Biogeochemical cycles

Biogeochemical cycles

Biogeochemical cycles are natural processes that involve the cycling of chemical elements and compounds between living organisms, the atmosphere, water bodies, rocks, and soils. These cycles are driven by biological, geological, and chemical processes and play a critical role in the functioning of ecosystems and the Earth’s biogeochemical system.

Note: the law of conservation of mass; matter is neither created nor destroyed.

The cyclic movement of chemical elements of the biosphere within organisms and the environment is referred to as the biogeochemical cycle (Vernadskii 1934). Movement through the biotic community can be viewed in terms of the food chain.

Biogeochemical cycles
Biogeochemical cycles

The flow of chemical elements through the food chains is the organic phase of the biogeochemical cycle. The biogeochemical cycles include the Abiotic phase.

The abiotic phase is very important to the ecosystem as a major reservoir of all nutrients. The flow of the food chain in the abiotic phase is much slower than the one in the organic or biotic phase.

There are several biogeochemical cycles, including:

  1. Carbon cycle: This cycle involves the movement of carbon between living organisms, the atmosphere, oceans, and soils. Carbon is stored in various forms such as CO2, organic matter, and fossil fuels.
  2. Nitrogen cycle: This cycle involves the transformation of nitrogen between various forms such as nitrogen gas, ammonium, nitrate, and organic nitrogen. It is critical for the production of plant and animal proteins and the maintenance of soil fertility.
  3. Phosphorus cycle: This cycle involves the movement of phosphorus between living organisms, rocks, soils, and water bodies. Phosphorus is a critical nutrient for plant growth and is often a limiting factor in ecosystem productivity.
  4. Sulfur cycle: This cycle involves the cycling of sulfur between living organisms, the atmosphere, and soils. Sulfur plays a critical role in the formation of amino acids, proteins, and other essential biomolecules.
  5. Water cycle: This cycle involves the movement of water between the atmosphere, oceans, land, and living organisms. Water is essential for life and plays a critical role in the Earth’s climate system.

Understanding biogeochemical cycles is important for managing natural resources, predicting and mitigating the impacts of human activities on the environment, and maintaining ecosystem services that support human well-being.

There is food two classes of abiotic phases in the biogeochemical cycles:

  1. Sedimentary phase ( found in all cycles)
  2. Atmospheric phase (found in some cycles)

In some cycles such as the nitrogen cycle, the atmospheric phase is more important than the sedimentary phase, and in some others such as the phosphorus cycle atmospheric phase does not exist. But in sulphur cycle, both phases are present but their importance depends on other environmental factors. Biogeochemical cycles that have a dominant atmospheric phase are called atmospheric reservoirs e.g. phosphorus cycle.

Biogeochemical cycles are also described as:

  1. Gas cycle: the main reservoir is the atmosphere and ocean. Carbon and nitrogen are representatives of such
  2. Sedimentary cycle: the main reservoir is the lithosphere (soil, rock other sediments) from which elements are released by weathering e.g. phosphorus and sulphur.

Also See: Population: Ecology, Size, Density, Growth Rate & Curve, Carry capacity, Sex ratio

In this cycle mentioned above they have:
  1. Both have abiotic and biotic factors
  2. Both are driven by the flow of energy.
  3. Both are tied to the water hydrological cycle

Hydrological cycle:

Water determines the structure and function of an ecosystem apart from its being vital for life. All elements depend on water for cycling as it provides a solvent medium for their uptake. The hydrological cycle is an important medium through which nutrients are introduced into autotrophic plants.

Hydrological cycle
Hydrological cycle

Water vapour gathers to form a cloud and moves with the wind over the earth. These vapour precipitate in the form of rain, snow, hail, dew, etc. over the surface of the earth.

Nitrogen Fixation                          

To be used biologically, the free molecular nitrogen must be fixed. It must be split into free nitrogen atoms, thus N2—2N.

Nitrogen Fixation
Nitrogen Fixation

The free nitrogen atom has to combine with hydrogen to form NH3 with the release of an electron. 2N+3H2 àNH3+energy

Note: Nitrogen combines with hydrogen to form ammonia to release energy.

Nitrogen Fixation is in two ways:

  1. High energy fixation
  2. Biological fixation

High energy fixation: high energy involved in high energy fixation are those such as cosmic radiation and lightning that provide the high energy needed to combine nitrogen with oxygen and hydrogen of H2O resulting in ammonia and nitrate. The materials produced are carried by rainwater.

Biological fixation:  it is the most significant fixation. Some bacteria, fungi, and blue-green algae can extract molecular nitrogen from the atmosphere. Free-living bacteria that are responsible for fixing nitrogen are nitrifying bacteria, acetobacter, and clostridium. These contributed to soil fertility.

Nostoc and Anabaena are important blue-green algae found in free land, fresh water, and marine water and they help in nitrogen fixation.


Ammonification is a biological process in which organic nitrogen compounds, such as proteins and nucleic acids, are converted into ammonium ions (NH4+) by decomposer microorganisms such as bacteria and fungi.

This process occurs during the decomposition of dead plants and animal material, such as fallen leaves or animal waste, and plays a critical role in the nitrogen cycle.

The ammonium ions produced during ammonification can then be further converted to nitrate by nitrifying bacteria or taken up by plants and microorganisms for use in the synthesis of amino acids, nucleic acids, and other nitrogen-containing compounds.

Ammonification is an important process for maintaining soil fertility, as it releases plant-available nitrogen from organic matter. However, excessive ammonification in soil can lead to the buildup of ammonia, which can be toxic to plants and soil microorganisms.

Nitrogen fixation by a symbiotic and non-symbiotic microorganism, in the soil and water, is one source of nitrogen.


Another source is an organic protein of dead organic materials decomposed by a group of microorganisms to produce amino acids and ammonia.

Ammonia is released in the atmosphere or retained in the soil to be absorbed by plants as ammonia salt


Nitrification is a biological process in which ammonia (NH3) and ammonium (NH4+) are converted to nitrite (NO2-) and then to nitrate (NO3-) by two groups of bacteria, known as ammonia-oxidizing bacteria and nitrite-oxidizing bacteria.

This process is a key component of the nitrogen cycle and occurs in soils, sediments, and aquatic environments. It is important because it converts ammonia, which is toxic to many organisms, into forms of nitrogen that can be used by plants to synthesize proteins and other essential compounds.

Nitrification is also important for maintaining water quality, as it helps to prevent the accumulation of toxic ammonia in aquatic ecosystems. However, excessive nitrification can lead to the over-enrichment of ecosystems with nutrients, a process known as eutrophication, which can cause harmful algal blooms, oxygen depletion, and other negative impacts on aquatic life.


The conversion of NH3 or NH4+ salt to NH3 is called Nitrification.

In the first step, Nitrites (NO2+) are formed and converted into NO3.

The conversion of NH3 to nitrate is done by Nitrosomonas

The bacteria Nitrobacter convert nitrite to Nitrate shown in the below equation

NH3 + 3O2 à 2NO + 2H2O + 2H(Nitrosonomas)

2NO2 + O2 à 2NO3 (Nitrobacter)

Nitrate forme can be taken up by plants at the begging of the food chain.


Denitrification is a biological process in which nitrate (NO3-) is converted to nitrogen gas (N2) and nitrous oxide (N2O) by microorganisms in an anaerobic (oxygen-free) environment.

This process is a natural part of the nitrogen cycle and occurs in various environments such as soil, water, and sediment. Denitrification is important because it reduces the amount of nitrate in ecosystems, preventing it from accumulating and causing environmental problems such as eutrophication, which is the excessive growth of algae and other aquatic plants due to high levels of nutrients like nitrate.

In agricultural systems, denitrification can be used as a management strategy to reduce the amount of nitrogen that leaches into groundwater and surface water. It can also be used in wastewater treatment to remove excess nitrogen from the effluent before it is released into the environment. However, denitrification can also result in the emission of greenhouse gases such as nitrous oxide, which is a potent contributor to climate change.

In certain conditions, nitrate is not produced in the nitrogen cycle but it is degraded into gaseous nitrogen (n2) nitrogen and ammonia.


The degradation of nitrogen is known as Denitrification. Bacteria Pseudomonas is important in this process.

Sulphur Cycle

Sulphur Cycle
Sulphur Cycle

The sulfur cycle is a biogeochemical process that involves the cycling of sulfur through various forms in the environment. It includes the following steps:

  1. Weathering and erosion: Sulfur is released into the environment through the weathering of rocks containing sulfur minerals such as pyrite.
  2. Sulfur assimilation: Sulfur is taken up by plants in the form of sulfate (SO42-) from the soil, and incorporated into organic compounds such as amino acids and proteins.
  3. Sulfur mineralization: When organisms die and decompose, sulfur in their organic matter is released back into the environment as hydrogen sulfide (H2S) or other sulfur-containing compounds.
  4. Sulfur oxidation: Sulfur compounds are oxidized by bacteria into sulfate, which can then be taken up by plants again, completing the cycle.
  5. Sulfur reduction: Under anaerobic conditions, some bacteria can reduce sulfate back to hydrogen sulfide, which can then be used as an energy source by other microorganisms.

The sulfur cycle is important because sulfur is a critical element for life, as it is a component of many essential biomolecules such as amino acids and vitamins. It is also important for the formation of acid rain and for regulating the Earth’s climate, as some sulfur compounds can reflect sunlight and cool the planet.

Sulphur can exist in a number of states namely:

  • Element sulphur
  • Sulphur monoxide
  • Sulphate
  • Sulphite
  • Sulphur dioxide
  • Sulphide

Note: Of all these sulphur, sulphide, sulphates are the most important in nature

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