Biogeochemical Cycles (part- 1)


A biogeochemical cycle or nutrient cycle is a pathway by which a chemical element or molecule moves through both biotic (biosphere) and abiotic ( lithosphere, atmosphere, and hydrosphere ) compartments  of  Earth.

★Nutrient cycle involves 2 general phases-

  1. The environmental phase – in which the chemical nutrient is present in the soil, water, or air.
  2. The organismic phase – in which the nutrient becomes part of the living tissues of producers and consumers.

These two phases are intimately linked an highly interdependent.

Since the total amount of each element of the environment  is essentially constant, some cycling process must take place. If it did not occur, shortage of minerals would limit the growth of plants and that would impose a limit on the size of dependent animal populations, Furthermore, there would be a never-ending build-up of dead organisms. When an organism dies the elements continue to move through a cycle, returning to the environment or to another organism.


Significance of Cycles

If cycling of nutrients did not occur then nutrients would replenish and run short.

If there is no nutrient cycling then there would be a pile of nutrients in one reservoir.


Types of Biogeochemical Cycle


Biogeochemical cycles can be divided mainly into two types:

A) Gaseous cycles :

In this cycle elements move through the atmosphere. Main reservoirs are the atmosphere and the ocean. Carbon and Nitrogen are the prime representatives of biogeochemical cycles with prominent gaseous phase.

B) Sedimentary cycle :

In this cycle elements move from land to water to sediment. Main reservoirs are the soil and sedimentary rocks. Phosphorus, sulfur and iodine are the examples of sedimentary cycles and they have the tendency to stagnate, i.e. a portion of the supply may get lost as in the deep ocean sediments.

A)Gaseous Cycles:






B) Sedimentary Cycles:




Carbon Cycle

(C) is an essential component of all organic molecules. The carbon (C) cycle is a gaseous cycle. The biological cycling of C is somewhat more direct. Two main forms namely CO2 and organic C compounds are involved in C cycle.

However, C also occurs in several inorganic pools. Atmosphere contains 0.0367% (367 ppm) of the air by volume. In addition, C is available in aquatic habitat either as bicarbonate (HCO3-) and carbonate (CO32-) ions dissolved in water.


Stages of Carbon Cycle

A. Fixation of Carbon and

B. Release of Carbon


A) Fixation of Carbon:

  • C fixation from the atmosphereic pool is an anabolic process in which complex organic molecules are formed from small and simpler ones. Two distinct groups of synthesizing (autotrophic) organisms carryout this process:

i) Photosynthetic organisms :

Green plants use light energy to reduce CO2 in the formation of carbohydrates, fats and proteins. Atmospheric C enters terrestrial and aquatic ecosystem. On land CO2 is absorbed by plants through stomata in the leaves. Within the plant, CO2 gas enters the cells, where it is combined with hydrogen (obtain from water molecules) during photosynthesis. Organic molecules are made in series of chemical reactions that are parts of photosynthesis.

6CO2 + 6H2O = C6H12O6


ii) Chemosynthetic organisms:

Nytrifying bacteria employ energy stored in chemical bonds, such as nitrate and nitrite, to reduce CO2. In both cases, CO2 is reduced by hydrogen from the donor (HA) an carbohydrate is formed.

CO2 + 2H2A = CH2O +H2O + 2A + Energy


  • 2H2A is the hydrogen donors
  • CH2O is the carbohydrate Organic-molecules formed in this way is transferred to higher trophic levels like primary, secondary and tertiary consumers.


B) Release of Carbon:

Carbon fixed in the organic matter is released into the atmosphere by the following processes:

i. Respiration (as CO2): In aerobic respiration CO2 is released back by complete oxidation of the carbohydrate.

C6H12O6+ 6O2 = 6CO2+ 6H2O + E

In anaerobic respiration, oxidation is incomplete resulting in the formation of various partially oxidized molecules, such as ethanol in plants and lactic acid in animals.

The C released in cellular respiration re-enters the environmental phase of the cycle.

ii. Decomposition: C also returns to the atmosphere through the decomposer food chain. Here decomposer organisms consume detritus and release CO2.


iii. Burning of fossil fuel: If the dead remians of plants and animasl accumulate without undergoing oxidation, fossil fuels may be eventually formed. Under acid conditions, in the absence of decomposers, debris accumulates layer upon layer to form deposit many meters in thickness. As pressure increases on the lower layer of debris, slow changes may occur, resulting in the successive formation of the following materials:

1. Peat ➡ Methane or other gases ➡ Oil

2. Peat ➡ Coal ➡ Anthracite ➡ Diamond


Re-use of Carbon

Carbon released back to the environment is re used by the autotroph.

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About Tarek Siddiki Taki

Currently studying at Department of Botany, University of Dhaka.

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