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Biogeochemical Cycles Or Nutrient Cycles Or Material Cycles

The process by which inorganic materials move from the atmosphere or soil into living organisms and back again is called biogeochemical cycle. Some materials including nitrogen, water, oxygen and carbon reach ecosystems via the atmosphere (Figure 10.4). Rocks and soil particles contain other important elements such as calcium and phosphorus, which are usually chemically bound as mineral salts. The weathering of rocks and soil by precipitation releases these minerals. For each element, the cycling process involves (1) a reservoir, that portion of earth that acts as a storehouse for the element; (2) an exchange pool, that portion of the environment from which the producers take their nutrients; and (3) the biotic community, through which chemicals move along food chains to and from the exchange pool.

There are two types of biogeochemical cycles:

  • The gaseous type of biogeochemical cycle is generally located in the atmosphere or the hydrosphere, e.g. carbon, oxygen, water, nitrogen and hydrogen (Figure 10.5).
  • In the sedimentary type, the reservoir exists in the earth’s crust, e.g. phosphorus, calcium and magnesium.
  • Carbon Cycle

Carbon is a basic element for all life. It is an essential constituent of all organic compounds that compose the protoplasm.



Atmospheric carbon dioxide serves as the source of carbon for the synthesis of organic compounds, particularly sugars, in autotrophic land plants.

In aquatic ecosystems, carbonates and bicarbonates dissolved in water serve as the source of carbon. Photosynthesis is, thus, responsible for the fixation of carbon into the nutrient pool.

The organic compounds manufactured by plants provide the source of carbon for animals. Through the food chains, carbon is passed on from plants to herbivores and from herbivores to carnivores. Thus, carbon circulates in the exchange pool. Much of this carbon dioxide is returned from the exchange pool to the reservoir pool, by way of respiration by plants, animals, bacteria and fungi. Besides, a part of the carbon is returned by the process of decomposition of bacteria and fungi.

Carbon assimilated by green plants (photosynthesis) is passed on to the bodies of animals (plants are eaten). During respiration of plants and animals and decomposition by microbes, this carbon dioxide is returned back to the atmosphere. This is called carbon cycle.

Nitrogen Cycle


Nitrogen makes up about 78% of the atmosphere. Most living things cannot use atmospheric nitrogen. They must rely instead on nitrogen compounds found in soils such as nitrite Description: 41133.png and nitrates Description: 41140.png. The process that converts nitrogen gas into compounds that plants and animals can use is called nitrogen cycle. The cycle involves five major processes, namely nitrogen fixation, nitrogen assimilation, ammonification, nitrification and denitrification.

A chemical process in which the atmospheric nitrogen chemically combines with other elements to form nitrogen compounds is nitrogen fixation. Nitrates (rarely ammonia) constitute the source of nitrogen for assimilation by most green plants. In the living cells of the green plants, nitrates reduced to ammonia enter as amino groups (−NH2) in the formation of the amino acid. Different animals get their supply of nitrogen in the form of plant or animal proteins through food chains. The animals digest these proteins into amino acids, which serve as the building blocks for their own proteins and nucleic acids.

Proteins of animals are broken down to urea, uric acid or ammonia that passes out through urine. The proteins of dead plants and animals are also broken by ammonification. Many heterotrophic bacteria, actinomycetes and fungi do ammonification by enzymatically degrading organic nitrogen into ammonia. The conversion of ammonia to nitrate is called nitrification. The soil bacteria Nitrosomonas and Nitrobacter bring about nitrification. The process of transforming nitrate to nitrous and nitric oxides and ultimately to gaseous nitrogen is called denitrification. Example includes Pseudomonas.

Oxygen Cycle


Oxygen is also an essential nutrient for living organisms. Oxygen cycle is the cycling of oxygen between the biotic and abiotic components of the environment. In the process of respiration, oxygen is taken in by living organisms and released into the atmosphere combined with carbon, in the form of carbon dioxide. Carbon dioxide enters the carbon cycle or is taken up by plants for photosynthesis. During photosynthesis, oxygen is evolved by the chemical splitting of water and returned to the atmosphere. In the upper atmosphere, ozone is formed from oxygen and dissociates to release oxygen.

Phosphorus Cycle


The cycling of phosphorus between the biotic and abiotic components of the environment represents the phosphorus cycle. Inorganic phosphates Description: 41147.png are absorbed by plants from the soil and bodies of water and eventually pass into animals through food chains. Within living organisms, phosphates are used to build up nucleic acids and other organic molecules. When plants and animals die, phosphates are released and returned to the abiotic environment through the action of bacteria. Phosphates in aquatic environments eventually become incorporated into and form a part of rocks, through a gradual process of erosion.These phosphates are returned into the soil, seas, rivers and lakes.

Hydrologic Cycle or Water Cycle


Water is essential to all organisms and its availability influences rates of ecosystem processes, particularly primary production and decomposition in terrestrial ecosystem.

Liquid water is the primary physical phase in which water is used, though some organisms can harvest water vapour. The freezing of soil water can limit water availability to terrestrial plants.

The main processes driving the water cycle are the evaporation of liquid water by solar energy, condensation of water vapour into clouds, and precipitation. Transpiration by terrestrial plants also moves significant volumes of water. Surface and groundwater flow can return water to the oceans, completing the water cycle.

We alter the water cycle by the following activities:

  • Withdrawing large amounts of fresh water.
  • Clearing vegetation and eroding soils.
  • Polluting surface and underground water.
  • Contributing to climate change.


Biogeochemical Cycle: Gaseous Type


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