Biogeochemical cycle

Biogeochemical cycle

The term 'biogeochemical cycle' comes from the biological, geological, and chemical processes. A biogeochemical cycle or inorganic-organic cycle is circulating or repeatable pathway by which either a chemical element or a molecule moves through both biotic (bio) and abiotic (geo) compartments of an ecosystem.

The cycling of the elements and inorganic compounds that are essential to life is known as nutrient cycle whereas the elements and dissolved salts essential to life is known as nutrients or biogenic salts. It is of two types: macronutrients which are needed in relatively larger quantities, being major components of the protoplasm like carbon, hydrogen, etc. and micronutrients are needed in very little quantities but necessary for life activities like iron, zinc, etc.

There are three types of biogeochemical cycles which are given below:

1. Gaseous cycle: Here, the reservoir of material is air or atmosphere. It shows little or no permanent change in the distribution and abundance of the element. It includes carbon, oxygen, and nitrogen cycle.
2. Sedimentary type: In this type of cycle, a major reservoir is a lithosphere from which the elements are released by weathering. These types are best exemplified by phosphorus, sulphur, and iodine cycles.
3. Water cycle: It was included with gaseous types of cycles but it was considered to be a separate major cycle, involving the movement of the compound while the others involve the movement of the element.

Nitrogen cycle:

Nitrogen is the essential component of DNA, RNA, and proteins, the building blocks of life. The movement of nitrogen between the atmosphere, biosphere, and geosphere in different forms is called the nitrogen cycle. Green plant obtains nitrogen from the soil solution in different forms. Atmospheric nitrogen is not directly available. Nitrogen cycles consist of the following steps.

1. Nitrogen fixation: It is the conversion of free atmospheric nitrogen into the biologically acceptable form or nitrogenous compounds. It is of two types:
   
   Non-biological nitrogen fixation: Here, the atmospheric nitrogen combines with oxygen during lightning or electric discharge producing different nitrogen oxides.
   
   N₂ + 2O →2NO
   2NO + 2O → 2NO₂
   2NO + O →N₂O₅
   
   Nitrogen oxides get dissolved in rainwater and react with mineral compounds to form nitrates and other nitrogenous compounds.
   N₂O₅+ H₂O→ 2HNO₃

2HNO₃ + CaCO₃→ Ca(NO₃)₂ + CO₂+ H₂O

1. Biological nitrogen fixation: Symbiotic bacteria (Rhizobium) present in root nodules of leguminous and non-leguminous plants and symbiotic blue-green algae (Nostoc, Anabene, etc.) fix atmospheric nitrogen. Bacteria like Azobacter, Clostridium also fix free N₂ of atmosphere in the soil.
2. Nitrogen assimilation: Inorganic nitrogen in the form of nitrates, nitrites, and ammonia are converted into the nitrogenous compound by green plants. Nitrates are converted into ammonia which combines with organic acids forming amino acids that are used in the synthesis of protein, acids etc.
3. Ammonification: Micro-organisms like Bacillus ramosus uses organic compounds of dead and decayed animals and plants in their metabolism and release ammonia.
4. Nitrification: Bacteria like Nitrosomonas converts ammonia into nitrites which are then converted into nitrates by Nitrobacter. These bacteria are called nitrifying bacteria and the process is called nitrification.
5. Denitrification: The process of conversion of ammonia and nitrates into free nitrogen by denitrifying bacteria like Bacillus denitrifications, Micrococcus denitrifications, etc. is known as denitrification.
6. Sedimentation: Nitrates that are washed away to the sea or deep earth along with percolating waters are locked up in the rocks. This is sedimentation of nitrogen and is only released when the rocks are exposed or weathered.

Carbon cycle:

Carbon is an essential constituent of all organic compounds found in living beings. Its sources are the atmosphere, water, carbonates in the rock and fossil fuels.

1. Carbon fixation: Carbon found in coal, gas and petroleum and carbonates in the earth's crust is not available till it is burnt or changed chemically. CO₂ is found in the free state in the atmosphere and in dissolved state in the water on earth which are used by green plants through photosynthesis and carbohydrate is formed.
2. Carbon assimilation: Carbohydrates, complex fats, and polysaccharides are formed in plants after using the free molecular carbon dioxide through the process of photosynthesis, that are utilized by animals (herbivorous) which are again used by carnivorous and the carbon compounds are again digested and converted into the other forms.

Release of carbon back to environment

Different microbes decompose the dead and decayed living beings into simple organic compounds, which in further destruction produce free molecular carbon dioxide and mix with the atmosphere. It is also released through respiration in living organisms, by combustion of wood, gas and fossil fuels, by dissolution of carbonate rocks and by volcanic activity of rocks.