Understanding Biogeochemical Cycles: Carbon, Nitrogen, Phosphorus, and Sulfur

Posted on Jan 24, 2025 in Geology

Biogeochemical Cycles

Biogeochemical cycles describe the exchange of certain chemical elements between the organisms that form the biocenosis and the biota.

Carbon Cycle

Carbon is the most abundant chemical element in all living things. It is located in the atmosphere, dissolved in water, in carbonate rocks, and in the form of fossil fuels.

• Through photosynthesis, autotrophic organisms convert atmospheric CO₂ into organic matter, which is used to feed animals.
• During respiration, organic matter is consumed, and the energy contained within it is released, forming CO₂ that is returned to the atmosphere.
• Carbon is incorporated into the soil when plants and animals die.
• Bacteria break down the remains, producing CO₂ that is released into the atmosphere.
• A portion of the organic remains found in soil are transformed into oil, which is used as fuel, and CO₂ is released through combustion.
• Volcanic activity also emits CO₂ into the atmosphere and hydrosphere.

Nitrogen Cycle

Nitrogen is a component of proteins and nucleic acids. It is mostly found in the atmosphere, where it constitutes almost 78% of atmospheric gases, and in the remains of decomposition.

• Atmospheric N₂ cannot be directly incorporated into plants. Only a limited number of organisms, like bacteria or cyanobacteria, can do this.
• Soil is enriched in nitrogen through the activity of certain bacteria that decompose organic matter and release it in the form of ammonia.
• Soil nitrifying bacteria convert ammonia to nitrates, and with them, plants create nitrogenous organic matter, which is incorporated into animals through the food chain.
• Nitrogen returns to the soil as ammonia from the droppings of animals and the decomposition of their corpses.
• A portion of soil nitrogen goes directly to the atmosphere through the action of denitrifying bacteria.

Phosphorus Cycle

Phosphorus is a component of important compounds such as nucleic acids.

• The main reservoir of phosphorus is sedimentary rocks located in the Earth’s crust, where it is released in the form of phosphates when phosphate rock is eroded. These phosphates are used by plants for the synthesis of organic matter and, through the food chain, are incorporated into animals.
• Bacterial decomposition of organisms returns phosphates to the soil, which are then used by new plants.
• A portion of phosphates are washed out to sea by runoff waters. These are used for the development of phytoplankton and all the food chains that depend on it, though the majority forms large deposits in the seabed. Only a minimal amount of marine phosphorus returns to land through the excrement that seabirds deposit on the shores.

Sulfur Cycle

Sulfur is found in living beings in the form of sulfhydryl groups in certain amino acids.

• Upon the death of an animal or plant, a group of bacteria known as putrefying bacteria release sulfur in the form of hydrogen sulfide.
• This acid, which causes the stench of putrefaction, is transformed into sulfate and is then absorbed by the soil.
• Sulfur, in the form of soluble sulfate, is taken up by plants and reduced to amino acids. These amino acids are passed to herbivores and from these to carnivores. Upon death, the cycle restarts.