Bio-Geochemical Cycle

A pathway by which a chemical substance moves through biotic (biosphere) & abiotic (lithosphere, atmosphere & hydrosphere) compartments of Earth.

Type of biogeochemical cycle: Gaseous & sedimentary

  • In the gaseous type of biogeochemical cycle, there is a prominent gaseous phase. Cycling of carbon and nitrogen represents gaseous biogeochemical cycles.
  • In sedimentary cycles, main reservoir is lithosphere from which nutrients are released largely by weathering of rocks. The sedimentary cycle is exemplified by phosphorus and sulphur.

Biogeochemical cycles are either perfect or imperfect.

  • A perfect nutrient cycle is one in which the nutrients are replaced as fast as they are used up. Most gaseous cycle’s arc is generally considered perfect.
  • In contrast, sedimentary cycles are considered relatively imperfect, as some nutrients are lost from the cycle into soil and sediments & become unavailable for immediate cycling.

Ecosystem – Carbon Cycle

The carbon cycle is an essential process for all living organisms and ecosystems on Earth. Carbon is a fundamental building block of life, and it cycles through the atmosphere, oceans, and terrestrial ecosystems in a complex series of processes known as the carbon cycle.

carbon cycle

The carbon cycle can be broken down into two main processes: the biological carbon cycle and the geological carbon cycle.

Biological Carbon Cycle

  • It refers to the processes by which carbon moves through living organisms and ecosystems.
  • It starts with photosynthesis, which is the process by which plants and other photosynthetic organisms convert atmospheric carbon dioxide (CO2) into organic matter through the use of sunlight.
  • This organic matter can be consumed by herbivores, which then become food for carnivores and other higher-level consumers.
  • During this process, carbon is transferred from one organism to another, and eventually, the carbon is released back into the atmosphere through respiration, decomposition, or combustion.

Geological Carbon Cycle

  • The geological carbon cycle refers to the processes by which carbon is transferred between the atmosphere, oceans, and terrestrial ecosystems over long periods of time.
  •  This process involves the weathering of rocks, which releases carbon into the oceans, where it can be stored in sedimentary rock or taken up by marine organisms. Over time, the carbon can be released back into the atmosphere through volcanic activity or erosion of sedimentary rock.

Ecosystem – Phosphorus Cycle

The phosphorus cycle is another important biogeochemical cycle that is critical for the functioning of ecosystems.

The phosphorus cycle can be broken down into several stages:

Phosphorus Cycle

Weathering and Erosion

Phosphorus is released into the environment through the weathering and erosion of rocks and minerals.


Plants absorb phosphorus from the soil and incorporate it into their tissues.


Herbivores consume plants, and carnivores consume herbivores, transferring phosphorus up the food chain.


When plants and animals die, their tissues decompose, releasing phosphorus back into the soil.


During decomposition, phosphorus is converted from an organic form to an inorganic form that can be taken up by plants.


Over time, phosphorus can accumulate in sedimentary rocks and become locked up in geological formations.

Geological Uplift

Geological uplift can bring phosphorus-rich rocks to the surface, restarting the phosphorus cycle.

The  two major and important differences between carbon and phosphorus cycle are

  • firstly, atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs
  • secondly, gaseous exchanges of phosphorus between organism and environment are negligible.

The Nitrogen Cycle

The nitrogen cycle is a vital process in ecology that involves the transformation and cycling of nitrogen in various forms within an ecosystem. Nitrogen is an essential element for the growth and development of living organisms, particularly in the form of proteins and nucleic acids. The nitrogen cycle consists of several key steps:

The Nitrogen Cycle

Nitrogen Fixation:

Certain bacteria, such as Rhizobium and Azotobacter, convert atmospheric nitrogen (N2) into ammonia (NH3) through a process called nitrogen fixation. This conversion can occur through symbiotic relationships with plants (as in legume nodules) or as free-living bacteria in the soil.


Ammonia (NH3) is further transformed by nitrifying bacteria into nitrite (NO2-) and then into nitrate (NO3-). This process is known as nitrification. Nitrite and nitrate are forms of nitrogen that can be readily used by plants.


Plants take up nitrate ions from the soil and incorporate them into organic compounds, such as amino acids and proteins. This assimilation of nitrogen by plants allows them to grow and develop.

Consumption and Decomposition

Animals obtain nitrogen by consuming plants or other animals. When plants or animals die, decomposers, primarily bacteria and fungi, break down the organic matter, releasing ammonia back into the soil.


Denitrifying bacteria convert nitrate (NO3-) back into atmospheric nitrogen (N2), completing the nitrogen cycle. This process occurs in oxygen-deprived environments, such as waterlogged soil or sediments, and results in the loss of nitrogen from the ecosystem.

The hydrological cycle

The hydrological cycle, also known as the water cycle, is a fundamental process in ecology that describes the continuous movement of water on Earth. It involves the circulation and transformation of water through various reservoirs, including the atmosphere, land, and oceans. The hydrological cycle consists of several key steps:

The hydrological cycle


Solar energy heats the surface of water bodies, causing water to change from a liquid to a gaseous state, forming water vapor. This process is known as evaporation.


Water vapor rises into the atmosphere and cools, forming tiny water droplets or ice crystals, leading to the formation of clouds. This process is called condensation.


Condensed water droplets in the clouds combine and grow in size until they become too heavy to remain suspended in the air. They then fall to the Earth’s surface as precipitation, which can take the form of rain, snow, sleet, or hail.


Precipitation that reaches the Earth’s surface can be absorbed into the soil through a process called infiltration. It percolates through the soil and may eventually reach underground aquifers, replenishing groundwater resources.

Surface Runoff

When the rate of precipitation exceeds the capacity of the soil to absorb water, excess water flows over the land surface as runoff. It eventually makes its way into rivers, streams, lakes, and eventually the oceans.


Plants absorb water through their roots and release it into the atmosphere through small openings in their leaves called stomata. This process, known as transpiration, contributes to the movement of water vapor into the atmosphere.


In addition to evaporation, water can also change directly from a solid (ice) to a gas (water vapor) through a process called sublimation. This occurs when ice or snow undergoes a phase transition without melting into liquid water.

The sulphur cycle

The sulphur cycle is a biogeochemical cycle that describes the movement and transformation of sulphur through various forms within ecosystems. Sulphur is an essential element for living organisms as it is a component of amino acids, proteins, and vitamins. The sulphur cycle involves several key processes:

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Sulphur compounds in rocks and minerals can be released through weathering processes, such as erosion and exposure to atmospheric elements.

Sulphur Mineralization

Sulphur-containing minerals in rocks and sediments can undergo mineralization, resulting in the release of elemental sulphur or sulphate ions into the environment.

Sulphur Assimilation

Plants absorb sulphate ions from the soil and assimilate them into organic compounds. Sulphur is an important component of plant proteins and plays a role in various metabolic processes.

Consumption and Decomposition

Animals obtain sulphur by consuming plants or other animals. When plants or animals die, decomposers break down organic matter and release sulphur compounds back into the environment.

Volcanic Activity

Volcanic eruptions release large amounts of sulphur dioxide (SO2) into the atmosphere, contributing to the sulphur cycle. Sulphur dioxide can react with atmospheric moisture to form sulfuric acid (H2SO4), which then returns to the Earth’s surface through precipitation.

Sulphur Oxidation and Reduction

Sulphur compounds in the atmosphere undergo oxidation and reduction reactions facilitated by various microorganisms. These reactions convert sulfur between different oxidation states, such as sulphate, sulphite , elemental sulphur, and hydrogen sulphide (H2S).

Sulphur Desorption

Sulphur compounds can be released from soil particles and transported through runoff or leaching, entering water bodies or groundwater.

Sulphur Gas Emissions

Certain bacteria, such as sulphate-reducing bacteria and sulphur-oxidizing bacteria, play a crucial role in the sulphur cycle. Sulphate-reducing bacteria convert sulfate ions into hydrogen sulphide gas (H2S), while sulphur-oxidizing bacteria oxidize hydrogen sulphide back to sulphate.

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