We all know that nutrients are the chemical substances that are required by all living organisms to grow and survive in the environment. Have you ever wondered how these nutrients are always available in the ecosystem? Don’t they vanish once used by a form of organism? If yes, then your answer is nutrient cycling. You might be amused to know that these nutrients are never produced newly; they are just moved in a cyclic pattern in our ecosystem, known as biogeochemical cycles or nutrient cycles. In this article, we are going to learn about important nutrient cycles that are carried in the environment around us.

What is Nutrient Cycle?

Circulation or exchange of nutrient elements between the living and nonliving components of an ecosystem is called the nutrient cycle or biogeochemical cycle. The term biogeochemical cycles represent the interactions among the organic (bio-) and inorganic (geo-) components and concentrate on the chemistry (chemical-) and movement (cycles) of chemical elements and compounds. Nutrient cycling represents the movement of elements through various forms and their return to their original state.

The amount of nutrients such as carbon, Nitrogen, Phosphorus, calcium, etc., present in the soil at any given time is referred to as the standing state. The soil has nutrients in a standing state; they are picked by plants, also known as producers and made part of organic matter. From producers, the nutrients pass to the higher trophic level in the form of organic matter. Nutrient cycling involves various factors such as biotic, abiotic, chemical and physical factors. Examples of nutrient cycles are the carbon cycle, oxygen cycle, nitrogen cycle, water cycle, phosphorus cycle, etc.

Types of Nutrient Cycle

There are three types of nutrient cycles that are observed in an ecosystem based on their reservoirs.

1. Gaseous Cycles: In these cycles, the main reservoirs of chemicals are the atmosphere and ocean. This type of cycle includes non-mineral chemicals. Nutrient cycles like the Nitrogen cycle, Carbon cycle, Hydrogen cycle and Oxygen cycle are examples of this type.
2. Sedimentary Cycles: In these cycles, the main reservoirs of chemicals are soil and rocks. These cycles include mineral chemicals. Phosphorus and Sulphur cycles are two examples of sedimentary cycles.
3. Hydrological Cycle: In this cycle, the reservoir may be in the atmosphere or in the soil. The water cycle is an example of this type.

We will learn about these cycles in detail in this article.

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Characteristics of Nutrient Cycle

The main characteristic features of nutrient cycles or biogeochemical cycles are:

1. All the subdivisions of the biosphere provide nutrients to the biotic components.
2. Materials involved in these cycles do not come from outside.
3. There is a specific rate of exchange of each biogenetic nutrient between biotic and abiotic components.
4. No amount is permanently lost from the biosphere.

Important Nutrient Cycle

The Earth receives energy only from the Sun, whereas all the other Earth elements remain within a closed system. These elements are the building blocks of life which means these are the raw materials used by all living organisms as nutrients to produce energy. These chemicals are named biogeochemical. The main elements that restore in a cyclic pattern include Carbon, Oxygen, Hydrogen, Nitrogen, Phosphorous, Sulphur, and Water. Let us now catch a look at some important nutrient cycles.

Gaseous Cycles

This type includes the carbon cycle, nitrogen cycle and oxygen cycle.

1. Carbon Cycle

The atmospheric carbon dioxide is virtually the only source of carbon. The carbon cycle is the process through which carbon elements are interchanged between the biosphere, geosphere, hydrosphere, and atmosphere in the Earth.

The complete carbon cycle looks like the figure below:

Fig: Carbon Cycle

Steps involved in the process of the carbon cycle:

1. Carbon present in the atmosphere is absorbed by the plants through photosynthesis.
2. All living organisms release carbon dioxide by respiration.
3. The carbon that is present in the plants is transferred to herbivores animals when they eat plants. Carnivores also get carbon through different food chains.
4. Dead bodies of animals and dead parts of plants decay (decompose), and carbon is transferred back to the soil.
5. Some amount of carbon that is not transferred to the soil is buried deep and forms fossil fuels. Carbon enters back to the atmosphere as carbon dioxide gas when these fossil fuels are burned.

2. Nitrogen Cycle

Nitrogen is also one of the important components in life. However, though Nitrogen is the most abundant gas present in the atmosphere, living organisms cannot utilize it in its elemental form. Therefore, Nitrogen must be converted into ammonia and other nitrogen compounds like nitrites and nitrates to be used by living organisms. This process of conversion is named Nitrogen Fixation. This process is carried out by various soil microorganisms in different steps.

Fig: Nitrogen Cycle

Steps involved in the process of the Nitrogen cycle:

1. Nitrogen-fixing bacteria fix atmospheric nitrogen into ammonia, and nitrifying bacteria present in the soil convert ammonia into nitrate, which is absorbed by the plants. This process is called Nitrogen fixation.
2. Atmospheric Nitrogen is converted into nitrates by the process of assimilation and lightning by plants.
3. Decomposers like bacteria present in the soil break down complex proteins and acids of decaying organic matter.
4. During the process of denitrification, denitrifying bacteria converts ammonia, nitrates into nitrogen. nitrogen gas is released into the atmosphere.

3. Oxygen Cycle

Oxygen is one of the most abundant elements on Earth after carbon. Around \(21\% \) of our atmosphere is formed of oxygen.

Fig: Oxygen Cycle

The steps involved in the Oxygen cycle are:

1. During the process of photosynthesis, all the green plants release oxygen into the atmosphere.
2. Aerobic organisms use oxygen present in the atmosphere for the respiration process.
3. Animals release carbon dioxide in the atmosphere, which is utilized by plants for photosynthesis, and the cycle continues.

Sedimentary Cycles

Mineral elements needed by living organisms are initially taken from inorganic sources. Most of the available forms occur as salts dissolved in water or soil. This type includes the Phosphorus cycle and Sulphur cycle.

1. Phosphorus Cycle

Phosphorus is a component of nucleic acids, ADP, ATP, NADP, phospholipids, etc. It is found in the soil in various forms like phosphate of rock, calcium, and iron or aluminium phosphate, fluorapatite, etc. Like many other mineral elements, Phosphorus also arrives in the oceans and settles down as sediment. A large proportion of Phosphorus percolates down to deep layers of soil. Biological processes like the formation of teeth and bones also keep Phosphorus locked up for some time.

Fig: Phosphorus cycle

The steps involved in the Phosphorus cycle are:

1. The dissolved Phosphorus is assimilated by plants and transformed into an organic form.
2. It travels to various trophic levels from plants in the form of organic phosphates.
3. The action of decomposers on dead parts of plants and animals releases Phosphorus into the environment. Consequently, this process proceeds cyclically.

2. Sulphur Cycle

The sulphur cycle connects soil, water, and air. Sulphur is found as sulphides or crystalline sulphates in the ground and rocks and as \({\rm{S}}{{\rm{O}}_2}\) and \({{\rm{H}}_2}{\rm{S}}\) in the atmosphere. Most of the organisms take sulphur as inorganic sulphates, except for a few organisms that require organic forms of sulphur like amino acids and cysteine. Most of the biologically fused sulphur is composed in the soil from the aerobic decomposition of proteins by microbes like bacteria and fungi. The sulphur cycle includes an excellent example of interaction and complex biochemical regulation between the different mineral cycles.

Fig: Sulphur Cycle

The steps involved in the Sulphur cycle are:

1. Green and purple photosynthetic bacteria utilize hydrogen molecules of \({{\rm{H}}_2}{\rm{S}}\) as the oxygen acceptor to reduce carbon dioxide.
2. Green bacteria can oxidize sulphide to elemental sulphur, whereas purple bacteria can lead to oxidation to the sulphate stage.
3. In the ecosystems, sulphur is carried from plants to animals, then to decomposers, and lastly, it returns to the environment through the breakdown of dead organic remains.
4. Sedimentary features of sulphur cycling include precipitation of sulphur in the presence of iron under anaerobic conditions.

Hydrologic or Water Cycle

Water plays a vital role in the existence of life. The exchange of water between atmosphere, land, sea, and living organisms and their habitats is achieved through the water cycle. The water cycle or hydrologic cycle involves various steps like evaporation, transpiration, cloud formation, and precipitation. The following diagram illustrates the steps involved in the water cycle.

Fig: Hydrologic or Water Cycle

The steps involved in the process of water or hydrologic cycle are:

1. Transfer of Water from the Earth takes place in the form of water vapour in the atmosphere by evaporation and transpiration. Evaporation occurs when water reaches its boiling point.
2. Water present in the lakes, oceans and other water reservoirs is converted into vapours through evaporation. Transpiration occurs from the surface of the plants.
3. After the water is converted into vapours, the vapours are converted back into liquid form due to the increase in temperature; this process is called condensation.
4. These tiny droplets of water fall down due to Earth’s gravity which is called precipitation.
5. Water falling on the ground in the form of rain fills the water bodies, which is called runoff or stored as groundwater.

Summary

All biotic components, cells and living organisms are made up of elements such as carbon, oxygen, nitrogen, hydrogen, sulphur and Phosphorus. These elements are essential in life. The flow of elements is regulated as the nutrient cycles are passed through each sphere, namely, biosphere, lithosphere, atmosphere and hydrosphere. Each sphere has a particular rate at which the flow of elements is calculated by the viscosity and density of the medium. Hence, the elements present in the nutrient cycles flow at different rates, and this maintains the flow of elements in those cycles. Ecosystems restore the equilibrium state to function properly through the nutrient cycles.

FAQs

Q.1. What is the nutrient cycle simple definition?
Ans: Circulation or exchange of nutrient elements between the living and nonliving components of an ecosystem is called the nutrient cycle or biogeochemical cycle.

Q.2. What are the \(3\) main nutrient cycles in an ecosystem?
Ans: There are many essential nutrient cycles in the ecosystem. Each one of these is important for all living organisms. Among these, the Nitrogen cycle, Carbon cycle and Water are the three main nutrient cycles in an ecosystem.

Q.3. What is the major function of a nutrient cycle?
Ans: The main function of a nutrient cycle is to restore, reform and maintain the balance of essential nutrients in an ecosystem so that they are available for all living organisms in various forms that can be used by them.

Q.4. Why are nutrient cycles important?
Ans: Nutrient cycles help in the storage of essential elements and maintain balance because certain organisms require a small amount of a particular nutrient to sustain life, whereas some nutrients are required in a large amount. Elements remain stored in their natural reservoirs in a nutrient cycle and are released to different organisms in an appropriate quantity.

Q.5. What are the three types of the nutrient cycle?
Ans: There are three types of nutrient cycles that are observed in an ecosystem based on their reservoirs.
a. Gaseous Cycles
b. Sedimentary Cycles
c. Hydrological Cycle

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