organisms of different species, functioning as an integral system in a certain physical-geographical space of the habitat.

The adaptive capabilities of a population are significantly higher than those of the individuals composing it. A population as a biological unit has a certain structure and functions.

The population has biological properties, inherent both to the population as a whole and to its constituent organisms, and group properties that appear only in the whole group. The biological properties of a population include, in particular, growth and participation in the cycle of substances. Unlike biological ones, group properties: fertility, mortality, age structure, spatial distribution, genetic fitness and reproductive continuity (i.e. the probability of leaving descendants over a long period of time) - can only characterize the population as a whole.

Below are the main population indicators.

Population density is the population size per unit of space. It is usually measured and expressed by the number of organisms (population size) or the total biomass of organisms (population biomass) per unit area or volume, for example, 500 trees per 1 hectare, 5 million microalgae per 1 m 3 of water or 200 kg of fish per 1 hectare of water surface .

Sometimes it is important to distinguish specific, or ecological density(number or biomass per unit of habitable space, i.e. actually accessible to organisms of a particular population) and average density(population size per unit of space within the geographic limits of the population’s habitat). For example, the average density of wood frogs is their number divided by the area of ​​the forest. However, these animals live only in wetlands of the forest, the areas of which are taken into account when calculating the specific population density.

Population density is not a constant value - it changes over time depending on living conditions, season of the year, etc. The distribution of organisms in the space occupied by a population can be random, uniform and group. Most often in nature there are various kinds of accumulations of organisms of the same species (group distribution: family groups and flocks in animals, group thickets in plants).

The most complete picture of population density is provided by the integrated use of indicators: the number of individuals well characterizes their average distance from each other; biomass - concentration of living matter; calorie content - the amount of energy bound in organisms. Typically, plant population densities are higher than herbivore population densities in the same area. The larger the organisms, the greater their biomass.

Density is one of the most important properties of a population. Respiration, nutrition, reproduction and many other functions of individual organisms in the population depend on population density. Excessive population density worsens the conditions of its existence, reducing the provision of organisms with food, water, living space, etc. The existence of a population is also negatively affected by its insufficient density, which makes it difficult to select individuals of the opposite sex, protect the population from predators, etc. (See more about mass and group effects in Lecture 6).

There are a number of mechanisms for maintaining population density at the desired level. The main one is self-regulation of population size based on the principle of feedback from the quantity and limited vital resources, in particular food. Thus, when food becomes scarcer, the growth of individuals slows down, mortality increases, sexual maturity (i.e., the ability to reproduce) occurs later, and as a result, the population size and its density decreases. The improvement in living conditions is accompanied by changes of the opposite nature, and the population density increases to a certain limit. The population size can fluctuate due to migration, generational change, the appearance of new individuals (due to birth and introduction from other populations) or as a result of death. Studying population dynamics is very important for predicting outbreaks of pests or commercial animals.

Population size is determined mainly by two opposing phenomena - fertility And mortality.

Fertility is the ability of a population to increase in size. It characterizes the birth of new organisms in the process of: birth in animals, seed germination in plants, formation of new cells as a result of division in microorganisms. The total number of new young individuals () that appeared in the population per unit of time (Δt) is called absolute (total) fertility. To compare the birth rate of different populations, the concept is used specific birth rate(b), expressed as the number of new individuals per individual per unit time:

Thus, for human populations, the number of newborn children born in 1 year per 1 thousand population is used as an indicator of specific fertility.

Maximum (potential) fertility- this is the theoretical maximum rate of appearance of new individuals under ideal conditions (when the reproduction rate does not decrease under the influence of limiting environmental factors). The maximum birth rate is a constant value for a given population. In real (natural) conditions of existence of a population, the birth rate is determined by various environmental factors that limit the rate of emergence of new individuals. Therefore, to assess population dynamics, the concept is used ecological (realized) fertility, representing an increase in the number of individuals in a population under specific environmental conditions. Ecological fertility is a variable value and varies greatly depending on population density and environmental conditions.

The difference between maximum and realized fertility can be illustrated by the following example. In experiments with mealy beetles, these beetles laid 12,000 eggs (maximum birth rate), of which only 773 larvae hatched (or 6%)—the value of the realized birth rate. In general, species that do not care for their offspring (for example, many insects, fish, amphibians) are characterized by high potential fertility and low realized fertility.

Mortality- the number of individuals in a population that died during a certain period. The concept of mortality is opposite to the concept of fertility. The total number of dead individuals (ΔN) per unit of time (Δt) is called absolute (total) mortality. Mortality can be expressed as the number of individuals killed per unit of time per individual - specific mortality(d):

Ecological (realized) mortality - the number of dead individuals in specific natural conditions. Like environmental fertility, it is not constant and depends on the characteristics of the environment. Theoretical minimal mortality- a constant value characterizing the death of individuals (from old age) under ideal environmental conditions (i.e., in the absence of the limiting influence of environmental factors). In specific conditions, the rate of population decline is determined by death from predators, animals, and old age.

Often, when describing population dynamics, the concept is used survivability, i.e., the inverse of mortality. If mortality d, then the survival rate is 1 - d.

As well as fertility, mortality, etc. Accordingly, survival rates vary greatly with age in many organisms. In this regard, the determination of specific mortality for different age groups is of great importance, since this allows ecologists to find out the mechanisms that determine the overall mortality in a population. The lifespan of individuals in a population can be estimated using survival curves(Fig. 11.1) By plotting the age of an individual on the x-axis as a percentage of total life expectancy, and on the y-axis the number of individuals surviving to a specific age, we can compare survival curves for species whose life expectancy varies significantly.

Figure 11.1. Types of survival curves; 1 - Drosophila; 2 - person; 3 - freshwater hydra; 4 - oyster.

Survival curves are divided into three general types (see Fig. 11.1)

The first type (convex curves 1 and 2) is characteristic of species in the population in which the highest mortality occurs at the end of life, i.e., mortality remains low almost until the end of the life cycle and increases sharply only in old individuals. Most individuals of the same population have approximately the same life expectancy, for example, large animals.

The other extreme (highly concave curve 4) corresponds to high mortality in the early stages of the life cycle and increased survival of older stages. This type of mortality is characteristic of most plants and animals. The maximum rate of death is characteristic of the larval phase of development or at a young age in animals, as well as in many plants at the stage of seed germination and seedlings. Upon reaching adulthood, organisms become more resistant to the adverse effects of environmental factors, and their mortality rate is significantly reduced (and survival rate increases). Thus, when the larval stages of fish develop to the sexually mature state of adults, as a rule, no more than 1...2% of the total number of spawned eggs survive. In insects, even fewer survive to sexual maturity: from 0.3 to 0.5% of the total number of eggs laid.

The intermediate type (line 3) includes survival curves for those species in which the specific survival rate for each age group is more or less the same (freshwater hydra). There are probably almost no populations in nature in which survival is constant throughout the entire life cycle.

The shape of the survival curve is related to the degree of care for the offspring and other means of protecting the young. Thus, the survival curves of bees and blackbirds (which care for their offspring) are much less concave than those of grasshoppers and sardines (which do not care for their offspring).

Age structure of the population is the ratio of individuals of different ages in a population.

Age composition is an important characteristic of a population that affects both fertility and mortality. Most populations in nature consist of individuals of different ages and sexes.

Simplified, three ecological age groups can be distinguished in the population:

pre-reproductive- young individuals who have not yet reached sexual maturity, i.e., not capable of participating in reproduction;

reproductive- sexually mature individuals capable of participating in reproduction;

post-reproductive- old individuals who have lost the ability to participate in reproduction.

The ratio of these ages to total lifespan in a population varies greatly among species. The quantitative ratio of different age groups in a population is influenced by overall life expectancy, time to reach sexual maturity, reproduction intensity, and mortality at different ages. In turn, the ratio of different age groups in a population determines its ability to reproduce at the moment and shows what can be expected in the future. Changes in the ratio of numbers of main age groups in populations are graphically depicted in the form of age pyramids (Fig. 11.2). In a rapidly growing population, a significant proportion is made up of young individuals (Fig. 11.2, a) of a population whose numbers do not change over time, the age composition is more uniform (Fig. 11.2, b), and in a population whose numbers are declining, the proportion of old individuals will increase ( Fig. 11.2, c).

Rice. 11.2. Three types of age pyramids characterizing populations

with high ( A), moderate ( b) and small ( V) relative abundance

young individuals (% of the total population):

1 - pre-reproductive, 2 - reproductive, 3 - post-reproductive age group

Population growth and growth curves. If the birth rate in a population exceeds the death rate, then an increase in population size is observed.

Each population and each species as a whole is characterized by biotic potential- maximum theoretically possible growth rate ( r) of the population, which is the difference between the specific birth rate ( b) and specific mortality ( d):

r = b-d.

The increase in population size can be described growth curves two main types - the J-shaped curve (exponential growth) and the S-shaped curve (decaying growth).

Exponential growth population size is characterized by a J-shaped growth curve and occurs when food spatial and other important life resources of the population are in excess, and mortality does not increase with increasing numbers of individuals (Fig. 11.3).

The equation for the J-shaped growth curve is

where N is the population size; t- time; r is the population growth rate constant associated with the maximum reproduction rate of an individual of a given species (biotic potential).