Nutrient Cycle – Meaning, Types, Characteristics, Importance

The Biology Topics of ecology involve studying the relationships between living organisms and their environment.

Biogeochemical Cycles – Basic 6 Types of Biogeochemical Cycles with Examples

Life cannot persist without the environment. All living organisms are composed of matter and overall the organisms require a number of elements for life process and growth. The elements are carbon, oxygen, nitrogen, sodium, potassium, copper, zinc, etc., from the nature. Those elements needed in large amounts are known as macro elements and those required in trace amounts, are called microelements. According to the law of conservation of matter, matter is neither created nor destroyed. The earth neither receives any great amount of matter from other parts of the universe nor does it lose significant amounts of matter to outer space. So, in the atmosphere, the ratio of carbon, oxygen, and nitrogen remains more or less constant.

The nutrient cycle, is the movement of nutrient elements through the various components of an ecosystem. Another name for the nutrient cycle is the biogeochemical cycle. A nutrient cycle is the movement and exchange of organic and inorganic matter back into the production of living matter. The process is regulated by the food-web pathways that decompose matter into mineral nutrients. The nutrient cycle occurs within the ecosystems which are interconnected systems where matter and energy flow and are exchanged as organisms feed, digest, and migrate about. Minerals and nutrients accumulate in varied densities and configurations across the planet.

Ecosystems recycle locally, converting mineral nutrients into the production of biomass, and on a larger scale, they participate in a global system of inputs and outputs where the matter is exchanged and transported through a larger system of biogeochemical cycles.

Living organisms continuously take their elements from the environment for different purposes but there is no sign of a deficit of these elements in the environment. Because

• Organisms cannot take the elements permanently or cannot store them inside the body for a long time.
• The organism returns these elements to the environment through excretion or through decomposition after their death.
• The elements like carbon, oxygen, nitrogen, etc., which have been used for a long time in the course of time are cyclically rotated from the environment to the organic body and from the organic body to the environment.
• The same carbon, oxygen, and nitrogen molecules are utilized by living organisms generation after generation which in the latter are slowly transferred to a cycling pool, e.g., phosphates in rocks.

Reservoir Pool

It is the reservoir of biogenetic nutrients from a large and usually abiotic store of a nutrient in a biogeochemical cycle. Exchanges between the reservoir pool and the active pool are typically slow by comparison with exchanges within the active pool. Human activity, such as the mining of mineral resources, may profoundly alter this exchange rate, generally releasing an excess into the active pool which can only be accommodated by establishing a new equilibrium. This may, in turn, produce unfavourable conditions, manifested as chemical pollution (e.g., excess phosphorus in eutrophication, excess sulphur in acid rainfall, and lake acidification). The potential for catastrophic change is such that a point could be reached where the re-establishment of the old equilibrium becomes improbable (e.g. if excess carbon dioxide in the atmosphere triggered a runaway greenhouse effect).

Cyclical Pool

In this pool, the abiotic and biotic compounds are exchanged continuously with the biogenetic nutrients. Particulate matter is recycled by biodiversity inhabiting the detritus in soils, water columns, and along particle surfaces (including ‘aeolian dust’). Ecologists may refer to ecological recycling, organic recycling, biocycling, bio-geochemical recycling, natural recycling, or just recycling in reference to the work of nature. Whereas the global biogeochemical cycles describe the natural movement and exchange of every kind of particulate matter through the living and non-living components of the earth, nutrient cycling refers to the biodiversity within community food web systems that loop organic nutrients or water supplies back into production. The difference is a matter of scale and compartmentalization with nutrient cycles feeding into global biogeochemical cycles.

Solar energy flows through ecosystems along unidirectional and noncyclic pathways, whereas the movement of mineral nutrients is cyclic. Mineral cycles include the carbon cycle, sulfur cycle, nitrogen cycle, water cycle, phosphorus cycle, and oxygen cycle, among others that continually recycle along with other mineral nutrients into productive ecological nutrition. Global biogeochemical cycles are the sum product of localized ecological recycling regulated by the action of food webs moving particulate matter from one living generation to the next. Earth’s ecosystems have recycled mineral nutrients sustainably for billions of years.

Different Types of Nutrient or Biogeochemical Cycles are described below.

Water Cycle

Living organisms, the atmosphere, and the earth maintain among themselves a cycle of water and moisture, which is referred to as the water cycle or hydrological cycle.

Explanation of Water Cycle

The water cycle is driven by solar energy and gravity. It is an alternation of evaporation and precipitation with the energy used to evaporate the water being dissipated as heat in the atmosphere as the water condenses. The process of the water cycle is very simple, in which water is evaporated from the surface of the oceans, rivers, ponds and different water bodies and then water vapors form the clouds which, when cool down, precipitate the water as rainfall. However several routes are open to precipitation that falls on land direct evaporation, transpiration, entry of water into the groundwater system, and runoff. Consequently, the routes of hydrological cycles on land can be divided into the following three main categories:

• The rapidly cycling portion or Evapotranspiration, which includes evaporation and transpiration.
• The less rapidly cycling water, or surface runoff.
• Very slowly cycling groundwater that seeps into the soil, can end up on any one of these categories.

The hydrological cycle on land, thus, includes evaporation of water from the earth’s surface and leaf surface → formation of clouds → precipitation → surface runoff + accumulation of water as groundwater → return of water to the sea via streams or direct evaporation and cloud formation, and so on.

Importance of Water Cycle

• Required for solubilization of chemicals and several biochemical reactions like hydrolytic digestion of polymeric nutrients, photosynthesis, etc.
• Important for the working of macromolecules, as a good ionizer, transport of materials, etc.
• Acts as a habitat for hydrophytic and aquatic animals and as an agent of geological change.
• Acts as an agent of energy transfer and use. Water is a tremendous factor in neutralizing heat radiations of sunlight, so also acts as a ‘temperature buffer’.

Oxygen Cycle

The cyclical process by which oxygen is taken by the organisms from the atmosphere and again is released by the organism to the atmosphere and maintains its equilibrium is called the oxygen cycle.

Explanation of Oxygen Cycle

The green plants assimilate carbon dioxide in photosynthesis and release oxygen from water to the atmosphere. The plants and animals use this free oxygen of the air in respiration and release carbon dioxide that is actually balanced by the uptake of carbon dioxide and the release of oxygen by the green plants in photosynthesis.

Different Process for Utilization of Oxygen from the Atmosphere

• Oxygen in the air is utilized by plants and animals during respiration.
• Aquatic plants and animals take their oxygen from water (dissolved oxygen).
• Oxygen combines with various compounds to form various oxides, i.e., nitrates, ferric oxide, etc.
• During volcanic eruptions, oxygen is absorbed from the environment.
• During the burning and combustion of different organic substances, a profuse amount of oxygen is utilized.
• Thus the amount of O₂ in the atmosphere is decreased and the balance of O₂ becomes disturbed.

Different Sources of Oxygen

• The main source of oxygen in the atmosphere is originated by the photosynthetic activity of green plants.
• During photosynthesis, water breaks into H⁺ and OH^– by photolysis of water in the presence of sunlight. This OH- part gives rise to oxygen which is released in the environment.
  H₂O → H⁺ + OH^–
  OH → [OH] + e^–
  4[OH] → 2H₂O + O₂
• Ozone gas (O₃) of the sea coast is also the source of oxygen.
• Water vapour in the atmosphere is broken into oxygen during lightning.
• So, the deficiency of O₂ in the atmosphere is fulfilled and the equilibrium of O₂ in the environment is maintained.

Atmospheric Balance of Oxygen
The amount of atmospheric oxygen is utilized for the respiration of aerobic organisms and the burning of organic and inorganic substances, that balance is maintained by the release of oxygen through photosynthesis.

Significance of Oxygen Cycle

• Oxygen in the air is utilized by living organisms during respiration. As a result, a shortage of O₂ takes place in the air. But this amount of oxygen is fulfilled by the oxygen cycle.
• The oxygen cycle is indirectly maintained by the balance of the carbon cycle. During the combustion of organic substances with the help of oxygen, the CO₂ is formed.
• Some parts of the atmospheric oxygen that reach the higher levels of the troposphere are reduced to ozone (O₃) by high energy ultraviolet radiation, which helps the living plant and animal kingdom from annihilation.

Nitrogen Cycle

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth’s atmosphere (78%) is nitrogen, making it the largest pool of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. Human activities such as fossil fuel combustion, the use of artificial nitrogen fertilizers, and the release of nitrogen in wastewater have dramatically altered the global nitrogen cycle.

Source of Nitrogen

The cyclical process by which organisms take their nitrogen from nature and from organisms to nature again and maintain its equilibrium is called the nitrogen cycle.
Or
The complete series of cyclical events that occur party in the micro-organisms of the solid and in the tissue of higher plants are collectively known as the nitrogen cycle.

Nitrogen is present in the environment in a wide variety of chemical forms including organic nitrogen, ammonium (NH⁺₄), nitrite (NO^–₂), nitrate (NO^–₃), nitrous oxide (N₂O), nitric oxide (NO) or inorganic nitrogen gas (N₂). Organic nitrogen may be in the form of a living organism, humus, or in the intermediate products of organic matter decomposition. The processes of the nitrogen cycle transform nitrogen from one form to another. Many of those processes are carried out by microbes, either in their effort to harvest energy or to accumulate nitrogen in a form needed for their growth. The diagram above shows how these processes fit together to form the nitrogen cycle.

Explanation of Nitrogen Cycle

Nitrogen is more essential than the other elements for the nutrition of organisms. Nitrogen is the source of proteins and nucleic acids and in that sense, it is essential for life. Nitrogen is the main component of organic substances like amino acid, DNA, RNA, chlorophyll, etc., present in the protoplasm. The atmosphere is the reservoir of free gaseous nitrogen as it constitutes nearly 78% of the atmosphere by volume.

But free nitrogen cannot be utilized directly by the organisms, with the exception of a few nitrogen-fixing bacteria, i.e., Azotobacter, Clostridium, Derxia, etc., and blue-green algae like Nostoc, Anabaena, Osillatoria, etc. Rhizobium living in root nodules of leguminous plants can also fix nitrogen symbiotically. These bacteria and algae convert gaseous nitrogen into organic compounds and finally to nitrates soluble in water. Nitrates are utilized by plants for the synthesis of amino acids and proteins. Because higher plants cannot assimilate nitrogen directly from air they get it from the nitrogenous compounds present in the soil. These soluble compounds mainly as nitrates are absorbed by the plants through root hairs. In nature, the nitrogen cycle is divided into three phases,

• Nitrogen Fixation
• Nitrification or Formation of Nitrate Compound
• Denitrification

1. Nitrogen Fixation

Atmospheric nitrogen must be processed, or “fixed” to be used by plants. Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make their own organic compounds. Most biological nitrogen fixation occurs by the activity of Monitrogenase, found in a wide variety of bacteria and some Archaea. Monitrogenase is a complex two-component enzyme that has multiple metal-containing prosthetic groups. Some nitrogen-fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbiosis. Today, about 30% of the total fixed nitrogen is manufac¬tured in ammonia chemical plants.

A. Natural Fixation:
During a flash of lightning in the sky, atmospheric nitrogen combines with oxygen to produce nitric oxide (NO).
N₂ + O₂ → 2NO
This nitric oxide again oxidized by oxygen produces nitrogen dioxide.
2NO + O → 2NO₂
This nitrogen dioxide which reacts with rain water or water vapour produces nitrous acid and nitric acid and comes down to the soil.
2NO₂ + H₂O → HNO₂ + HNO₃
These two acids which react with different metallic salts (potassium, calcium, etc.) produce nitrate compounds and increase the amount of nitrogen in the soil.
2HNO₃ + CaCO₃ → Ca(NO₃)₂ + CO₂ + H₂O

B. Biological Nitrogen Fixation:
The process by which microbes of the soil fix the atmospheric nitrogen through a process of nitrogen fixation and convert them into nitrogenous compounds (NH₃), is called biological nitrogen fixation. It takes place in two ways:

• By Free Living Bacteria: Some free-living heterotrophic bacteria like Azotobacter, Clostridium (anaerobic), Chromatium, Pseudomonas, etc., and blue-green algae (autotrophic) like Nostoc, Anabaena, etc., can fix the molecular nitrogen from the atmosphere and combine it with hydrogen to form ammonia (NH₃). This NH₃ in the form of NH₄+ ions is absorbed by root hairs of higher plants.
• By Symbiotic Bacteria: Symbiotic nitrogen fixation is mainly carried out by the activities of Rhizobium species. These rod-shaped gram-negative bacteria, unable to fix atmospheric nitrogen themselves, can do this in combination with cells either from the root of leguminous plants like peas, beans, alfa-alfa, or the non-leguminous angiospermic plants like Casuarina, Myrica, Alanus, etc. These bacteria invade the roots and stimulate the formation of root nodules. The ‘leg-hemoglobin’ is present in leguminous plants which are able to fix the atmospheric nitrogen and by oxidation and reduction process nitrogen is converted to ammonia (NH₃).

Assimilation: Plants take nitrogen from the soil, by absorption through their roots in the form of either nitrate ions or ammonium ions. All nitrogen obtained by animals can be traced back to the eating of plants at some stage of the food chain.

Plants can absorb nitrate or ammonium ions from the soil via their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll. In plants that have a symbiotic relationship with Rhizobium, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. It is now known that there is a more complex cycling of amino acids between Rhizobium bacteroids and plants. The plant provides amino acids to the bacteroids so ammonia assimilation is not required and the bacteroids pass amino acids (with the newly fixed nitrogen) back to the plant, thus forming an interdependent relationship. While many animals, fungi, and other heterotrophic organisms obtain nitrogen by ingestion of amino acids, nucleotides, and other small organic molecules, other heterotrophs (including many bacteria) are able to utilize inorganic compounds, such as ammonium as sole nitrogen sources. The utilization of various nitrogen sources is carefully regulated in all organisms.

2. Nitrification or Formation of Nitrate Compound

The conversion of ammonia to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria. In the primary stage of nitrification, the oxidation of ammonium ion (NH⁺₄) is performed by bacteria such as the Nitrosomonas species, which convert ammonia to nitrites (NO₂). Other bacterial species, such as the Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO^–₃). It is important for the ammonia to be converted to nitrates because accumulated nitrites are toxic to plant life.

Due to their very high solubility, and because soils are largely unable to retain anions, nitrates can enter groundwater. Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and cause methemoglobinemia or blue-baby syndrome. Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to eutrophication, a process that leads to high algal, especially blue-green algal populations. While not directly, toxic to fish life, like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication. Nitrogen has contributed to severe eutrophication problems in some water bodies. Since 2006, the application of nitrogen fertilizer has been increasingly controlled in Britain and the United States. This occurs along the same lines as the control of phosphorus fertilizer, the restriction of which is normally considered essential to the recovery of eutrophic water bodies.

Ammonia is formed by the process of different nitrogen fixation and ultimately added to the soil. In the nitrification process, the ammonia is first converted into nitrites (NO₂) by a group of micro-organisms like Nitrosomonas, Nitrosococcus, Nitrospira, etc. Nitrites are then converted to nitrates mostly by Nitrobacter.
NH₃ + O₂ → NO₂ (Nitrite) + H₂O + H
NO₂ + O₂ → NO₃ (Nitrate)
The majority of the land plants get their principal source of nitrogen from the nitrate present in the soil. Sundew, Drosera, and pitcher plants like insectivorous plants collect nitrogen from the body of insects after trapping these insects inside the plant body. After the death of these plants, the nitrogen again is mixed with the soil.

3. Denitrification

Denitrification is the process in which ammonia and oxides of nitrogen are reverted back to nitrogen by different forms of denitrifying bacteria, namely Thiobacillus denitrificans, Pseudomonas, Bacillus cereus, etc. The gaseous nitrogen is released into the atmosphere and the cycle continues. Denitrification is the reduction of nitrates back into the largely inert nitrogen gas (N₂), completing the nitrogen cycle. This process is performed by bacterial species such as Pseudomonas and Clostridium in anaerobic conditions. They use the nitrate as an electron acceptor in place of oxygen during respiration. These facultatively anaerobic bacteria can also live in aerobic conditions.

1. Ammonification	Nitrogen compound → Ammonia	Bacillus ramosus
2. Nitrification	Ammonia → Nitrate	Nitrosomonas
3. Denitrification	Nitrate → Nitrogen	Pseudomonas
4. Nitrogen-fixing free-living bacteria	Azotobacter, Clostridium
5. Nitrogen-fixing symbiotic bacteria	Rhizobium
6. Nitrogen-fixing algae	Blue-green Algae (Nostoc, Anabaena)
7. Nitrifying bacteria	Nitrobacter, Nitrosomonas
8. Ammonifying bacteria	Bacillus mycoides
9. Denitrifying bacteria	Pseudomonas, Thiobacillus

Sedimentation (Leaching)
Nitrates of the solid are washed away to the sea or go deep into the earth along with percolating water. Nitrates thus lost are locked up in the rocks. This is the sedimentation of nitrogen. Nitrogen of rock is only released when the rocks are exposed and weathered.

Significance of Nitrogen Cycle

• In nature, the balanced component of nitrogen is maintained through the nitrogen cycle.
• Nitrogen is taking place for the new cell formation in the living body which comes through the nitrogen cycle over a great length of time.
• It is an essential component of all the cells (unit of life) and is made up of protein, Hence life cannot exist without nitrogen.
• Nitrogen assimilation: The process by which inorganic nitrogen in the form of nitrates, nitrites, and ammonia are absorbed by green plants and converted into nitrogenous organic compounds, is called nitrogen assimilation.
• Ammonification: The process in which nitrogen in organic matter is converted to ammonia and amino acid (eg., Bacillus mycoides) is called ammonification.
• Nitrification: The process in which ammonia is converted into nitrites and nitrates mostly by microorganisms, (eg., Nitrobacter, Nitrosomonas) is called nitrification.
• Denitrification: The process in which ammonia and oxides of nitrogen are reverted back to nitrogen by different forms of bacteria, (eg., Pseudomonas, Thiobacillus) is called denitrification.

Carbon Cycle

The carbon cycle is a perfect cycle in the sense that carbon is returned to the atmosphere as soon as it is removed. The cyclical process by which carbon from the atmosphere passes on to the organisms and from the organisms to the atmosphere and maintains its equilibrium is called the carbon cycle.

Explanation of Carbon Cycle

Carbon is the basic constituent of all organic compounds and a major element in the fixation of energy of photosynthesis Carbon constitutes about 49% dry weight. The source of all carbons both in living organism and fossil deposits are CO₂. In nature, maximum parts of the carbon remain dissolved in water either sea or other large bodies of water. The CO₂ is present in water as bicarbonate form but in the atmosphere, it occurs as gaseous CO₂ form. Only 0.03% CO₂ is present in the atmosphere. The main component of organic substances (carbohydrate, protein, fat) is carbon. Any organic body is composed of 24% of carbon.

Sources of Carbon

• A major source of carbon for the living world is carbon dioxide. The atmosphere contains about 700 × 10⁹ metric tonnes of carbon dioxide while ocean water contains about 35,000 × 10⁹ metric tonnes of carbon dioxide.
• Carbonates of the earth’s crust are derived from rocks, which by chemical reactions give rise to carbon dioxide.
• Fossil fuels like peat, coal, and petroleum products found in the lithosphere contain about 2.81 × 10²¹ kg of carbon.
• Oceans, where carbon remains stored as carbonates in the form of limestone and marble rocks.

Carbon dioxide Utilization
The major process that brings carbon from the atmosphere into the living world is photosynthesis, where producers take in carbon dioxide from the atmosphere and convert it into organic compounds. Oxygen is released as a by-product.

Carbon fixed by the producers enters the food chain and is passed to herbivores, carnivores, and decomposers. About 4 × 10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually.

Carbon dioxide Production

• Carbon dioxide is released back into the environment by the respiration of producers and consumers.
• Released by the decomposition of organic wastes and dead bodies by the action of bacteria and fungi on decay.
• Burning of wood and fossil fuels adds a considerable amount of carbon dioxide into the atmosphere.
• Volcanic eruptions and weathering of carbonate rocks by the action of acids.

The estimated amount of carbon fixed by photosynthesis is nearly 7 × 10¹³ kg/year. A large number of organisms buried deep in the layers of the earth, transform into coal, petroleum, and natural gas, and remain locked up till man uses them. Thus, natural exchange between the lithosphere and hydrosphere or atmosphere is a slow process and is a self-regulated feedback system. Recently this self-regulated system has been upset by man’s activities such as large-scale deforestation and excessive burning of fossil fuels. As a result, the carbon dioxide content of the atmosphere is increasing, affecting the greenhouse effect in nature.

Cyclical Ways or Carbon Balance

• During photosynthesis, carbon from atmospheric CO₂ is incorporated into the production of carbohydrates (C₆H₁₂O₆) that subsequently may be converted into organic compounds. This CO₂ is again released from the organic body by the oxidation of organic substances into CO₂, H₂O, and energy by respiration.
• Decomposers help in breaking down dead materials with the release of carbon back into the carbon cycle.
• The exchange of carbon between air and water depends on the partial pressure of CO₂ in air and water.
• Felspar and limestone rocks absorb CO₂ from the atmosphere to form metallic carbonate.
• Again by the process of weathering, the carbon of rocks is transformed into CO₂ or replaced by CO₂ released from volcanic eruption and other intense geological activities.
• The carbon of hot water springs becomes CO₂ due to the great heat of the water and mixes with the atmosphere.
• CO₂ is dissolved in the ocean water to form carbonic acid, which converts into carbonates (CaCO₃). This carbonate is used by oysters, protozoa, and sea algae for shell construction. After the death of these animals, these carbonates again dissolve in water.
• Land and aquatic plants utilize atmospheric carbon for photosynthesis. These plants are taken by the land and aquatic animals as their food and store carbon in their body. When this food is oxidized, CO₂ is released into the atmosphere.
• After the death of living organisms, the dead bodies are decomposed by the microorganism, and CO₂ is released.
• During the burning of petrol, coal, wood, etc. CO₂ is formed and increases the atmospheric carbon.

Atmospheric Balance of Carbon:
The carbon of CO₂ taken from the environment through photosynthesis by the plants is returned to the environment through respiration, decomposition, and burning of organic substances.

Significance of Carbon Cycle

• The balance of carbon in nature is maintained through the carbon cycle.
• The carbon cycle and oxygen cycle are inseparably mixed up with each other and one cycle always assists the other cycle indirectly.
• So, the carbon and oxygen cannot be thoroughly exhausted at any time from the environment.

Phosphorus Cycle

The cyclical process by which phosphorus is taken by the organisms from nature and from the organisms to nature again and maintains its equilibrium is called the phosphorus cycle. Phosphorus is an essential nutrient to biological systems. Its requirement is mainly seen in nucleic acid, cell membranes, bones, and teeth.

Sources of Phosphate

• The greatest reservoirs of phosphate are the insoluble ferric and calcium phosphates in rocks (rock phosphate) in combination with calcium, iron, and aluminium. The slow process of weathering releases phosphates into the soil.
• Phosphates are also added to the soil by man in the form of artificial fertilizers.
• Some quantity of phosphorus is deposited in deep sediments in oceans and is brought to the cycle by upwelling (but not extensive).

Phosphate Utilisation

• A considerable amount of phosphorus from the soil rock is washed into the sea by rains and floods (an estimated amount of 2 million tonnes of phosphatic rock is lost to the sea) where seaweeds take up phosphorus and is then passed onto fishes and sea birds.
• Terrestrial plants absorb the phosphorus as the phosphate ions (orthophosphate ions) from the soil.
• Animals obtain phosphates by consuming plants as food (as organic phosphate through the food chain).

Phosphorus Production

• Some amount of phosphorus is returned to earth in the form of bird excreta-Guano deposits (excreta of marine birds) and dead fish (around 60,000 tonnes which is less than 0.5% of phosphorus discharged from rivers).
• Death and decay of organisms and decomposition of organic matter by microorganisms release the phosphates into the soil, making them available to plants once again.
• Zooplankton excrete phosphorus into water (negligible amount).

Once phosphorus becomes a part of the soil water as phosphate or in a dissolved state in any aquatic system, it re-enters the cycle through producers. When phosphates form compounds with metals like aluminium, iron, or calcium, phosphorus becomes unavailable to plants (becomes sediment and is deposited in the deep ocean floor), and is lost to the phosphorus cycle until chemically changed.

Explanation of Phosphorus Cycle

The phosphorus cycle has no atmospheric phase. It occurs naturally in the environment as phosphate (PO₄-) or one of its analogs (HPO^–₄ or H₂PO^–₄). The ultimate source of phosphate in the ecosystem is crystalline rocks. By the process of weathering phosphate is made available to living organisms, generally, as ionic phosphate, which is absorbed by autotrophic plants through their roots. From autotrophs, it is passed along the grazing food chain in the same fashion as nitrogen and sulphur with excess phosphate being excreted in the feces.

In the detritus food chain, phosphorus-containing organic molecules are degraded, and the phosphate is liberated as inorganic ionic phosphate. In this form, it can be immediately taken up by autotrophs, or it can be incorporated into a sediment particle, either in the soil of a terrestrial ecosystem or in the sediment of an aquatic ecosystem.

Importance of Phosphorus Cycle

• Phosphorus is essential to strengthen the bones, and teeth of animals. Besides that phosphorus helps in the formation of cell membranes, DNA, RNA, etc.
• It is the main constituent of energy-rich compounds like ADP, ATP, GTP, etc.
• It is essential for the metabolic reactions to release energy.
• It is required for encoding the information in genes.

Phosphorus incorporated in bones and teeth also remains outside the natural cycle for a long time as the bones and teeth are resistant to decay. Therefore, the phosphorus cycle is an imperfect cycle and shows a way flow which can be represented as:
Phosphate rocks → Land ecosystem → Oceans → Ocean sediment.

Sulphur Cycle

The cyclical process by which sulphur is taken by organisms in various ways and then organisms to nature again and maintains its equilibrium, is called sulphur cycle.

Explanation of Sulphur Cycle

The sulphur cycle is both sedimentary and gaseous. The sedimentary phase of sulphur cycle is long termed and in it, sulphur is tied up to inorganic and organic deposits. From these deposits, it is released by weathering and decomposition and is carried to terrestrial and aquatic ecosystems in the form of salt solution. The gaseous phase of sulphur cycle is less pronounced. Sulphur enters the atmosphere from several sources, namely, the combustion of fossil fuels, volcanic eruption, the surface of the oceans, and gases released by decomposition. Initially sulphur releases into the atmosphere as hydrogen sulphide (H₂S).

This H₂S then quickly oxidizes into sulphur dioxide (SO₂) in the presence of atmospheric oxygen. Then, SO₂ mixes with rain water and forms sulphurous acid (H₂SO₃) and sulphuric acid (H₂SO₄), and through rain, it is carried back to earth. Whatever its source, sulphur in a soluble form, mostly as sulphate (SO₄⁼) or sulphite (SO₃⁼) is absorbed through plant roots, where it is incorporated into certain organic molecules, such as some amino acids (e.g., cysteine) and proteins. And from the producers, the sulphur is transferred to the consumer animals. Excretion and death carry sulfur in living material back to the soil and to the bottom of the ponds, lakes, and oceans, where organic materials are splitted up by bacteria of the detritus food chain, and sulphur gets back into the environment. Some bacteria also play an important role in sulphur cycle.

Importance of Sulphur Cycle

• Like nitrogen, sulphur is also an essential part of protein and amino acids and is characteristic of organic compounds.
• Sulphur plays an important role in plant nutrition.

Differences between the Gaseous and Sedimentary Nutrient Cycles:

Gaseous Cycle	Sedimentary Cycle
1. The biogenetic compound is gaseous.	1. The biogenetic compound is non-gaseous.
2. The primary reservoir pool is the atmosphere.	2. The reservoir pool is lithosphere.
3. It occurs quickly.	3. It occurs very soon.
4. It includes nitrogen, oxygen, carbon and water cycle.	4. It includes iron, calcium, phosphorous, and other earth-bound elements.

Difference between the Carbon Cycle and the Phosphorus Cycle:

Carbon Cycle	Phosphorus Cycle
1. The main compound is gaseous.	1. The main compound is non-gaseous.
2. Carbon dioxide is released from carbon due to respiration.	2. Phosphorus is not released by respiration.
3. The cycle occurs in the hydrosphere and atmosphere.	3. The cycle occurs in the lithosphere.