The main biogeochemical cycles of carbon, nitrogen, phosphorus, water. Biogeochemical nitrogen cycle and consequences of anthropogenic activity impact on it. Biogeochemical cycles of the most important chemical elements: carbon, oxygen, nitrogen, sulfur, phosphorus,

The carbon cycle.

The most intense biogeochemical cycle is the carbon cycle. IN

nature, carbon exists in two main forms - in carbonates

(limestones) and carbon dioxide. The content of the latter is 50 times greater than

in the atmosphere. Carbon is involved in the formation of carbohydrates, fats, proteins and

nucleic acids.

The bulk is accumulated in carbonates on the ocean floor (1016 t), in

crystalline rocks (1016 tons), coal and oil (1016 tons) and

participates in a large cycle of circulation.

The main link in the great carbon cycle is the interconnection of processes

photosynthesis and aerobic respiration (Fig. 1).

Another link in the large carbon cycle is

anaerobic breathing (without oxygen); various types of anaerobic

bacteria convert organic compounds into methane and other substances

(for example, in wetland ecosystems, in waste dumps).

The small cycle of the cycle involves carbon contained in

plant tissues (about 1011 tons) and animal tissues (about 109 tons).

Oxygen cycle.

In quantitative terms, the main component of living matter is

oxygen, the circulation of which is complicated by its ability to enter into

various chemical reactions, mainly oxidation reactions. IN

as a result, there are many local cycles occurring between

atmosphere, hydrosphere and lithosphere.

(sedimentary calcites, iron ores), is of biogenic origin and should

considered as a product of photosynthesis. This process is the opposite

the process of oxygen consumption during breathing, which is accompanied by

destruction of organic molecules, interaction of oxygen with hydrogen

(split off from the substrate) and the formation of water. In a way

the oxygen cycle resembles the reverse cycle of carbon dioxide. IN

it mainly occurs between the atmosphere and living organisms.

The consumption of atmospheric oxygen and its replacement by plants in

the process of photosynthesis is quite fast. Calculations show

that for the complete renewal of all atmospheric oxygen, about

two thousand years. On the other hand, in order for all water molecules

hydrospheres were subjected to photolysis and synthesized again by living

organisms, it takes two million years. Most of the oxygen

produced during geological eras, did not remain in the atmosphere, but

was fixed by the lithosphere in the form of carbonates, sulfates, iron oxides, and its

the mass is 5.9 * 1016 tons. The mass of oxygen circulating in the biosphere in

in the form of gas or sulfates dissolved in oceanic and continental

waters, several times less (0.4 * 1016 tons).

Note that, starting from a certain concentration, oxygen is very

toxic to cells and tissues (even in aerobic organisms). And alive

the anaerobic organism cannot withstand (this has been proven in the past

century L. Pasteur) oxygen concentration exceeding atmospheric by 1%.

The nitrogen cycle

Nitrogen gas results from the oxidation reaction of ammonia,

formed during volcanic eruptions and decomposition of biological waste:

4NH3 + 3O2 (2N2 + 6H2O.

The nitrogen cycle is one of the most difficult, but at the same time the most

ideal gyres. Despite the fact that nitrogen is about 80%

atmospheric air, in most cases it cannot be

directly used by plants, because they do not digest gaseous

nitrogen. The interference of living beings in the nitrogen cycle is subject to strict

hierarchies: only certain categories of organisms can influence

into separate phases of this cycle. Nitrogen gas is continuously supplied to

atmosphere due to the work of some bacteria, while other bacteria

- fixatives (together with blue-green algae) constantly absorb it,

converting to nitrates. Nitrates are also formed inorganically in the atmosphere.

as a result of electrical discharges during thunderstorms.

The most active nitrogen consumers are bacteria on the root system

plants of the legume family. Each type of these plants has its own special

bacteria that convert nitrogen to nitrates. In the process of biological

cycle nitrate ions (NO3-) and ammonium ions (NH4 +), absorbed by plants from

soil moisture, converted into proteins, nucleic acids, etc. Further

waste is generated in the form of dead organisms that are objects

the vital activity of other bacteria and fungi that convert them into ammonia. So

a new cycle of circulation arises. There are organisms capable of

convert ammonia to nitrite, nitrate and nitrogen gas. Main links

the nitrogen cycle in the biosphere is shown schematically in Fig. 3.

The biological activity of organisms is complemented by industrial

methods of obtaining nitrogen-containing organic and inorganic substances,

many of which are used as fertilizers to increase

productivity and growth of plants.

The anthropogenic impact on the nitrogen cycle is determined by the following

processes:

1.Fuel combustion leads to the formation of nitrogen oxide and then

reactions:

2.2NO + O2 (2NO2,

3.4NO2 + 2H2O. + O2 (4HNO3,

4. contributing to acid rain;

5.As a result of the action of some bacteria on fertilizers and waste

livestock production, nitrous oxide is formed - one of the components,

creating a greenhouse effect;

6. mining of minerals containing nitrate ions and ammonium ions,

for the production of mineral fertilizers;

7. during harvesting, nitrate and ammonium ions are removed from the soil;

8. Runoff from fields, farms and sewers increases the amount of nitrate

ions and ammonium ions in aquatic ecosystems, which accelerates growth

algae and other plants; decomposition of the latter consumes

oxygen, which ultimately leads to the death of fish.

Phosphorus cycle

Phosphorus is one of the main components (mainly in the form and

) of living matter and is part of nucleic acids (DNA and RNA),

cell membranes, adenosine triphosphate (ATP) and adenosine diphosphate (ADP),

fats, bones and teeth. The cycle of phosphorus, like other biogenic

elements, occurs in large and small cycles.

The phosphorus reserves available to living things are completely concentrated in

lithosphere. The main sources of inorganic phosphorus are igneous or

sedimentary rocks. In the earth's crust, the phosphorus content does not exceed 1%, which

limits the productivity of ecosystems. From the rocks of the earth's crust, inorganic

phosphorus is recirculated by continental waters. It is absorbed

plants, which, with its participation, synthesize various organic

compounds and thus are included in the trophic chains. Then

organic phosphates together with corpses, waste and excreta of living creatures

return to the ground, where they are again exposed to microorganisms and

turn into mineral forms used by green plants.

In the ocean ecosystem, phosphorus is brought in by flowing waters, which

promotes the development of phytoplankton and living organisms.

In terrestrial systems, the phosphorus cycle takes place at optimal

natural conditions with a minimum of losses. This is not the case in the ocean. This is

associated with the constant sedimentation (sedimentation) of organic matter.

Organic phosphorus settled at a shallow depth returns to the cycle.

Phosphates deposited at great depths do not participate in shallow

the cycle. However, tectonic movements contribute to the rise of sedimentary

rocks to the surface.

Thus, phosphorus is slowly moved out of phosphate deposits.

on land and shallow oceanic sediments to living organisms and back

Considering the cycle of phosphorus on the scale of the biosphere for a relatively

short period, we can conclude that it is not completely closed. Stocks

phosphorus on earth are small. Therefore, it is believed that phosphorus is the main factor,

limiting growth of the primary production of the biosphere. It is even believed that phosphorus -

the main regulator of all other biogeochemical cycles, it is the most

the weak link in the chain of life that ensures the existence of a person.

The anthropogenic influence on the phosphorus cycle is as follows:

1.the extraction of large quantities of phosphate ores for mineral fertilizers and

detergents leads to a decrease in the amount of phosphorus in

biotic circulation;

2.flows from the field, farms and municipal waste lead to an increase in

phosphate ions in water bodies, to the sharp growth of aquatic plants and

imbalance in aquatic ecosystems.

The sulfur cycle

Sulfur from natural sources enters the atmosphere in the form of hydrogen sulfide,

sulfur dioxide and particles of sulfate salts (Fig. 5).

About one third of sulfur compounds and 99% of sulfur dioxide are anthropogenic

origin. In the atmosphere, reactions occur that lead to acidic

2SO2 + O2 (2SO3,

SO3 + H2O (H2SO4.

The water cycle

Water, like air, is the main component necessary for life. IN

quantitatively, it is the most common inorganic

component of living matter. Seeds of plants in which the water content is not

exceeds 10%, refer to forms of slow life. The same phenomenon

(anhydrobiosis) is observed in some species of animals, which, when

adverse external conditions can lose most of the water in their

Water in three states of aggregation is present in all compound

parts of the biosphere: atmosphere, hydrosphere and lithosphere. If the water located

in various hydrogeological forms, evenly distributed over

corresponding regions of the globe, then layers of the following

thickness: for the World Ocean 2700 m, for glaciers 100 m, for groundwater

15 m, for surface fresh water 0.4 m, for atmospheric moisture 0.03 m.

The main role in the circulation and biogeochemical circulation of water is played by

atmospheric moisture, despite the relatively small thickness of its layer.

Atmospheric moisture is unevenly distributed over the Earth, which causes

large differences in the amount of precipitation in different regions of the biosphere. Average

geographic latitude. For example, at the North Pole it is 2.5 mm (in

column of air with a cross section of 1 cm2), at the equator - 45 mm.

The mechanism of the hydrogeological cycle was mentioned above - in the section

concerning the description of the features of the hydrosphere. Water dropped onto land, then

spent on seepage (or infiltration), evaporation and runoff.

Percolation is especially important for terrestrial ecosystems as it contributes to

water supply to the soil. In the process of infiltration, water enters the aquifers

horizons and underground rivers. Evaporation from the soil surface also plays a role

important role in the water regime of the area, but a more significant amount

the water is emitted by the plants themselves with their foliage. Moreover, the amount of water

excreted by plants, the more, the better they are supplied with it. Plants,

producing one ton of plant matter absorb at least 100 tonnes

The main role in the water cycle on the continents is played by the total

evaporation (trees and soil).

The last component of the water cycle on land is runoff. Surface

runoff and resources of underground aquifers provide water supply

streams. At the same time, with a decrease in the density of the vegetation cover, the runoff

becomes the main cause of soil erosion.

As already noted, water also participates in the biological cycle, being

a source of oxygen and hydrogen. However, its photolysis during photosynthesis is not

plays an essential role in the cycle.

Biogeochemical cycles

Unlike the energy once used by the body,

turns into heat and is lost to the ecosystem, substances circulate in

biosphere, which is called biogeochemical cycles. Out of 90+

elements found in nature, about 40 are needed by living organisms.

The most important for them and required in large quantities: carbon,

hydrogen, oxygen, nitrogen. Oxygen enters the atmosphere as a result

photosynthesis and is consumed by organisms during respiration. Nitrogen is extracted from

atmosphere due to the activity of nitrogen-fixing bacteria and returns to

her with other bacteria.

The cycles of elements and substances are carried out at the expense of

self-regulatory processes, in which all the constituent parts are involved

ecosystems. These processes are waste-free. There is nothing in nature

useless or harmful, even from volcanic eruptions there is a benefit,

since the necessary elements enter the air with volcanic gases,

for example nitrogen.

There is a law of global closure of the biogeochemical cycle in

biosphere, acting at all stages of its development, as well as the rule of increasing

the closedness of the biogeochemical circulation during the succession. In the process

evolution of the biosphere, the role of the biological component in the closure

biogeochemical circulation. An even greater role in biogeochemical

the circulation is rendered by a person. But his role is carried out in the opposite

direction. A person violates the established cycles of substances, and in this

its geological power is manifested, destructive in relation to the biosphere

to date.

When life appeared on Earth 2 billion years ago, the atmosphere

consisted of volcanic gases. It contained a lot of carbon dioxide and little

oxygen (if at all), and the first organisms were anaerobic. As

production on average exceeded respiration, for geological time in

the atmosphere accumulated oxygen and decreased the content of carbon dioxide.

burning large quantities of fossil fuels and reducing absorption

abilities of the "green belt". The latter is the result of a decrease

the number of green plants themselves, and also due to the fact that dust and

Pollutants in the atmosphere reflect the rays entering the atmosphere.

As a result of anthropogenic activity, the degree of isolation

biogeochemical circulation decreases. Although it is quite high (for

different elements and substances, it is not the same), but nevertheless it is not

absolute, which is shown by the example of the emergence of an oxygen atmosphere.

Otherwise, evolution would be impossible (the highest degree of isolation

biogeochemical cycles are observed in tropical ecosystems -

the most ancient and conservative).

Thus, we should not talk about a person's change of what is not

should change, but rather about the human influence on speed and direction

changes and to expand their boundaries, violating the rule of measures of transformation

nature. The latter is formulated as follows: during operation

natural systems can not be exceeded some of the limits that allow these

systems to preserve the properties of self-support. Breaking the measure aside

increase, and in the direction of decrease leads to negative

results. For example, excess fertilization is just as harmful as

flaw. This sense of proportion has been lost by modern man who believes that

everything is allowed to him in the biosphere.

Hopes for overcoming environmental difficulties are pinned, in

in particular, with the development and commissioning of closed

technological cycles. Human-Made Material Cycles

it is considered desirable to arrange them so that they are similar to natural

cycles of the circulation of substances. Then the problems would be solved simultaneously

providing humanity with irreplaceable resources and the problem of protecting

the natural environment from pollution, since now only 1 - 2% of the weight of natural

resources are disposed of in the final product.

Theoretically, closed cycles of transformation of matter are possible. but

complete and final restructuring of the industry according to the cycle principle

substance in nature is not real. At least a temporary violation of isolation

technological cycle is almost inevitable, for example, when creating

synthetic material with new properties unknown to nature. Such

the substance is first comprehensively tested in practice, and only then can

methods of its decomposition be developed with the aim of introducing its constituent parts

into natural cycles.

Similar information.

The term "biogeochemistry" was proposed by the Russian scientist VI Vernadsky and means the field of science about the exchange of substances between living and nonliving matter of the biosphere ("bio" refers to living organisms, and "geo" - to rocks, air and water). 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 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 (mobile) fund- a smaller, but more active mass of a substance, which is characterized by a 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 readily available; therefore, such cycles are relatively stable. Sedimentary biogeochemical cycles are generally less stable.

The amazing constancy of the percentage 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 an opportunity for self-regulation (homeostasis) of the system and maintaining its stability.

The processes of new formation of organic matter in the course of 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 the solar energy coming from outside. Consequently, the speed and direction of the cyclic movement of elements in the ecosystem are determined by the flows of energy passing through the biological community.

The generalized scheme of biogeochemical cycles in combination with a simplified scheme of energy flow (Fig. 10.1) shows how a unidirectional flow of energy drives the circulation of matter. Attention is drawn to the fact that the chemical elements involved in the process of circulation repeatedly pass the same path, and the energy flows only in one direction.

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 a reserve fund is called unavailable, and an active exchange fund is called available. For example, agronomists usually 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 due to substances that return to the circulation in two main ways - either as a result of vital excretions of metabolic products by animals and plants into the external environment, or during the destruction (mineralization) of dead organic matter (detritus) by microorganisms.

The influence of man on biogeochemical cycles is that with anthropogenic interference, 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 any substances. Ultimately, measures to protect natural resources should be aimed at preventing cyclical disruptions, i.e. balancing the cycles of the most important elements in the biosphere. Knowledge of the features of biochemical cycles is a necessary condition for the rational use of natural resources and the preservation of natural ecosystems.

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

Р G - gross primary production; Р N - net primary production (can be consumed by heterophiles in the system itself or exported); Р - 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 are involved in the cycles of mineral substances in an ecosystem: living organisms, dead organic detritus, and available inorganic substances in the habitat.

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

The biogeochemical cycle of sulfur is an example of a functional relationship between the atmosphere, water and the earth's crust, since sulfur actively circulates in each of these "reservoirs" and between them. Microorganisms play a key role in the nitrogen and sulfur cycle.

The nitrogen cycle, including both gas and mineral phases, despite the large number of organisms participating in it, provides a rapid circulation of nitrogen in various ecosystems (Fig. 10.2).

Rice. 10.2... Nitrogen cycle diagram (gray rectangle - nitrogen reserve fund)

The main source and reservoir of nitrogen is the atmosphere, the mass of which is 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 the 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 assimilate nitrogen only in the composition of nitrate ions (NO 3 -) or ammonium ions (NH 4 +). Nitrates are formed mainly as a result of the vital activity of microorganisms - nitrogen fixers, which include symbiotic bacteria of the genus Rhizobium, living in nodules on the roots of legumes, bacteria of the genus Azotobacter, dwelling in the soil; and cyanobacteria (blue-green). All nitrogen-fixing microorganisms are capable of fixing atmospheric nitrogen due to a very complex metabolism, which includes molybdenum and hemoglobin as catalysts. Symbiotic nitrogen-fixing microorganisms penetrate into the tissues of the root system of leguminous plants. Plants provide the symbiotic bacteria with habitat and food (sugars), and these supply the plant with organic nitrogen, which they synthesize from nitrogen gas. Free living non-symbiotic microorganisms - nitrogen fixers ( Azotobacter in cyanobacteria) also assimilate gaseous nitrogen 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 the nitrate form (NO 3 -) enriches the soil.

Animals can only absorb nitrogen 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 take place in soils ammonification(formation of ammonium ions) and nitrification(formation of nitrate - ions), consisting of a series of successive reactions, during which, with the participation of different groups of microorganisms, the destruction of dead organic matter occurs.

Molecular nitrogen returns to the atmosphere and the biogeochemical nitrogen cycle is closed during the life of bacteria - denitrifiers kind Pseudomonas, reducing nitrates to free nitrogen and oxygen in 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. Then these nitrates fall out with rains on the soil surface. Another source of atmospheric nitrogen input into the biogeochemical cycle is volcanoes, which compensate for the loss of nitrogen cut off 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 input into the biogeochemical cycle, it is necessary to bear in mind the following: the average annual input 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, 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 (N 2) and returned to the atmosphere. Only about 10% of mineral nitrogen is absorbed from the soil by higher plants and is at the disposal of multicellular organisms.

The phosphorus cycle. Phosphorus is a part of the energy-rich organic substances - adenosine nitriphosphate (ATP) and adenosine diphosphate (ADP), which are carriers and accumulators of energy in plant and animal cells. The main source of phosphorus for plants is phosphate ions (PO 4 -). Plants absorb phosphate ions from the environment (soil or water) and in the process of biosynthesis include phosphorus in the composition of organic substances that form the biomass of plants. Animals, eating plants, receive 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 the terrestrial parts of plants and algae varies from 0.01 to 0.1%, and in animals from 0.1% to several percent, in the process of circulation it gradually passes from organic compounds to phosphates, which can again be used by plants (Figure 10.3 ).

Rice. 10.3. Phosphorus cycle diagram (gray rectangle - phosphorus reserve fund)

If we compare the phosphorus content in the living and inanimate matter of the biosphere, it turns out that the disproportion is very large. Therefore, phosphorus is one of the most scarce 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, in the process of leaching, water-soluble phosphates (PO 4 3-) enter terrestrial and aquatic ecosystems. Getting into the ecosystems of the land, phosphorus is absorbed by plants from an aqueous solution in the form of inorganic phosphate - ion (PO 4 3-) and is included in the composition of various organophosphorus compounds. Through the food chains, phosphorus-containing organic matter passes 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 is 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. The river runoff annually carries out about 2 million tons of phosphorus into the World Ocean.

In marine ecosystems, mineral phosphorus is converted into phytoplankton, which serves as food for other organisms in the sea, and accumulates in the tissues of marine animals, such as fish. Part of organic phosphorus compounds migrate along food chains within shallow depths, while the other part sinks to great depths in the process of sedimentation of dead organic matter. The dead remains of organisms lead to the accumulation of phosphorus at different depths. It follows that phosphorus, getting into water bodies in one way or another, saturates, and often oversaturates their ecosystems. The reverse movement of phosphorus from the oceans to land and into 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.

The 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 of sedimentary nature and occurs in soil and water.

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

Figure 10.4. Sulfur cycle diagram

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 enter living organisms only in dissolved form. Plants, absorbing sulfates, restore them and produce essential sulfur-containing amino acids (methionine, cysteine, cystine), which play an important role in the creation of the tertiary (spatial) structure of proteins. Animals and microorganisms, consuming plant biomass for food, assimilate sulfur-containing organic compounds.

With the decomposition of dead organic matter (fallen leaves, dead organisms, excretion products) by heterotrophic bacteria, sulfur again passes into an 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, thereby again increasing the supply of sulfur in a form available to plants. Such bacteria are called chemosynthetic, since they synthesize organic substances due to the oxidation energy of simple chemicals (in this case, hydrogen sulfide). By this circumstance, they differ from photosynthetic organisms that create organic substances due to the energy of light.

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. Thus, the process ends with a slow and gradual accumulation of sulfur in deep sedimentary rocks.

In the whole ecosystem, in comparison with nitrogen and phosphorus, much less sulfur is required. Therefore, sulfur is less often a limiting factor for the development of plants and animals. At the same time, the sulfur cycle is one of the key ones in the general process of creating the decomposition of organic matter of biomass in the biosphere. For example, when iron sulfides are formed in sediments, phosphorus from an insoluble form passes into a soluble one and becomes available for photosynthetic organisms. This is a clear confirmation that one cycle is connected with another and regulated by it.

The carbon cycle... Carbon, as the most important structural element, is a part of any organic matter; therefore, its circulation 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 (limestone) and in the form of a mobile form of carbon dioxide (carbonic acid, CO 2). In the biochemical carbon cycle, the atmospheric carbon dioxide fund is relatively small (711 billion tons) in comparison with the carbon reserves in the oceans (39,000 billion tons), in fossil fuels (12,000 billion tons) and terrestrial ecosystems (3100 billion tons).

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

All these compounds are interconnected and can transform into each other in the process of chemical reactions when environmental conditions change. For example, in the case of an increase in the acidity of water (at low pH values), carbonic acid molecules decompose 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 (СОЗ 2–), poorly soluble calcium carbonates (CaCO 3) and magnesium (MgCO 3), which sink to the bottom in the form of a 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) in the process of photosynthesis is included in the organic matter of plants and then along the food chain enters the organisms of animals and microorganisms. The reverse process of the transition of carbon from an organic to a mineral form occurs during the respiration of all organisms of animals and plants (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 soil, the biogeochemical cycle of carbon 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 a 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 - carbon reserve funds)

The oceanic branch of the biogeochemical carbon cycle has its own characteristics, which, given the significant volume 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 unicellular microscopic algae floating in the water column (phytoplankton).

The vital 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 generate significant reserves of organic carbon in the biomass of living organisms. Most of the organic carbon in the hydrosphere is re-consumed and eventually oxidized to a mineral form - carbon dioxide (CO 2). Another part of the dead organic matter (detritus) under the action of gravity settles into the deep layers of the water column and is deposited at the bottom, where it can be stored for a long time in the form of organic sediments.

A small part of organic matter and carbon contained in it, according to V.I. Vernadsky, escapes the cycle and "goes into geology" - in 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 CO2 balance in the atmosphere

Source: A.M. Tarko Stability of biospheric processes and Le Chatelier principles // Doklady RAN. 1995. T. 343. No. 3. S. 123.

Thus, 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 the carbon dioxide content in the atmosphere by more than 25% and has caused the stimulation of the greenhouse effect. In turn, a change in the Earth's climatic regime can and is already leading to global climate change.

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

Control questions and tasks.

1. What are called 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 biogeochemical cycles of elements pass.

4. In the cycle of which biogenic elements 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 the same 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 set of populations of organisms of different species, functioning as an integral system in a certain physical and geographical space of the habitat.

The adaptive capacity of the population is much higher than that of its constituent individuals. The population as a biological unit has a certain structure and functions.

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

Below are the main indicators of the population.

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

Sometimes it is important to distinguish specific, or ecological density(the number or biomass per unit of inhabited space, i.e., actually available for organisms of a particular population) and medium density(the size of the population referred to a unit of space within the geographical limits of the population's habitat). For example, the average density of wood frogs is their number per forest area. However, these animals live only in swampy areas 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 habitat 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 clusters of organisms of the same species (group distribution: family groups and flocks in animals, group thickets in plants).

The most complete picture of the population density is provided by the complex use of indicators: the number of individuals characterizes well their average distance from each other; biomass - the concentration of living matter; calorie content - the amount of energy bound in organisms. Typically, the plant population density is higher than the herbivore population density 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 supply of organisms with food, water, living space, etc. Negatively affects the existence of the population and its insufficient density, which makes it difficult to select individuals of the opposite sex, protect the population from predators, etc. (see Lecture 6 for more details on mass and group effects).

There are a number of mechanisms for maintaining the population density at the required level. The main one is self-regulation of the population size according to the principle of feedback with the amount and limited life resources, in particular, food. So, when there is less food, the growth of individuals slows down, mortality increases, sexual maturity (that is, the ability to reproduce) occurs later, and as a result, the size of the population and its density decreases. The improvement in living conditions is accompanied by changes of the opposite character, and the population density increases to a certain limit. The size of a population may fluctuate due to migration, generational change, the appearance of new individuals (due to the birth and introduction from other populations) or as a result of death. The study of population dynamics is very important for predicting outbreaks of pest or game animals.

The population size is determined mainly by two opposite phenomena - fertility and mortality.

Fertility is the ability of a population to increase in size. It characterizes the emergence of new organisms in the process: birth in animals, germination of seeds in plants, the 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 time (Δt) is called absolute (total) fertility... To compare the fertility of different populations, the concept is used specific fertility(b) in terms of the number of new individuals per individual per unit of time:

So, for human populations, the number of newborns born in 1 year per 1,000 population is used as an indicator of specific fertility.

Maximum (potential) fertility is the theoretical maximum of the rate of emergence of new individuals under ideal conditions (when the rate of reproduction does not decrease under the influence of limiting environmental factors). Maximum fertility is a constant value for a given population. In real (natural) conditions of the existence of a population, the birth rate is determined by various environmental factors that limit the rate of appearance of new individuals. Therefore, to assess the dynamics of the population size, 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 and highly variable depending on population density and environmental conditions.

The difference between maximum and realized fertility can be illustrated by the following example. In experiments with the flour beetle, these beetles laid 12,000 eggs (maximum fertility), of which only 773 larvae (or 6%) hatched - the value of the realized fertility. In general, biological species that are not inherent in caring for offspring (for example, many insects, fish, amphibians) are characterized by high potential fertility and low realized fertility.

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

Ecological (realized) mortality - the number of dead individuals in specific natural conditions... Like ecological 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). Under specific conditions, the rate of population decline is determined by the death from predators, more useful and old age.

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

Like fertility, mortality, etc. accordingly, survival in many organisms varies greatly with age. In this regard, it is of great importance to determine the specific mortality for different age groups, since this allows ecologists to elucidate the mechanisms that determine the total mortality in the population. The lifespan of individuals in a population can be estimated using survival curves(Fig. 11.1) Plotting on the abscissa the age of an individual as a percentage of the total life span, and on the ordinate - the number of individuals who survived to a particular age, one can compare the survival curves for species, the life expectancy of individuals of which varies significantly.

Fig 11.1. Survival curve types; 1 - fruit fly; 2 - person; 3 - freshwater hydra; 4 - oyster.

Survival curves are classified into three general types (see Figure 11.1)

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

The other extreme option (strongly concave curve 4) corresponds to a high mortality rate in the early stages of the life cycle and an increase in the survival rate of the older stages. This type of mortality is common in 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 decreases significantly (and survival increases). So, with the development of the larval stages of fish 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 less survives to sexual maturity: from 0.3 to 0.5% of the total number of laid eggs.

The intermediate type (line 3) includes the survival curves for those species in which the specific survival rate for each age group is more or less the same (freshwater hydra). Probably, in nature, there are almost no populations in which the survival rate 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 ways of protecting the juveniles. Thus, the survival curves of bees and thrushes (which take care of the offspring) are significantly less concave than those of grasshoppers and sardines (which do not take care of the offspring).

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

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 sex.

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

pre-reproductive- young individuals who have not yet reached sexual maturity, that is, they are not able to participate in reproduction;

reproductive- sexually mature individuals capable of participating in reproduction;

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

The ratio of these ages to total life expectancy in a population varies greatly from species to species. The quantitative ratio of different age groups in the population is influenced by the total life expectancy, the time to puberty, the intensity of reproduction, and mortality at different ages. In turn, the ratio of different age groups in a population determines its ability to reproduce at a given moment and shows what can be expected in the future. Changes in the ratio of the number 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 are young individuals (Figure 11.2, a) of the population, the number of which does not change over time, the age composition is more uniform (Figure 11.2, b), and in the population, the number of which is decreasing, the proportion of old individuals will increase ( Fig.11.2, c).

Rice. 11.2... Three types of age pyramids that characterize populations

with high ( but), moderate ( b) and small ( in) relative number

young individuals (in% of the total population):

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

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

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

r = b-d.

Population growth can be described growth curves two main types - J-shaped curve (exponential growth) and S-shaped curve (damped growth).

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

The equation for the J-shaped growth curve is

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

Nitrogen is one of the elements that separated in the gas phase already at the stage of the formation of the Earth in the process of shock degassing. Subsequently, the release of gaseous nitrogen compounds from the interior of the Earth continued during volcanic eruptions, the removal of hydrothermal fluids and gas jets. Gaseous molecular nitrogen, due to its chemical inertness, is the most stable form of this element. For this reason, N 2 initially accumulated in the atmosphere, and did not concentrate in the form of dissolved compounds in ocean water, like chlorine, or in the form of insoluble compounds in ocean sediments, like carbon in carbonate strata. At present, the annual inflow of gaseous nitrogen compounds from the Earth's interior into the atmosphere is 1.0 × 10 6 tons. In the ocean, nitrogen is present in the form of dissolved ions, in the composition of dissolved and dispersed-suspended organic matter. The mass of nitrogen in the form of dissolved ions NH 4 +, NO 2 -, NO 3 - is 685 × 10 9 tons. In the granite layer of the earth's crust, the nitrogen concentration is 0.002%, and the total mass of the element is 165 × 10 12 tons. In the sedimentary shell nitrogen is fixed in organic matter. The mass of nitrogen in the sedimentary shell is approximately 0.6 × 10 15 tons, that is, in the sedimentary shell there is three times more nitrogen, and in the atmosphere 23 times more than in the granite layer of the Earth.

So, the main supplier of nitrogen to the biosphere is the bowels of the Earth, the main storage is the atmosphere, more precisely, the troposphere. The composition of the atmospheric gas is continuously renewed due to the cyclical processes of mass transfer that connect the atmosphere with the World land, the pedosphere, the ocean and its precipitation.

The modern structure of the global cycle of nitrogen mass transfer is very complex and consists of several interconnected cycles (Fig. 32).

The wonderful property of nitrogen is its strongly expressed polyvalence. Organisms receive energy for their life, transferring nitrogen from one form to another, changing its valence under different conditions. It is possible that, not without the influence of this circumstance, nitrogen is a necessary component of proteins.

There are some types of bacteria that can activate chemically inactive molecular nitrogen and bind it into chemical compounds. This process is called nitrogen fixation.

Nitrogen fixation is carried out by individual specialized bacteria of the Azotobacteracea family and, under certain conditions, by blue-green algae. The most productive are nitrogen-fixing nodule bacteria that form symbiosis with legumes.

Rice. 32. Schematic diagram of the nitrogen cycle

The mass of nitrogen fixed from the air by soil bacteria, before the start of human economic activity per year, ranges from (30-40) ∙ 10 6 to 200 × 10 6 tons. artificial biological fixation obtained with the help of leguminous agricultural plants (about 20 × 10 9 t), as well as industrial nitrogen fixation from the air, which exceeded 60 × 10 6 tons.

The first interconnected bacterial process in soil is ammonification- microbiological transformation of nitrogen of organic compounds (mainly amino acids) into ammonium ion or ammonia. The decomposition of organic matter takes place under aerobic conditions and is accompanied by the active formation of CO 2. Ammonium undergoes the following transformation process. IN aerobic conditions happens nitrification: conversion of ammonia to nitrite ion by some bacteria, and then to nitrate by others. IN anaerobic conditions processes are developing denitrification, as a result of which nitrates and nitrites are reduced to nitrous oxide or to gaseous molecular nitrogen. The amount of nitrous oxide is several times less than the mass of N 2 fixed by bacteria. As a result, molecular nitrogen, after various biochemical transformations, returns to the atmosphere. The nitrogen cycle, caused by its bacterial fixation and further transformation, is closely related to another powerful cycle of this element. Large masses of nitrate and ammonium nitrogen are captured from the pedosphere into the biological cycle, which occurs due to the activity of photosynthetic plants and microorganisms that destroy plant residues... Part of the nitrogen is removed from the biological cycle and accumulates in dead organic matter. This unique reserve of nitrogen in forest litter, peat and soil humus is constantly maintained in the pedosphere and indicates a certain inhibition of the biological cycle on land. Forest fires make a significant contribution to the annual release of nitrogen oxides into the atmosphere, due to which from 10 × 10 6 to 200 × 10 6 tons of nitrogen enter the atmosphere.

In the ocean, the same processes of transformation and migration of nitrogen compounds occur as on land, but the ratio of these processes is different. The life cycles of photosynthetic organisms in the ocean are much faster than on land.

In small quantities, atmospheric nitrogen binds with oxygen during lightning discharges in the atmosphere, and then falls to the soil surface with rains.

After analyzing Figure 32, highlight the stages of the nitrogen function of living organisms ...

1) biological nitrogen fixation; 2) ammonification; 3) nitrification; 4) denitrification;

1) nitrification; 2) ammonification; 3) protein synthesis; 4) photochemical binding;

1) photosynthesis; 2) decomposition by bacteria; 3) nitrogen fixation; 4) ammonification;

1) ammonification; 2) nitrification; 3) denitrification; 4) electrochemical bonding.

Nitrogen gas(N2) is extremely inert in the atmosphere, in other words, a very large amount of energy is required 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 a few 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 pathway for 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. For this, bacteria use the enzyme nitrogenase and the energy contained in ATP. 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 is so important for soil fertility, and the need for it in agriculture is so great that every year chemical plants produce colossal amounts of ammonia, which is used in nitrogen fertilizers such as ammonium nitrate (NH4NO3) or urea.

Now scale of industrial nitrogen fixation are comparable with natural ones, but we still have a poor 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 bind to the atmospheric pool.

The 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%) gives ionization in the atmosphere. The resulting nitrogen oxides, interacting with rainwater, give the corresponding acids, which, once in the soil, are ultimately converted to nitrates.

Probably, main natural source of fixed nitrogen- representatives of the legume family, such as 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 from the bacteria in the form of ammonia, 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. Unsurprisingly, legumes are often sown to enrich the soil with this element, while also producing 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 nitrogen source use nitration. Animals, in turn, directly or indirectly receive assimilable nitrogen from plants. In fig. 10.11 shows how nitrates are formed after the degradation of the protein of dead tissues by saprotrophic bacteria and fungi. This process involves oxidative reactions involving oxygen and aerobic bacteria. Proteins are first broken down to amino acids, and then amino acids give ammonia. The same product is formed during the decomposition of animal excreta and faeces. Chemosynthesizing bacteria Nitrosomonas and Nitrobacter carry out the so-called nitrification - stage by stage ammonia is oxidized 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 nitrates are used in respiration instead of oxygen as an oxidizer for organic compounds (electron acceptor). In this case, the nitrates themselves are reduced, usually to nitrogen. Therefore, denitrifying bacteria are facultative aerobes.

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

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

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

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

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

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

There are several types of water cycles in nature:

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

Small, or oceanic, cycle - water vapor formed over the surface of the ocean condenses and falls as precipitation back into the ocean.

Intracontinental circulation - water that has evaporated above the land surface falls back onto land in the form of atmospheric precipitation.

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

Oxygen cycle

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

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

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

The carbon cycle.

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

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

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

Up to 50% (according to some data - up to 90%) of carbon in the form of dioxide is returned to the atmosphere by soil decomposers. Bacteria and fungi make an equal contribution to this process. The return of carbon dioxide during the respiration of all other organisms is thus less than during the activity of decomposers.

Some bacteria produce methane in addition to carbon dioxide. The release of methane from the soil increases with waterlogging, when anaerobic conditions are created that are favorable for the activity of methane-forming bacteria. For this reason, the release of methane from the forest soil sharply increases if the stand is cut down and, due to a decrease in transpiration, it becomes waterlogged. Rice fields and livestock give off a lot of methane.

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

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

The phosphorus cycle.

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

In the process of cellular respiration, organic compounds containing phosphorus are oxidized, while organic phosphates enter the environment as excretions. Reducing organisms mineralize organic matter containing phosphorus into inorganic phosphates, which can be reused by plants and thus re-involved in the circulation.

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

The sulfur cycle

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