What is meant by nutrient cycling?

The chemical nutrients that are essential for the synthesis of living matter are taken from the physical environment. After the death and decomposition of living organisms, they are returned to the environment to be used over and again. This cyclic back and forth regenerative movement of chemical elements between organisms and their physical environment is known as the biogeochemical cycle or nutrient cycle. Since these elements serve as the essential chemical nutrients of organisms, their cyclic movements are also called nutrient cycling or mineral cycling. Minerals are not uniformly distributed all over the ecosystems but are more concentrated in specific compartments, called pools. The major biogeochemical cycles include the water cycle, nitrogen cycle, carbon cycle, phosphorus cycle, calcium cycle, sulfur cycle, etc.

Characteristics of the nutrient cycle

• Nutrient cyclings is maintained and regulated by feedback mechanisms. So they constitute a self-regulating system.
• They differ from energy flow in being cyclic and bi-directional; energy flow is non-cyclic, linear and unidirectional.
• A nutrient cycle has two phases, biotic and abiotic. The biotic phase exists in living organisms and the abiotic phase in the physical environment. The flow of nutrients and energy through the food chain forms the biotic phase, and the dispersal and distribution of elements in the physical environment form the abiotic phase.
• Functionally, a nutrient cycle has two components, namely a reservoir pool and an exchange pool or cycling pool. The reservoir pool is the storehouse and the primary source of the element. Generally, it is in the physical environment. In the reservoir pool, the exchange of materials with organisms is very slow. The exchange pool is represented by the biotic phase and, in some cases, by a part of the abiotic phase also. In it, there is the quick and active transfer of the element from one source to another.
• Water and mineral nutrients are cycled in the biosphere through the movement and exchange of organic and inorganic matter through recycling pathways. It involves microbial decomposition, animal and plant-animal respiration, autolysis of organic matter, direct recycling through symbiosis, direct action of solar energy, the direct effect of volcanic activity, etc.

Types of nutrient cycles

Gaseous cycles and sedimentary cycles

Based on the nature of the reservoir pool, the two kinds of biogeochemical cycles can be recognized as gaseous cycles and sedimentary cycles. In the former, there is a prominent gaseous phase and the reservoir pool is in the atmosphere or hydrosphere. But, in the latter, the reservoir pool is in the lithosphere from where minerals are released by processes, such as weathering of rock. The nitrogen cycle, carbon cycle, oxygen cycle, etc., are gaseous cycles. The phosphorus cycle, the sulfur cycle, the calcium cycle, etc., are sedimentary cycles.

Perfect cycles and imperfect cycles

Perfect nutrient cycles are the cycles in which nutrients are added to the reservoir pool as fast as they are removed from it and so they will never accumulate anywhere in appreciable quantities. Examples are gaseous cycles. On the other hand, imperfect nutrient cycles are the cycles in which the discharge of nutrients into the reservoir pool is either incomplete or very slow so that their removal from the reservoir pool will not be readily compensated by replenishment. Sedimentary cycles are examples.

Open cycles and closed cycles

At any stage of nutrient cycling, there is a high possibility of the loss of nutrients from the system. The nutrient cycles from which nutrients are lost in considerable amounts are called open cycles. They are common in highly erosive ecosystems where large quantities of soil may be washed off or blown away, causing a heavy loss of nutrients. Nutrient cycles with virtually no leakage or loss of nutrients are called closed cycles. Most tropical forests have more or less closed nutrient cycling. In them, decomposition is very quick so that nutrients are reabsorbed into the vegetation before they get washed away by running waters.

Nitrogen cycle

Living organisms require nitrogen for synthesizing amino acids, proteins, purines, pyrimidines, porphyrins, nucleic acids, nucleotides, vitamins, alkaloids, and many other protoplasmic compounds. Atmospheric air, which contains nearly 78% of nitrogen, is the richest primary reservoir of nitrogen. So, the nitrogen cycle is a typical gaseous cycle. For most terrestrial organisms, soil serves as a secondary source of nitrogen. In an ecosystem, the cyclic back and forth movement of nitrogen between the atmosphere and living organisms is the fundamental pathway of the nitrogen cycle.

Most organisms are incapable of directly utilizing atmospheric nitrogen. This is because gaseous nitrogen is extremely stable and very inert. So, for biological purposes, it has to be fixed into a chemically usable inorganic form. The nitrogen cycle is defined by the fixation of atmospheric nitrogen to nitrogenous compounds, the transformation and circulation of these compounds in ecosystems, and the ultimate release of molecular nitrogen back into the atmosphere.

Chemical processes of the nitrogen cycle

The nitrogen cycle involves five major events, namely nitrogen fixation, denitrification, ammonification or mineralization, nitrification, and nitrate assimilation. Nitrogen fixation is the removal and reduction of atmospheric molecular nitrogen and its subsequent incorporation with other elements to form nitrogenous compounds by nitrogen-fixing bacteria (eg: Azotobacter). Denitrification, on the other hand, is the release of nitrogen from nitrogenous compounds back into the atmosphere using denitrifying bacteria (eg: Pseudomonas). Ammonification is the synthesis of ammonia (NH₃) from nitrogenous organic compounds such as amino acids, using an ammonifying bacterium (eg: Clostridium). Nitrification is the formation of nitrites (NO₂^-) and nitrates (NO₃^-) from NH₃ by using a nitrite bacterium (eg: Nitrosomonas). Nitrate assimilation is the utilization of inorganic nitrates by organisms to synthesize their organic nitrogenous compounds.

The carbon cycle

Carbon is an important element of living matter, and it forms nearly 49% of the dry weight of organisms. Also, it is very actively involved in the fixation and flow of energy in the biosphere. The carbon cycle is a perfect gaseous cycle because the transfers and transformations of carbon compounds are quick, and the replenishment of carbon is as fast as its removal so that no appreciable amount of carbon is lost from biospheric circulation. The reservoirs of all the fixed carbon include the atmosphere and the hydrosphere.

The circulation of carbon in the biosphere involves two distinct cycles, one on land, and the other in the sea. These two are perfectly balanced and dynamically interlinked. The oceanic carbon cycle is the cyclic movement of carbon between the atmosphere and seawater (hydrosphere), It is almost self-sufficient. In it, the reservoir of carbon is the carbon dioxide (CO₂) dissolved in seawater. The terrestrial carbon cycle is the cyclic back and forth movement of carbon between the atmosphere and organisms.

The Phosphorus cycle

The phosphorus cycle is an example of a sedimentary cycle. Phosphorus is an essential element for organisms to grow and develop normally. It is a necessary constituent of nucleic acids, energy-rich phosphates, bone, teeth, etc.

The phosphate rocks, the excrement deposits of fish-eating sea birds, and the fossil deposits of the bones of extinct animals are the main reservoirs of phosphorus. Some phosphorus from these reservoirs gets distributed into the soil through soil erosion and running water. This causes the formation of the phosphorus pool in the soil in the ecosystem.  From this pool, plants absorb phosphorus. Phosphorus enters the ecosystem's food chain from plants and reaches animals. A small amount of phosphorus is returned to the soil from animal excreta. A large amount of phosphorus is returned to the soil for recycling after the death and decomposition of plants and animals.

Context and Applications

This topic is significant in the exams at school, graduate and post-graduate levels, especially for,

• Bachelors in Zoology/Botany
• Masters in Zoology/Botany

Practice Problems

Question 1: What are the two phases in nutrient cycling?

1. Biotic and abiotic
2. Reservoir pool and exchange pool
3. Gaseous and sedimentary
4. None of the above

Answer: Option 1 is correct.

Explanation: Nutrient cycling has two phases, biotic and abiotic. The biotic phase exists in living organisms and the abiotic phase in the physical environment.

Question 2: The phosphorus cycle is an example of ______.

1. Gaseous cycle
2. Sedimentary cycle
3. Both 1 and 2
4. None of the above

Answer: Option 2 is correct.

Explanation: The phosphorus cycle is sedimentary because the reservoir pool in the phosphorus cycle is in the lithosphere, from which minerals are released.   

Question 3: The cyclic movement of carbon between the atmosphere and the seawater ecosystem is known as _____.

1. Terrestrial carbon cycle
2. Oceanic carbon cycle
3. Ecosystem cycling
4. None of the above

Answer: Option 2 is correct.

Explanation: The oceanic carbon cycle is the cyclic movement of carbon between the atmosphere and the seawater ecosystem. In it, the reservoir of carbon is the carbon dioxide (CO₂) dissolved in seawater.

Question 4: The nitrogen cycle involves _____.

1. Nitrogen fixation
2. Ammonification
3. Nitrate assimilation
4. All of the above

Answer: Option 4 is correct.

Explanation: The nitrogen cycle involves five major events, namely nitrogen fixation by a nitrogen-fixing bacterium, denitrification by a denitrifying bacterium, ammonification by a mineralizing bacterium, nitrification by a nitrifying bacterium, and nitrate assimilation.

Question 5: In nutrient cycling, recycling pathways involve _____.

1. Plant-animal respiration
2. Autolysis of organic matter
3. Action of solar energy
4. All of the above

Answer: Option 4 is correct.

Explanation: In nutrient cycling, the recycling pathways involve microbial decomposition, animal and plant-animal respiration, autolysis of organic matter, direct recycling through symbiosis, direct action of solar energy, the direct effect of volcanic activity, etc.