Biogeochemical cycles of the main chemical elements
Introduction
The emergence of living matter on Earth made possible the continuous circulation of chemical elements in the biosphere, their transition from the external environment into organisms and vice versa. This circulation of chemical elements is called biogeochemical cycles. Biogeochemical cycle is a part of the biotic cycle, including the exchange cycles of chemical elements of abiotic origin, without which living matter cannot exist (carbon, oxygen, hydrogen, nitrogen, phosphorus, sulfur, and many others). Usually, three main types of biogeochemical cycles are distinguished: the water cycle, cycles of gaseous substances with a reserve fund in the atmosphere or hydrosphere (ocean), sedimentary cycles of chemical elements with a reserve fund in the earth's crust.
The water cycle
Water is the basic element necessary for life. In quantitative terms, this is the most common inorganic component of living matter.
97% of the total mass of water in the biosphere is concentrated in the oceans. It is assumed that evapotranspiration is balanced by precipitation. More water evaporates from the ocean than comes into it with precipitation, on land - on the contrary. “Extra” precipitation falling on land falls into ice caps and glaciers, replenishes groundwater (from there plants draw water for transpiration), and finally ends up in lakes and rivers, gradually returning with runoff to the ocean. Basically, the water cycle occurs between the atmosphere and the ocean.
The presence in the atmosphere of a significant reserve fund favors the fact that the cycles of some gaseous substances are capable of fairly rapid self-regulation in various local imbalances. Thus, an excess of carbon dioxide accumulated somewhere as a result of increased oxidation or combustion is quickly dissipated by the wind; in addition, the intensive formation of carbon dioxide is compensated by its large consumption by plants or its conversion into carbonates. Ultimately, as a result of self-regulation by the type of negative feedback, the cycles of gaseous substances on a global scale are relatively perfect. The main such cycles are the cycles of carbon (in the composition of carbon dioxide), nitrogen, oxygen, phosphorus, sulfur and other biogenic elements.
The carbon cycle
On land, it begins with the fixation of carbon dioxide by plants through photosynthesis to form organic matter and by-production of oxygen. Part of the bound carbon is released during the respiration of plants as part of CO2
Soil fungi, depending on the growth rate, emit from 200 to 2000 cm 3 CO 2 per 1 g of dry mass. A lot of carbon dioxide is emitted by bacteria, which, in terms of live weight, respire 200 times more intensively than a person. Carbon dioxide is also emitted by plant roots and numerous living organisms. Microorganisms decompose obsolete plants and dead animals, as a result of which the carbon of dead organic matter is oxidized to carbon dioxide and re-enters the atmosphere.
Carbon migration processes are constantly going on between land and the World Ocean, in which its removal in the form of carbonate and organic compounds from land to the ocean predominates. From the oceans to land, carbon comes in small quantities in the form of CO2 released into the atmosphere. Carbon dioxide of the atmosphere and hydrosphere is exchanged and renewed by living organisms for 395 years.
nitrogen cycle
Just like the carbon cycle and other cycles, it covers all areas of the biosphere. In the cycle of nitrogen compounds, microorganisms play a key role: nitrogen fixers, nitrifiers and denitrifiers. Other organisms affect the nitrogen cycle only after it enters the composition of their cells. As is known, legumes and representatives of some genera of other vascular plants (for example, alder, araucaria, goof) fix nitrogen with the help of symbiont bacteria. The same is observed in some lichens that fix nitrogen with the help of symbiotic blue-green algae. Obviously, the biological fixation of molecular nitrogen by free-living and symbiotic organisms occurs both in autotrophic and heterotrophic parts of ecosystems.
Of the huge reserves of nitrogen in the atmosphere and the sedimentary shell of the lithosphere, only fixed nitrogen, assimilated by living organisms on land and ocean, participates in its cycle. The category of the exchange fund of this element includes: nitrogen of annual biomass production, nitrogen of biological fixation by bacteria and other organisms, juvenile (volcanogenic) nitrogen, atmospheric (fixed during thunderstorms) and technogenic
It is easy to see that, with the exception of tundra vegetation, where the content of nitrogen and ash elements is approximately the same, in vegetation of almost all other types, the mass of nitrogen is 2 ... 3 times less than the mass of ash elements. The number of elements circulating during the year (i.e., the capacity of the biological cycle) is greatest in tropical forests, then in chernozem steppes and broad-leaved forests of the temperate zone (oak forests).
Oxygen cycle
The active geochemical activity of living matter and its primary role in this process are clearly expressed in the oxygen cycle. The biogeochemical oxygen cycle is a planetary process that links the atmosphere and hydrosphere to the earth's crust. The key links in this cycle are: the formation of free oxygen during photosynthesis in green plants, its consumption for the implementation of respiratory functions by all living organisms, for the oxidation of organic residues and inorganic substances (for example, fuel combustion) and other chemical transformations leading to the formation of such oxidized compounds, like carbon dioxide and water, and their subsequent involvement in a new cycle of photosynthetic transformations.
Consideration should also be given to the use of oxygen for combustion and other human activities. It is assumed that in the foreseeable future, the annual total oxygen consumption will reach 210...230 billion tons. Meanwhile, the annual production of this gas by the entire phytosphere is 240 billion tons.
Phosphorus cycle
The clarke of this element in the earth's crust is 0.093 %, which is several tens of times greater than the clarke of nitrogen. However in unlike the latter, phosphorus does not play the role of one of the main elements of the Earth's shells. Nevertheless, the geochemical cycle of phosphorus includes various migration routes in the earth's crust, an intense biological cycle, and migration in the hydrosphere. Phosphorus is one of the main organogenic elements. His organic compounds play an important role in the life processes of all plants and animals, are part of nucleic acids, complex proteins, membrane phospholipids, are the basis of bioenergetic processes. Phosphorus is concentrated in living matter, where its content is almost 10 times higher than in the earth's crust. On land, an intensive cycle of phosphorus occurs in the soil-plants-animals-soil system.
A fairly developed process of cyclic transformations of sulfur and its compounds has been formed in the biosphere. Reserve funds of this element are allocated in soil and sediments (rather extensive), as well as in the atmosphere (small). In the exchange fund of sulfur, the main role belongs to specialized microorganisms, some species of which perform an oxidation reaction, others - reduction. The nitrogen and sulfur cycles are increasingly affected by industrial air pollution. The burning of fossil fuels significantly increases the release into the atmosphere (and, of course, the content in it) of volatile oxides of nitrogen (NO and NO2,) and sulfur (SO2), especially in cities. The current concentration of these ingredients is already becoming dangerous for the biotic components of ecosystems.
Potassium cycling
Potassium, as is known, takes part in the processes of photosynthesis, affects carbohydrate, nitrogen and phosphorus metabolism, significantly affects the osmotic properties of cells. It is concentrated in fruits and seeds, in intensively growing tissues and organs of plants.
So far, the potassium cycle in the aquatic environment remains poorly understood. Every year, about 90 million tons of this element enter the World Ocean with water runoff. Some of it is absorbed by aquatic organisms, but a significant amount is not recorded anywhere, and its subsequent movement is unknown.
An important component of the cycles is ionic and solid runoff. The circulation of chemical elements takes place, as a rule, in several adjacent shells of the Earth at once (atmosphere and hydrosphere, hydrosphere and pedosphere) or in all three geospheres simultaneously. The reliability and constancy of the implementation of the cycles are provided by the regular exchange of matter and energy between the geospheres. This kind of directional relationship is clearly manifested in the example of ion sink, which is the process of removal of chemical elements in the ionic dissolved state by rivers from the land to the World Ocean. Chemical elements received in ionic form, as well as on land, in the aquatic environment are exposed to living organisms, continuing the cycle. The migration of chemical elements in a dissolved state is a gigantic planetary process.
The solid matter of the Earth's surface does not remain motionless. It also participates in migration, moving by the surface waters of the land. Surface waters, along with elements migrating in a dissolved state or with colloidal particles, carry huge masses of rock fragments and minerals, called solid runoff (by analogy with water runoff). A significant part of the solid runoff moves within the land, but the volumes that enter the seas are quite large. Every year, 22.13 billion tons of clastic and clayey material enters the World Ocean from the continents, which is approximately 7 times the amount of dissolved substances carried out.
Biotechnosphere and noosphere
Peculiarities of biogeochemical cycles of migration. The biosphere is not only perfect organized system, but a kind of “mechanism”, in which the connection and relationship between living and inert matter obey strict laws, as immutable as the laws of motion of heavenly bodies. Geochemically, these functions of life are carried out due to the reproduction of organisms. Living matter overcomes the resistance of the environment, strives to spread to the free territory.
The rate of reproduction is the rate of transfer of geochemical energy in the biosphere. It depends not only on astronomical parameters, but also on the speed of propagation of a solar beam in the environment, on the size of organisms, and on the geochemical energy contained in them.
An essential feature of living matter is its difference from the "inert" environment in terms of spatial and temporal characteristics. Living matter corresponds to special space and time inherent only to it.
The time of individual existence of living organisms is associated with the steadily ongoing process of aging and death, which have a positive value for the evolutionary process, since the fragility of living beings ensures not only a long and continuous cycle of biogenic material, but also a significant variability of morphological forms.
Human impact on the biosphere
With the growth in the use of natural resources due to the industrial revolution, the anthropogenic impact on the biosphere and its components objectively increases. The natural and multilateral process of the growth of productive forces has significantly expanded the range of human impact on nature (including negative ones). Vernadsky noted that human production activity is acquiring a scale comparable to geological transformations. Thus, the reduction of forests, the plowing of virgin lands, erosion and salinization of soils, and the decrease in biodiversity have added new permanent mechanical and physico-chemical factors that aggravate the environmental risk.
Man already exploits more than 55% of the land, uses about 13 % river waters, the rate of deforestation reaches 18 million hectares per year.
The impact on the biosphere comes down to four main forms:
Changing the structure of the earth's surface (plowing steppes, deforestation, land reclamation, creation of artificial lakes and seas, other changes in the regime of surface waters, etc.):
Changes in the composition of the biosphere, the cycle and balance of its constituent substances (removal of minerals, the formation of dumps, the release of various substances into the atmosphere and water bodies, changes in moisture circulation);
Changes in the energy, in particular heat, balance of individual regions of the globe, dangerous for the entire planet;
Changes made to the biota (the totality of living organisms) as a result of the extermination of some species, the creation of new animal breeds and plant varieties, and their movement to new habitats.
Considering the role of man in the evolution of the biosphere, they characterize the violation by man of the basic principles of the natural structure of the biosphere.
2. Biogeochemical cycles of biogenic elements involved in natural cycles have been worked out evolutionarily and do not lead to waste accumulation. Man, on the other hand, uses the substance of the planet extremely inefficiently; in this case, a huge amount of waste is generated, many of which are transferred from the passive form in which they were in the natural environment to an active, toxic form. As a result, the biosphere is “enriched” with compounds unusual for it, i.e. the natural ratio of chemical elements and substances is violated.
3. With a huge variety of species, competitive and predatory relations between them contribute to the establishment of biological balance. The path of mankind, unfortunately, is marked by the death of many representatives of flora and fauna. According to some reports, one biological species disappears on Earth every day.
4. The activity of people has led to a violation of population stability. The number of species associated with humans (rats, cockroaches, etc.) is growing, while the number of many other populations, on the contrary, is declining, and sometimes in catastrophic proportions, which puts the species in danger of complete extinction.
5. Expanding economic activity, people in a short time change the parameters of environmental factors; many species do not have time to adapt to such rapid changes.
The complex of anthropogenic factors influencing the state of the biosphere and the health of the population is extremely diverse.
Biotechnosphere
Biotechnosphere is an area of our planet in which living matter and man-made urban-technical objects exist and where their interaction and influence on external environment. The biotechnosphere is a complex conglomeration of many subsystems that are controlled by a person. These subsystems do not accumulate, but consume the energy, biomass and oxygen of the biosphere.
The biotechnosphere and its technogenic subsystems are located in the biosphere, but they do not have most of the properties and functions that are inherent in natural ecosystems.
As long as humanity exists, the biotechnosphere will develop. But the biotechnosphere should be in a state of ecological self-sufficiency, consistent with the laws of nature and satisfying the needs of human society. At the same time, society must purposefully and reasonably influence the forces of nature.
Noosphere
Noosphere- the highest stage of development of the biosphere, characterized by the preservation of all natural patterns inherent in the biosphere (with a high level of development of productive forces, the scientific organization of the impact of society on nature), the maximum possibilities of society to satisfy the material and cultural needs of man.
The noosphere is a new state of the biosphere, based on the universal connection between nature and society, when the further evolution of the planet Earth becomes guided by reason.
He considers the need to transfer the biosphere to the noosphere as a guarantor of the survival of modern man.
The transition to the noosphere is a difficult and slow process of developing the principles of concerted action, new behavior of people, changing standards, restructuring the whole being. Mankind must begin to reasonably regulate its population and significantly reduce the negative pressure on nature, and subsequently develop deeply substantiated technologies for building the noosphere based on the preservation of the biosphere as a prerequisite for life.
There is a constant exchange of chemical elements between the lithosphere, hydrosphere, atmosphere and living organisms of the Earth. This process is cyclical: having moved from one sphere to another, the elements again return to their original state. The circulation of elements has taken place throughout the history of the Earth, numbering 4.5 billion years.
Giant masses of chemicals are carried by the waters of the oceans. First of all, this applies to dissolved gases - carbon dioxide, oxygen, nitrogen. Cold water high latitudes dissolves the gases of the atmosphere. Acting with ocean currents in the tropical zone, it releases them, since the solubility of gases decreases when heated. The absorption and release of gases also occurs during the change of warm and cold seasons of the year.
The emergence of life on the planet had a huge impact on the natural cycles of some elements. This, first of all, refers to the circulation of the main elements of organic matter - carbon, hydrogen and oxygen, as well as such vital elements as nitrogen, sulfur and phosphorus. Living organisms also influence the circulation of many metallic elements. Despite the fact that the total mass of living organisms on the Earth is millions of times less than the mass of the earth's crust, plants and animals play a crucial role in the movement of chemical elements.
Human activities also affect the cycle of elements. It has become especially noticeable in the last century. When considering the chemical aspects of global changes in the cycles of chemical elements, one should take into account not only changes in natural cycles due to the addition or removal of chemicals present in them as a result of normal cyclic or human-induced impacts, but also the release of chemicals into the environment that did not previously exist in nature. Let us consider one of the most important examples of the cyclic movement and migration of chemical elements.
Carbon, the basic element of life, is found in the atmosphere in the form of carbon dioxide. In the ocean and fresh waters of the Earth, carbon is in two main forms: in the composition of organic matter and in the composition of interconnected inorganic particles: bicarbonate ion -, carbonate ion and dissolved carbon dioxide. A large number of carbon is concentrated in the form of organic compounds in animals and plants. A lot of "non-living" organic matter is present in the soil. The carbon of the lithosphere is also contained in carbonate minerals (limestone, dolomite, chalk, marble). Part of the carbon is found in oil, coal and natural gas.
The link in the natural carbon cycle is carbon dioxide (Fig. 1).
Simplified diagram of the global carbon cycle. The numbers in the boxes reflect the size of the tanks in billions of tons - gigatons (Gt). The arrows show the fluxes and the associated numbers are in Gt/yr.
The largest reservoirs of carbon are marine sediments and sedimentary rocks on land. However, most of this matter does not interact with the atmosphere, but cycles through the solid part of the Earth on geological time scales. Therefore, these reservoirs play only a secondary role in the relatively fast carbon cycle that takes place with the participation of the atmosphere. The next largest reservoir is sea water. But even here, the deep part of the oceans, where the main amount of carbon is contained, does not interact with the atmosphere as quickly as their surface. The smallest reservoirs are the land biosphere and the atmosphere. It is the small size of the latter reservoir that makes it sensitive to even small changes in the percentage of carbon in other (large) reservoirs, such as when burning fossil fuels.
The modern global carbon cycle consists of two smaller cycles. The first of these is the binding of carbon dioxide during photosynthesis and its new formation during the life of plants and animals, as well as during the decomposition of organic residues. The second cycle is due to the interaction of atmospheric carbon dioxide and natural waters:
In the last century, significant changes have been made to the carbon cycle by human economic activity. The burning of fossil fuels - coal, oil and gas - has led to an increase in the release of carbon dioxide into the atmosphere. This does not greatly affect the distribution of carbon masses between the Earth's shells, but can have serious consequences due to the increased greenhouse effect.
The activity of living organisms in the biosphere is accompanied by the extraction of large quantities of minerals from the environment. After the death of organisms, their constituent chemical elements are returned to the environment. This is how a biogenic (with the participation of living organisms) circulation of substances in nature arises, that is, the circulation of substances between the lithosphere, atmosphere, hydrosphere and living organisms. The cycle of substances is understood as a repetitive process of transformation and movement of substances in nature, which has a more or less pronounced cyclical character.
All living organisms take part in the circulation of substances, absorbing some substances from the external environment and releasing others into it. Thus, plants consume carbon dioxide, water and mineral salts from the external environment and release oxygen into it. Animals inhale the oxygen released by plants, and when they eat them, they assimilate organic substances synthesized from water and carbon dioxide and release carbon dioxide, water and substances from the undigested part of the food. When dead plants and animals are decomposed by bacteria and fungi, an additional amount of carbon dioxide is formed, and organic substances are converted into minerals that enter the soil and are again absorbed by plants. Thus, the atoms of the main chemical elements are constantly migrating from one organism to another, from the soil, atmosphere and hydrosphere to living organisms, and from them to the environment, thus replenishing the inanimate substance of the biosphere. These processes are repeated an infinite number of times. So, for example, all atmospheric oxygen passes through living matter in 2 thousand years, all carbon dioxide - in 200-300 years.
The continuous circulation of chemical elements in the biosphere along more or less closed paths is called the biogeochemical cycle. The need for such circulation is explained by the limited availability of their reserves on the planet. To ensure the infinity of life, the chemical elements must move in a circle. The circulation of each chemical element is part of the general grandiose circulation of substances on Earth, that is, all cycles are closely interconnected.
The cycle of matter, like all processes occurring in nature, requires a constant supply of energy. The basis of the biogenic cycle that ensures the existence of life is solar energy. The energy bound in organic substances decreases along the steps of the food chain, because most of it enters the environment in the form of heat or is spent on the implementation of processes occurring in organisms. Therefore, a flow of energy and its transformation are observed in the biosphere. Thus, the biosphere can be stable only under the condition of a constant circulation of substances and an influx of solar energy.
"Circulation
in nature."
The activity of living organisms is accompanied by the extraction of large amounts of mineral substances from the inanimate nature surrounding them. After the death of organisms, their constituent chemical elements are returned to the environment. This is how the biogenic circulation of substances in nature arises, i.e. circulation of substances between the atmosphere, hydrosphere, lithosphere and living organisms.
Let's give some examples.
The water cycle. Under the influence of solar energy, water
evaporates from the surface of water bodies and is transported by air currents over long distances. Falling out on
the surface of the land in the form of precipitation, it contributes to the destruction of rocks and makes the minerals that make them up
foot for plants, microorganisms and animals. She is
erodes the upper soil layer and leaves along with the
chemical compounds created in it and suspended organic and inorganic particles into the seas and oceans. The circulation of water between ocean and land
essential link in sustaining life on Earth.
Plants participate in the water cycle in two ways: they extract it from the soil and evaporate it into the atmosphere; Part of the water in plant cells is broken down during photosynthesis. In this case, hydrogen is fixed in the form of organic
chemical compounds, and oxygen enters the atmosphere.
Animals consume water to maintain osmotic and salt balance in the body and release it into the external environment along with metabolic products.
The carbon cycle. Carbon enters the biosphere in
as a result of its fixation during photosynthesis. The amount of carbon annually bound by plants is estimated
amounts to 46 billion tons. Part of it enters the body of animals
and released as a result of respiration in the form of CO2, which
re-enters the atmosphere. In addition, carbon stocks
in the atmosphere are replenished due to volcanic activity and human combustion of fossil fuels. Although
the main part of the carbon dioxide entering the atmosphere is absorbed by the ocean and deposited in the form of carbon-
rises.
- 2 -
The nitrogen cycle. Nitrogen is one of the main biogenic
elements - is found in huge quantities in the atmosphere, where it is 80% of the total mass of its gaseous
components. However, in molecular form, it cannot
used by higher plants or animals.
into a usable form, atmospheric nitrogen
transfer electrical discharges (which form
nitrogen oxides, which in combination with water give nitrous and nitric acids), nitrogen-fixing bacteria and blue-green algae. At the same time, ammonia is formed, which
other chemosynthetic bacteria successively
lead to nitrites and nitrates. The latter are the most digestible for plants. Biological nitrogen fixation on land
makes about 1 g / m 2, and in fertile areas it reaches
Gaet 20 g/m 2 .
After the organisms die, putrefactive bacteria decompose nitrogen-containing compounds to ammonia. part of it
goes into the atmosphere, part is restored by denitrification
reducing bacteria to molecular nitrogen, but the main
the mass is oxidized to nitrites and nitrates and reused. A certain amount of nitrogen compounds settles in the deep
side water sediments and for a long time (millions of years)
comes from the cycle. These losses are compensated for by the
the release of nitrogen into the atmosphere with volcanic gases.
Sulfur cycle. Sulfur is found in proteins and also
is a vital element. In the form of co-
compounds with metals - sulfides - it occurs in the form of ores
on land and is part of deep-sea sediments. In to-
foot for assimilation soluble form of these compounds
translated by chemosynthetic bacteria capable of
receive energy by oxidizing reduced compounds
sulfur. As a result, sulfates are formed, which
used by plants. deep-seated sulfates
are involved in the cycle by another group of microorganisms that reduce sulfates to hydrogen sulfide.
Phosphorus cycle. The reservoir of phosphorus is
lie its compounds in rocks. Due to washout
it enters river systems and is partly used by plants, and partly carried away to the sea, where it settles in the deep
water deposits. In addition, in the world annually
from 1 to 2 million tons. phosphorus-containing rocks. Big
some of this phosphorus is also washed out and eliminated from
circulation. Thanks to fishing, part of the phosphorus is returned
comes to land in small sizes (about 60 thousand tons of elements
mental phosphorus per year).
From the above examples, it can be seen how significant
living organisms play a role in the evolution of inanimate nature. Their activities significantly affect the formation of the composition of the atmosphere and the earth's crust. Great contribution to
understanding the relationship between living and non-living things
was introduced by the outstanding Soviet scientist V.I. Vernadsky. He
revealed the geological role of living organisms and showed
that their activities are the most important factor
transformations of the mineral shells of the planet.
Thus, living organisms, experiencing the influence of factors of inanimate nature, by their activity
change environmental conditions, i.e. their habitats. This leads to a change in the structure of the entire community - the biocenosis.
It has been established that nitrogen, phosphorus and potassium can have the greatest positive effect on crop yields.
plants, and therefore these three elements in the largest quantities are introduced into the soil with fertilizers used in agriculture. Therefore, nitrogen and phosphorus turned out to be the main
This is another reason for the accelerated eutrophication of lakes in countries with intensive agriculture. Eutrophication is the process of
quenching water bodies with nutrients. She pre-
is a natural phenomenon in lakes, since rivers
bring nutrients from surrounding drainage
areas. However, this process usually proceeds very slowly, over thousands of years.
Unnatural eutrophication, leading to a rapid increase in lake productivity, occurs as a result of runoff from agricultural land, which can be enriched with fertilizer nutrients.
There are also two other important sources of phosphorus - sewage and detergents. Wastewater, both in its original form and processed, are enriched with phosphates. Household detergents contain 15% to 60% biodegradable phosphate. Briefly, it can be summarized that eutrophication ultimately leads to the depletion of oxygen resources and the death of most living organisms in lakes, and in extreme situations in rivers.
Organisms in an ecosystem are bound together by a commonality of energy and nutrients, and it is necessary to clearly distinguish between these
two concepts. The whole ecosystem can be likened to a single
khanizma, consuming energy and nutrients to do work. Nutrients originally come from the abiotic component of the system,
to which they eventually return either as
waste products, or after the death and destruction of organisms. Thus, in an ecosystem
constant nutrient cycling
Both living and non-living components are involved. Such cycles are called biogeochemical cycles.
The flow of energy and cycles of biogenic elements in
ecosystem.
Energy Biotic Thermal
light component energy
Sun Biogenic
elements
Abiotic
component
Energy flow
Nutrient cycling
At a depth of tens of kilometers, rocks and minerals are exposed to high pressures and dark
peratur. As a result, metamorphism (change) of their structure, mineral, and sometimes chemical composition occurs.
va, which leads to the formation of metamorphic rocks.
genera can melt and form magma. Internal
Earth's energy (i.e. endogenous forces) lifts magma to the surface. With molten rocks, i.e. magma, chemical elements are brought to the surface of the Earth during volcanic eruptions, freeze in the thickness of the earth's crust in the form of intrusions. Mountain building processes raise deep rocks and minerals to the surface of the Earth. Here, rocks are exposed to the sun, water, animals and plants, i.e. are broken down, transported and deposited as precipitation in a new location. As a result, sedimentary rocks are formed. They accumulate in the mobile zones of the earth's crust and, when bending down, again descend to great depths (over 10 km).
The processes of metamorphism, ferrying,
crystallization, and chemical elements return to the surface of the Earth. Such a "route" of chemical elements is called a large geological cycle. The geological cycle is not closed, because part of the chemical elements leaves the circulation: it is carried away into space, fixed by strong bonds on the earth's surface, and part comes from outside, from space, with meteorites.
The geological cycle is a global journey of chemical elements within the planet. They make shorter trips on Earth within individual
her plots. The main initiator is living matter. Organisms intensively absorb chemical elements from the soil, air, water. But at the same time they return. Chemical elements are washed out of plants by rainwater, released into the atmosphere during respiration, and deposited in the soil after the death of organisms. The returned chemical elements are again and again involved in "journeys" by living matter. All together constitutes the biological, or small, cycle of chemical elements. He is also not a lock-whip.
Some of the "travelers" elements are carried away beyond its limits with superficial and groundwater, some - for different times "turns off" from the cycle and lingers in trees, soil, peat.
Another route of chemical elements runs from top to bottom from peaks and watersheds to valleys and riverbeds, depressions, depressions. On watersheds, chemical elements
cops arrive only with atmospheric precipitation, and are carried down both with water and under the influence of gravity. The consumption of matter prevails over the intake, as evidenced by the very name of watershed landscapes - eluvial.
On the slopes, the life of chemical elements changes. The speed of their movement increases dramatically, and they "pro
the slopes ride like passengers comfortably seated in the compartment of a train. The landscapes of the slopes are called transit landscapes.
Chemical elements can "rest" from the road only in accumulative (accumulating) landscapes,
laid in depressions of the relief. In these places, they often remain, creating good feeding conditions for the vegetation. In some cases, vegetation has to deal with an excess of chemical elements.
Already many years ago, man intervened in the distribution of chemical elements. Since the beginning of the twentieth century, human activity has become the main way of their travel. During the extraction of minerals, a huge amount of substances is removed from the earth's crust. Their industrial
The work is accompanied by emissions of chemical elements with production waste into the atmosphere, water, and soil. It pollutes the habitat of living organisms. New areas with a high concentration of chemical
elements - man-made geochemical anomalies. They are common around non-ferrous metal mines (copper,
lead). These areas sometimes resemble lunar landscapes, because they are practically devoid of life due to the high content of harmful elements in soils and waters. It is impossible to stop scientific and technological progress, but a person must remember that there is a threshold in the pollution of the natural environment, which cannot be crossed, beyond which human diseases and even the extinction of civilization are inevitable.
Having created biogeochemical "dumps", nature, perhaps, wanted to warn a person against ill-conceived, immoral activities, to show him by a clear example what a violation of the distribution of chemical elements in the earth's crust and on its surface leads to.
Tutoring
Need help learning a topic?
Our experts will advise or provide tutoring services on topics of interest to you.
Submit an application indicating the topic right now to find out about the possibility of obtaining a consultation.
BIOGENIC CYCLE
Let us consider the circulations that play the greatest role in the biosphere, which include the biogeochemical cycles of carbon, nitrogen, oxygen, sulfur, and phosphorus.
The carbon cycle. The sources of carbon in nature are as numerous as they are varied. Meanwhile, only carbon dioxide, which is either in a gaseous state in the atmosphere or in a dissolved state in water, is the source of carbon that serves as the basis for processing
it into the organic matter of living beings. Absorbed by plants during photosynthesis, it turns into sugars, and in other biosynthetic processes it is converted into proteins, lipids, etc. These various substances serve as carbohydrate food for animals and non-green plants. Saprophage animals and microorganisms living in the soil transform dead plants and animal remains into a new formation of organic matter, a more or less powerful layer of brown or black mass - humus. The rate of action of decomposing organisms on humus is far from being the same, and the chains of fungi and bacteria leading to the final mineralization of carbon are of different lengths. Sometimes the chain can be short and incomplete: organic residues accumulate in the form of peat and form peat bogs. In some swamps with a thick cover of sphagnum mosses, the peat layer can reach 20 m or more. This is where the carbon cycle stops. Deposits of fossil organic compounds in the form of coal and oil indicate the stagnation of the circulation on the scale of geological time (Fig. 3).
Stagnation of the carbon cycle also occurs in water, as carbon dioxide accumulates in the form of CaCO 3 (chalk, limestones or corals) of chemical or biogenic origin. Often these masses of carbon remain out of circulation for entire geological periods, until CaCO3 rises above the sea surface in the form of mountain ranges. From this moment, the entry of carbon and calcium into the circulation begins due to the leaching of limestone by precipitation, under the influence of lichens, as well as the roots of flowering plants. Human activities play an important role in the carbon cycle. Mankind annually consumes about 6 · 10 9 tons of fossil carbon. If the carbon dioxide formed as a result of combustion was not removed from the atmosphere, the annual increase in its content in the air would be 2.3 million tons. Over the past 100 years, the carbon dioxide content has increased from 290 to 320 million tons, and more than 1/5 of this increase falls for the last decades. Thus, the total increase in the content of carbon dioxide in the atmosphere is approximately only 1/3 of the amount of gas released during combustion (in absolute mass - 200 · 10 9 t). The rest of the carbon dioxide goes into plant growth (because it is known that plants grow faster if the CO2 content in the atmosphere is higher); part of it dissolves in the waters of the ocean. Although, according to some estimates, land biomass could have increased by 15 · 10 9 tons over the past 100 years, but there is no direct evidence for this.
The intensity of human activity is increasing. Increases year by year and the rate of consumption of fossil fuels. In 15 years, the content of CO 2 in the atmosphere will increase from 320 to 375 -
400 million tons. An increase in the content of CO 2 in the atmosphere will inevitably lead to an increase in the temperature of the Earth's surface, and, consequently, to the melting of glaciers, an increase in the level of the ocean and other equally serious consequences. Therefore, humanity is faced with the task of finding such energy sources and technological processes, at which the content of carbon dioxide in the air will not grow at such a significant rate. It is also known that deforestation, the use of land for roads and buildings reduce the area of the green cover of the Earth and reduce the rate of assimilation. When using natural phytocenoses and replacing them with cultural ones, one should keep in mind the need to maintain the overall level of photosynthesis, and even better, to ensure its rise.
nitrogen cycle- difficult process. Although nitrogen accounts for 70% of the atmosphere, it is necessary to fix it,
so that it is in the form of certain chemical compounds. Ways of nitrogen fixation are very diverse (Fig. 4). Nitrogen fixation occurs during volcanic activity, during lightning discharges in the atmosphere, when its ionization takes place, at the moment of combustion of meteorites. However, an incomparably large role in the process of nitrogen fixation belongs to microorganisms, both free-living and living on the roots in special nodules, and sometimes on the leaves of some plants.
The huge reservoir of free molecular nitrogen in the atmosphere is not directly used by higher plants, since it takes a lot of energy to break strong bonds between atoms in the N 2 molecule. Only 0.001% of biosphere nitrogen is bound in biomass and metabolites of organisms. The conversion of molecular nitrogen into a bound state is carried out in nature by nitrogen-fixing microorganisms, which form compounds from it with the amino group NH 2 - the main product of nitrogen fixation, which is included in the biogenic cycle by all other organisms: microbes, plants, fungi, animals. Subsequently, nitrogen-rich compounds (ammonia, ammonium ions, amino acids) are oxidized in water and in soils by nitrite- and nitrate-forming bacteria to nitrogen oxides NO 2 and NO 3 , and at the last stage of the cycle, these oxides are converted by denitrifying bacteria again into molecular nitrogen entering the atmosphere. Every year, bacteria convert at least 1 billion tons of nitrogen into a bound form, while the amount of bound nitrogen in mineral fertilizers does not exceed 90 million tons per year.
Nitrogen-fixing organisms on plant roots are represented by bacteria, less often by fungi. Nodules with nitrogen-fixing organisms develop on the roots of representatives of the legume family and other plants of various systematic affiliations. The output of fixed nitrogen for nodule bacteria living on the roots of legumes is often 350 kg/ha per year, i.e. about 100 times higher than for free-living nitrogen-fixing organisms.
Probably the largest human intervention in the natural cycle is the industrial fixation of nitrogen. According to K. Delvich (1972), industry annually fixes as much nitrogen as it was fixed by living organisms before the introduction of modern agricultural technology.
The oxygen cycle. Undoubtedly, most of the oxygen in the atmosphere is of biogenic origin, only a small fraction of it appeared as a result of photolysis (the decomposition of water into oxygen and hydrogen by the energy of light). The role of living beings and organic matter in the formation of atmospheric carbon dioxide is also indisputable. It can be stated with certainty that the life that arose
Rice. 4. Evaluation of the amount of fixed nitrogen lost and acquired by the biosphere in various processes (P.Dyuvino, M.Tang, 1968). During the year, almost 92 million tons of fixed nitrogen enters the biosphere (unshaded bars), and about 83 million tons returns to the atmosphere as a result of denitrification (shaded bars). "Missing" about 9 million tons, apparently, are deposited annually in the biosphere in soil, groundwater, lakes, rivers and the ocean
on Earth, gradually led to the appearance of the modern composition of the atmosphere, which is supported by the activity of living beings. In quantitative terms, oxygen is the main component of living matter. If we take into account the water contained in the tissues, then, for example, the human body contains 62.8% oxygen and 19.4% carbon. If we consider the biosphere as a whole, this element, compared with carbon and hydrogen, is the main element among simple substances.
The oxygen cycle is greatly complicated by the ability of an element to form numerous chemical compounds, presented in various forms. As a result, there are many epicycles occurring between the lithosphere and the atmosphere, or between the hydrosphere and these two media.
Oxygen contained in the atmosphere and numerous surface minerals (sedimentary calcite, iron ores) is of biogenic origin. The huge post-Cambrian deposits of iron oxides testify to the great activity of primitive organisms, which sometimes bound all the free oxygen of the hydrosphere in their biomass and metabolites. The formation of an ozone screen in the atmosphere, capable of retaining the most dangerous ultraviolet radiation, began from the moment oxygen reached a concentration of approximately 1% of its current content. After that, autotrophic eukaryotic organisms were able to develop in the upper layers of the water (where the solar flux was the most powerful), which increased the intensity of photosynthesis and, accordingly, oxygen production.
Atmospheric oxygen consumption and its replacement by primary producers is quite fast. It has been calculated that it takes 2,000 years to completely renew all atmospheric oxygen. On the other hand, it takes 2 million years for all water molecules in the hydrosphere to undergo photolysis and be synthesized again by living organisms. As for atmospheric carbon dioxide, its complete cycle occurs very quickly, since it takes only 300 years for its complete renewal. Most of the oxygen produced during geological epochs did not remain in the atmosphere, but was fixed in the lithosphere in the form of carbonates, sulfates, iron oxides, etc. This mass is 590 · 10 14 tons against 39 · 10 14 tons of oxygen circulating in the biosphere in the form of gas or sulfates dissolved in oceanic and continental waters.
Sulfur cycle. The predominant part of the cycle of this element is sedimentary in nature and occurs in soil and water in the presence of numerous gaseous sulfur compounds, such as hydrogen sulfide and sulfur dioxide.
The main source of sulfur available to living beings is various sulfates. Good water solubility of many sulfates
facilitates the access of inorganic sulfur to ecosystems. Absorbing sulfates, plants restore them and produce sulfur-containing amino acids (methionine, cysteine, cystine).
All kinds of organic residues in the biocenosis are decomposed by heterotrophic bacteria, which eventually form hydrogen sulfide from sulfoproteins contained in the soil.
Black silts, which naturally occur at the bottom of some seas (for example, the Black Sea), lakes, as well as in various freshwater continental reservoirs after human pollution, are rich in sulfur-decomposing organisms that function under anaerobic conditions. Some types of bacteria, such as beggiatoa, can reduce hydrogen sulfide to elemental sulfur. However, there are bacteria that can again oxidize hydrogen sulfide to sulfates, which again increases the supply of sulfur available to producers.
The last phase of the sulfur cycle is entirely sedimentary. It consists in the precipitation of this element under anaerobic conditions in the presence of iron. Various stages of this process, especially reversible ones, further allow the use of sedimentary rock reserves.
Thus, the last phase of the sulfur cycle ends with its slow and gradual accumulation in deep sedimentary rocks.
Phosphorus cycle. This element is one of the main components of living matter, in which it is contained in a fairly large amount.
The reserves of phosphorus available to living beings are completely concentrated in the lithosphere. The main sources of inorganic phosphorus are igneous (eg, apatite) or sedimentary (eg, phosphorite) rocks. Mineral phosphorus is a rare element in the biosphere; in the earth's crust, its content does not exceed 1%, which is the main factor limiting the productivity of ecosystems. Inorganic phosphorus from the rocks of the earth's crust is drawn into circulation by leaching and dissolution in continental waters. It enters terrestrial ecosystems and is absorbed by plants, which, with its participation, synthesize various organic compounds, and thus is included in the trophic chains. Then organic phosphates, together with the remains, wastes and secretions of living beings, return to the earth, where they are again exposed to microorganisms and converted into mineral orthophosphates, ready for consumption by green plants and other autotrophs.
Phosphorus is brought into aquatic ecosystems by flowing waters. Rivers continuously enrich the oceans with phosphate, which promotes the development of phytoplankton and living organisms located at different levels of the freshwater or marine food chains.
water bodies The return of mineral phosphates to water is carried out by means of bioreducers. In all aquatic ecosystems, as well as in continental ones, phosphorus occurs in four forms, respectively insoluble or soluble
Having traced all the transformations of phosphorus on the scale of the biosphere, one can notice that its cycle does not close (Fig. 5) In terrestrial ecosystems, the phosphorus cycle takes place under optimal natural conditions with a minimum of losses due to leaching the phosphorus cycle does not deserve attention) In the ocean, this is far from the case. This is due to the incessant sedimentation of organic substances, in particular, phosphorus-enriched fish remains, the fragments of which, not used for food by detritophages and destructors, constantly accumulate on the bottom of the seas Organic phosphorus settled in tidal band and in shallow waters, can
be returned to the cycle after mineralization, but this does not apply to sediments at the bottom of deep-sea zones, which occupy 85% of the total area of the oceans. Phosphates deposited at great sea depths are turned off from the biosphere and can no longer participate in the cycle. Of course, as V.A. Kovda (1968), elements of the biogeochemical sedimentary cycle cannot accumulate indefinitely on the ocean floor. Tectonic movements contribute to the slow rise of sedimentary rocks accumulated at the bottom of geosynclines to the surface. Thus, a closed cycle of sedimentary elements has a duration measured in geological periods, i.e. tens and hundreds of millions of years.
