6 Biogeochemical Cycles

Saleha Jamal

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Structure

 

1.1 Introduction

 

1.2 Significant features of Biogeochemical Cycles

 

1.3 Importance of Biogeochemical cycles

 

1.4 Mechanism of Biogeochemical Cycles

 

1.5 Types of Biogeochemical Cycles

 

1.5.1 Water/Hydrological Cycle

 

1.5.2 Carbon cycle

 

1.5.3 Oxygen cycle

 

1.5.4  Nitrogen cycle

 

1.5.5  Phosphorous cycle

 

1.5.6  Sulphur Cycle

 

1.6 Conclusion

 

 

Objectives:

 

At the end of this module the student should be able to:

  • Define biogeochemical cycle.
  • Understand significant features and importance of biogeochemical Cycles
  • Explain the mechanism of biogeochemical Cycles
  • Have clear understanding about the Hydrological Cycle, Carbon cycle, Oxygen cycle, Nitrogen cycle, Phosphorous cycle, Sulphur Cycle

 

1 Introduction

      Living organisms, from viruses, bacteria to plants and animals require various kinds of chemical elements such as carbon, hydrogen, nitrogen, sulphur and phosphorus for their growth and life processes. The absorption and utilization of such elements by organisms is compensated by their recycling and regeneration back into the environment. The cyclic path of these elements from environment to organisms and then into the environment are called biogeochemical cycles. In other words, the organic and inorganic substances of the biosphere i.e. various gasses (Carbon, nitrogen, oxygen etc.) from atmosphere, chemical elements (iron, nickel, oxygen, nitrogen, carbon, hydrogen) from lithosphere and water from hydrosphere, move reversibly through various closed system or cycles in such a way that total mass of these substances remain almost the same and these substances are always available for use by the biotic communities. This circulation of various elements in a cyclic manner is termed as biogeochemical cycle. These cycles are called biogeochemical cycles, because they include a variety of biological, geological, and chemical processes. . The first part of the word, bio, involves biological organisms, like bacteria, plants, and animals. The next part of the word, geo, involves geological processes, like weathering of rocks. The last part of the word indicates chemical processes, such as the formation of molecules.

 

    1.2 Significant features of Biogeochemical Cycles

 

Biogeochemical cycles are pathways by which nutrients flow between the biotic and abiotic compartments of the Earth. The abiotic portion of the Earth includes the lithosphere (the geological component of the Earth) and the hydrosphere (the Earth’s water) while biotic components include flora and fauna.

 

Ecosystems survive due to biogeochemical cycles. Many of the nutrient elements on which living things depend, such as carbon, nitrogen, and phosphorous are in constant circulation.

 

Essential elements are often stored in reservoirs, where they can be taken out of circulation for years. For example, coal is a reservoir for carbon.

 

Humans can affect biogeochemical cycles. Humans extract carbon and nitrogen from the geosphere and use them for energy and fertilizer. This has increased the amount of these elements in circulation, which has detrimental effects on ecosystems.

 

 

 

1.3  Importance of Biogeochemical cycles

 

An ecosystem survives by combination of energy flow and matter recycling that’s why biogeochemical cycles are important in following ways.

 

Biogeochemical cycles are a form of natural recycling that allows the continuous survival of ecosystems

 

Biogeochemical cycles are pathways by which nutrients flow between the abiotic and abiotic compartments of the Earth.

 

Biogeochemical cycles regulate the elements necessary for life on Earth by cycling them through the biological and physical aspects of the world.

 

 

1.4 Mechanism of Biogeochemical Cycles:

 

Various elements of the atmosphere i.e. hydrogen, oxygen, carbon and nitrogen and of lithosphere i.e. potassium, calcium, magnesium, phosphorus are stored into soil. Energy is responsible for the flow of these elements in the biosphere. The elements of the atmosphere are brought to the soils through the precipitation while lithospheric elements are made available to soil due weathering and erosion of rocks. The elements are taken up by plants in solution form through the process of root osmosis. The plants then convert these elements into such forms which are easily used in the development of plant tissues and plant growth by photosynthesis. Thus these elements or nutrients pass into various components of the biotic community through the process named as biogeochemical cycles. (Fig. 1)

 

Fig. 1 Biogeochemical Cycle

(Figure – By Eva Horne and Robert A. Bear; source article is CC BY 4.0)

 

In Fig. 1 a very simple form of biogeochemical cycle is shown. Here we can see that inorganic elements (inorganic nutrient pool) i.e. carbon, nitrogen, oxygen, sulphur, phosphorus etc. are used by producers i.e. green plantsand then these nutrients incorporated into plants are taken by consumer. Further nutrient from consumers are released in the atmosphere through decomposers and energy is required for each process shown by yellow arrows and lost to the atmosphere shown by red arrows.

 

     1.5 Types of Biogeochemical Cycles

 

Biogeochemical cycle is circulating or repeatable pathway by which an element moves through both the biotic (biosphere) and abiotic (lithosphere, atmosphere, and hydrosphere) components of Earth.Biogeochemical cycles are basically of two types:

 

a) Gaseous cycles: In gaseous cycles, the elements have a main reservoir in the gaseous phase, and the reservoir pool is the atmosphere or water. The biogenetic materials involved in circulation pass through a gaseous phase before completing the cycle. E.g. carbon cycle, nitrogen cycle, oxygen cycle

 

b)  Sedimentary cycle: In sedimentary cycles, the elements main reservoir pool is lithosphere and the biogenetic materials involved in circulation are non-gaseous. The sedimentary cycles are generally very slow as the elements may get locked up in rocks and go out of circulation for long periods. E.g. sulphur cycle, phosphorus cycle.

 

Major biogeochemical cycles are discussed below.

 

 

1.5.1 Water cycle/Hydrological cycle

 

Water is the basis of all the earth processes. In both liquid and frozen forms, it covers approximately 75% of the Earth’s surface.The human body is more than 1/2 water and human cells are more than 70 percent water. Thus, most land animals need a supply of fresh water to survive. However, when examining the stores of water on Earth, 97.5 percent of it is non-potable salt water and only 2.5 percent is the fresh water. Of the remaining water about 99 percent is locked underground as water or as ice. And almost 1 percent of fresh water is easily accessible from lakes and rivers. (Fig. 2) Many living things, such as plants, animals, and fungi, are dependent on the small amount of fresh surface water supply, a lack of which can have massive effects on ecosystem dynamics.

Fig. 2 Distribution of Earth’s Water

 

The amount of water in the atmosphere is only enough to supply the needs of the earth for about for 10 days. Because of the limited supply of water on the earth, there must be a constant interchange of water between the atmosphere and the surface of the planet. Aided by the energy from the sun and the force of gravity, water moves in an endless cycle from the oceans to the atmosphere and back to the oceans again. This circulation is known as hydrological or water cycle. Water cycling is extremely important to ecosystem dynamics. Water has a major influence on climate and, thus, on the environments of ecosystems.

 

Steps of Water Cycle: The water or hydrological cycle at global scale involves the following steps (Fig. 3).

 

Evaporation. Evaporation is the process by which oceanic water changes from liquid to gaseous state i.e. into water vapour and moisture.

 

Evapotranspiration: In general, evapotranspiration is the sum of evaporation and transpiration. Evapotranspiration is defined as the water lost to the atmosphere from the ground surface, evaporation from the capillary fringe of the groundwater table, and the transpiration of groundwater by plants whose roots tap the capillary fringe of the groundwater table.

 

Condensation. Condensation  is  the  process  in  which  water  vapour  in  the  air  is changed into liquid water. Condensation is responsible for the formation of clouds.

 

Precipitation: Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.

 

Infiltration: Infiltration is the process by which water on the ground surface enters into the soil. Some water that infiltrates will remain in the shallow soil layer, where it will gradually move through the soil and might enter a river by seepage into the river bank. Some of the water may infiltrate deeper, recharging ground-water aquifers.

 

Runoff: Runoff can be described as the part of the water cycle that flows over land as surface.

 

Fig. 3: Water Cycle (Source:WEATHERNET.COM)

 

 

Mechanism of water cycle: Oceanic water is heated by insolation and thus a small fraction of water is transformed into gaseous form i.e. water vapour or moisture. Water is also transformed into vapour from ground water and plants by the process of evapotranspiration.

This moisture is transported across the oceans and over the continents by atmospheric circulation or winds. The air is cooled because of its ascent and moisture and results in formation of clouds, fog and dew. On further cooling precipitation occurs in the form of rainfall or snowfall over the oceans and continents. Some precipitation falls directly in the streams or lakes known as direct fall while some proportion reaches the ground directly as through fall. The portion of rainfall reaching the ground surface flow as surface runoff and reaches to various water bodies. Some portion of surface water infiltrates through the soil. Some of the infiltrated water will remain in the shallow soil layer, where it will gradually move through the soil and might enter a river by seepage into the river bank. Some of the water may infiltrate deeper, recharging ground-water aquifers. (Fig. 4)

 

Fig. 4 the Water Cycle

Source credit: modification of work by John M. Evans and Howard Perlman, USGS)

 

Importance of Water Cycle: Water Cycle is very important mechanism for the various biological processes in the biosphere because no life is possible without water. Water cycle carries freshwater to people, animals and plants all around the world, it also transports things like nutrients, pathogens and sediment in and out of aquatic ecosystems. Rain and surface runoff are main ways in which minerals such as carbon, nitrogen, phosphorus, and sulphur, are cycled from land to water and then water to land.

 

1.5.2 Carbon Cycle

 

Carbon is important for all life on Earth. All living things are made up of carbon. Carbon is produced by both natural and human-made (anthropogenic) sources.

 

Natural Sources

 

1. Atmosphere: Carbon is found in the atmosphere mostly as carbon dioxide. Animal and plant respiration place carbon into the atmosphere.

 

2. Lithosphere: Carbon is found in the lithosphere in the form of carbonate rocks. Carbonate rocks came from ancient marine plankton that sunk to the bottom of the ocean hundreds of millions of years ago that were then exposed to heat and pressure. Carbon is also found in fossil fuels, such as petroleum (crude oil), coal, and natural gas. Carbon is also found in soil from dead and decaying animals and animal waste

 

3. Hydrosphere: Carbon is found in the hydrosphere dissolved in ocean water and lakes. Carbon is used by many organisms to produce shells. Marine plants use carbon for photosynthesis. The organic matter that is produced becomes food in the aquatic ecosystem.

 

4. Biosphere: Carbon is found in the biosphere stored in plants and trees. Plants use carbon dioxide from the atmosphere to make the building blocks of food during photosynthesis.

 

Anthropogenic Sources

 

1. Deforestation: After deforestation, plants are no available to remove carbon dioxide from the air. Thus deforestation results in extra carbon dioxide placed in the atmosphere.

 

2. Wood burning: When wood is burned, the carbon that is stored in the trees converts into carbon dioxide and enters into the atmosphere.

 

3. Combustion of fossil fuels: When fossils fuels (coal, oil, and natural gas) are extracted from the ground and burn for energy at power plants, this burning of fossil fuels is called combustion which releases carbon dioxide into the atmosphere.

 

Steps of the Carbon Cycle: Carbon cycle in the biosphere involve following steps. (Fig. 5)

  1. Carbon enters the atmosphere as carbon dioxide from respiration by animals and combustion of fuels
  2. Carbon dioxide is absorbed by producers and converted into carbohydrates by process called photosynthesis.
  3. Animals consume the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxideformed during respiration.
  4. After the death of the animals and plants, they are eaten by decomposers and the carbon in their bodies is released to the atmosphere as carbon dioxide. In some conditions decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion.

Fig. 5:  Carbon Cycle

Source: https://www.sciencelearn.org.nz/image_maps/3-carbon-cycle

    1.5.3 Nitrogen Cycle

 

Nitrogen is very important for all life forms in the biosphere because it is an essential part of the amino acids which make up proteins. Although nitrogen constitutes the largest proportion of atmospheric gasses by volume, but living organisms cannot use nitrogen directly rather they obtain nitrogen in the form of ammonia salts and nitrates.

 

Sources of Nitrogen: Natural sources of nitrogen include some of the nitrogen in the atmosphere, instream sources (e.g. salmon carcasses and litter fall), and vegetation (e.g. from N-fixing plants). Nitrogen is also a by-product of many natural biogeochemical processes that occur in watersheds, rivers and streams, such as the decomposition of plants and organisms.

 

Steps of Nitrogen Cycle

 

In general, the nitrogen cycle has five steps: (Fig.6)

 

1. Nitrogen Fixation

 

Nitrogen fixation is the process by which gaseous nitrogen (N2) is transformed to ammonia (NH3 or NH4+) through biological fixation or nitrate (NO3-) through high-energy physical processes. Gaseous Nitrogen can be converted directly into nitrate through processes that exert a tremendous amount of heat, pressure, and energy such as combustion, volcanic action, lightning discharges, and industrial means. However, a larger amount of biologically available nitrogen is naturally created through the biological fixation of Nitrogen to Ammonia and Ammonium. A small group of bacteria Known as Rhizobium that live in nodules on the roots of legumes and some other woody plants are capable of breaking bonds among the molecular nitrogen and combine it with hydrogen, using the enzyme nitrogenese.

 

2. Nitrification

 

Nitrification is a two-step process in which ammonia is converted to Nitrate. First, the soil bacteria Nitrosomonas and Nitrococcus convert Ammonia to Nitrite, and then another soil bacterium, Nitrobacter, oxidizes Nitrite to nitrate. These bacteria gain energy through these conversions, both of which require oxygen to occur.

 

3. Assimilation

 

In the assimilation process plants and animals incorporate the Nitrate and ammonia formed through nitrogen fixation and nitrification. Plants take these forms of nitrogen by their roots, and integrate them into plant proteins and nucleic acids. After assimilation, animals are then capable to use nitrogen from the plant tissues.

 

4. Ammmonification

Ammonification is the process in which microorganisms such as bacteria break down organic nitrogen including proteins and amino acids into ammonia.The ammonia produced by this process is excreted into the environment and is then available for either nitrification or assimilation.

 

5. Denitrification

 

Denitrification is the conversion of Nitrates to gaseous Nitrogen by anaerobic bacteria. This process only occurs with the little or no presence of oxygen, such as deep in the soil near the water table. Hence, areas such as wetlands provide a valuable place for reducing excess nitrogen levels via denitrification processes.

 

Fig.6: Nitrogen Cycle

Source:http://ib.bioninja.com.au/options/option-c-ecology-and-conser/c6-nitrogen-and-phosphorus/nitrogen-cycle.html

     1.5.4 Oxygen Cycle

 

Oxygen plays a significant role in the biosphere and is very essential element for the living organisms because it supports life. Oxygen gas makes up about 30% of the earth and about 21% of the air in the earth’s atmosphere. Oxygen forms about 70 percent atoms in the living and plays a major role in the formation of carbohydrates, fats and proteins. It is required for the respiration process by animals including man.

 

Sources of Oxygen

 

Green Plants: One of the major source of oxygen are green plants that produce oxygen by photosynthesis.

 

Atmosphere :Some oxygen is produced when sunlight reacts with water vapour in the atmosphere.

 

Steps of Oxygen Cycle: Oxygen cycle involves following steps (Fig. 7)

 

1. Photosynthesis: During the day time, green plants take energy from the sun, carbon di oxide from the air, and water from the soil to make glucose which is their food. This process is called photosynthesis. Photosynthesis is also done in the marine environment by phytoplankton. In the process of photosynthesis, Oxygen is released into the air as a by-product. During the night, this process is reversed asplants take in oxygen and release carbon di oxide, to uphold their metabolism that is why sleeping under tree during night is not advisable.

 

2. Respiration: The oxygen that is released by plants is used by humans, animals, and other organisms for respiration, i.e. breathing. Animals use oxygen to break down simple sugars and generate energy to sustain themselves. During the respiration process, animals take in oxygen and release carbon di oxide into the air.

 

3. Repeat: This carbon di oxide is again, taken up by plants, for photosynthesis.

Fig. 7 Oxygen Cycle

Source: https://www.smore.com/6k7zg-carbon-dioxide-oxygen-cycle

 

1.5.5 Phosphorus Cycle

 

Phosphorus is the second most important element in the biosphere and found on earth in different compound forms, such as the phosphate ion (PO43-), located in water, soil and sediments. Phosphorus is found in phosphate rocks and it enters into the environment cycling thorough water, soil and sediments. Phosphorus cannot be found in air in the gaseous state like other compounds of biogeochemical cycles. The phosphorus cycle is the slowest of the biogeochemical cycles.

 

Importance of Phosphorus for animals and plants

 

Phosphorus is the most essential for the growth of the organisms and being found in the soil in very small quantity it often limits plant growth. Animals take up phosphates by eating plants or plant-eating animals. Phosphorus is a crucial nutrient for both animals and plants. It plays an important role in cell development and is a key component of molecules that store energy, such as ATP (adenosine triphosphate), DNA and lipids (fats and oils). Deficiency of phosphorus in the soil can end up in a decreased crop produce.

 

Steps of the phosphorus cycle

 

Phosphorus moves in a cycle through rocks, water, soil, sediments and organisms. Here are the key steps of the phosphorus cycle (Fig. 8).

  1. Phosphorus enters the soil and water through rain and weathering that causes phosphate rocks release phosphate ions and other minerals. These phosphate ions are then distributed in soils and water.
  2. Plants take up phosphate ions from the soil.
  3. The plants may then be consumed by herbivorous animals.
  4. These animals are further consumed by carnivores. In this way, in the plant or animal, the phosphate is included into organic molecules such as DNA and lipids.
  5. This phosphates that is absorbed by the animals are returned to the soil through excretion and from decay and decomposition of plants and dead material by bacteria and is released out to the environment by this process.
  6. The phosphate in the soil is leached out or eroded into water and it eventually reach to oceans. The phosphate in the water is again used by algae and plants as nutrients.

 

Fig. 8 Phosphorus Cycle

Source: Biogeochemical Cycles: The Phosphorus Cycle https://www.youtube.com/watch?v=DM9MEsVRLjs

     1.5.6 Sulphur Cycle

 

Sulphur is the tenth most abundant element in the universe, and is essential part of all living matter like nitrogen and carbon. Sulphur is one of the components that make up proteins and vitamins. Proteins consist of amino acids that contain sulphur atoms. Sulphur is important for the functioning of proteins and enzymes in plants, and in animals that depend upon plants for sulphur.

 

Sources of Sulphur

 

Most of the earth’s sulphur is tied up in rocks and salts or buried deep in the ocean in oceanic sediments.

 

Sulphur can also be found in the atmosphere. It enters the atmosphere through both natural and human sources.

 

Natural recourses can be for instance volcanic eruptions, bacterial processes, evaporation from water, or decaying organisms

 

Human sources can be burning of fossil fuels that release oxides of sulphur in the atmosphere

 

Sulphur occurs as elemental sulphur also.

 

 

Steps of Sulphur Cycle

 

 

1. The sulphur cycle involves both atmospheric and terrestrial processes.

 

2. As the main source of sulphur are rocks, the cycle begins with the weathering of rocks, releasing the stored sulphur.

 

3. The sulphur then comes into contact with air where it is combined to oxygen and becomesulphate (SO4).

 

4. The sulphate is taken up by plants and microorganisms and is converted into organic forms;

 

5. Animals then consume these organic forms through foods they eat, thereby moving the sulphur through the food chain.

 

6. As organisms die and decompose, some of the sulphur is again released as a sulphate and some enters the tissues of microorganisms.

Fig. 9 Sulphur Cycle

Source: https://www.canstockphoto.com/sulfur-cycle-31929868.html

 

1.6 Conclusion

 

Biogeochemical cycles are of immense importance as an ecosystem survives by combination of energy flow and matter recycling. It is a form of natural recycling that allows the continuous survival of ecosystems and is pathways by which nutrients flow between the abiotic and biotic compartments of the Earth. It also regulates the elements necessary for life on Earth by cycling them through the biological and physical aspects of the world.

 

 

you can view video on Biogeochemical Cycles

 

References