BIO-GEOCHEMICAL CYCLE

The term Nutrient cycle or Biogeochemical cycle is used for the exchange or circulation of biogenetic nutrients between the living and non-living components of the biosphere. Biogenetic nutrients or biogeochemicals are essential elements required by the organism for their body building and metabolism.

The Biogeochemical cycles are classified into two main types

1– Gaseous cycle

1. Carbon cycle
2. Nitrogen cycle 
3. Oxygen cycle
4. Water cycle (Hydrological cycle)

2– Sedimentary cycle

1. Sulphur cycle
2. Phosphorus cycle

NITROGEN CYCLE

It is a biogeochemical process which transforms the inert Nitrogen present in the atmosphere to a more usable form of living organisms. The Nitrogen gas exists in both organic and inorganic forms. Organic Nitrogen exists in living organisms, and they get passed through the food chain by the consumption of other living organisms. Inorganic form of nitrogen are found in abundance in the atmosphere. This Nitrogen is made available to plants by symbiotic bacteria which can convert the inert nitrogen into a usable form- such as Nitrites and Nitrates.

Process of Nitrogen cycle consists of the following steps- Nitrogen fixation, Nitrification, Assimilation, Ammonification, and Denitrification. These processes take place in several stages and are explained below:

NITROGEN FIXATION

It is the initial step of Nitrogen cycle. Here, atmospheric Nitrogen(N₂) is converted into Ammonia(NH₃). During the process of Nitrogen fixation, N₂ is deposited into soils from the atmosphere and surface waters. Later, the Nitrogen undergoes a set of changes, in which two Nitrogen atoms get separated and combines with Hydrogen to form Ammonia(NH₄⁺).

The entire process of Nitrogen fixation is completed by symbiotic bacteria which are known as Diazotrops. Azotobacter and Rhizobium also have a major role in this process. These bacteria consists of a Nitrogenase enzyme which has the capability to combine gaseous Nitrogen with Hydrogen to form Ammonia.

NITRIFICATION

In this process, the Ammonia is converted into Nitrate by the presence of bacteria in the soil. Nitrites are formed by the oxidation of Ammonia with the help of Nitrosomonas bacterium species. Later, the produced Nitrites are converted into Nitrates by Nitrobacter.

The reaction involved in the process of Nitrification is as follows:

2NH₄⁺ + 3O₂ —— 2NO₂^– + 4H^– + 2H₂O

2NO₂^–  + O₂ —— 2NO₃^–

ASSIMILATION

Plants take in the Nitrogen compounds from the soil with the help of their roots, which are available in the form of Ammonia, Nitrite ions, Nitrate ions or Ammonium ions and are used in the formation of the plant and animal proteins. This way, it enters the food web where the primary consumers eat the plants.

AMMONIFICATION

When plants or animals die, the Nitrogen present in the organic matter is released back into the soil. The decomposers namely bacteria or fungi present in the soil, converts the organic matter back into Ammonium. This process of decomposition produces  Ammonia which is further used for other biological processes.

DENITRIFICATION

It is the process in which the Nitrogen compounds makes it way back into the atmosphere by converting Nitrate into gaseous Nitrogen. This process occurs in the absence of Oxygen. Denitrification is carried out by the de-nitrifying bacterial species- Clostridium and Pseudomonas, which will process nitrate to gain Oxygen and gives out free Nitrogen gas as a byproduct.

CARBON CYCLE

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere and atmosphere of the earth. The carbon cycle was initially discovered by  Joseph Priestly and Antoine Lavoisier.

The carbon cycle occurs in nature as follows:

 

OXYGEN CYCLE

It is the biogeochemical cycle that describes the movement of oxygen within its three main reservoirs, the atmosphere, the  total content of biological matter within the biosphere and the lithosphere.

The diagrammatic presentation of oxygen cycle is shown below:

WATER CYCLE 

The Water cycle is also known as the hydrological cycle or H₂O cycle. The water constantly circulates through evaporation, condensation and precipitation and the nutrients dissolved in water also circulates within it. It describes the continuous movement of water on, above and below the surface of the earth.

The water cycle influences climate which leads to temperature changes. Human activities that alter the water cycle includes agriculture, industry, dams, urbanization, use of ground water etc.

PHOSPHORUS CYCLE

It is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere and biosphere. Unlike other biogeochemical cycles, atmosphere does not play a significant role in the movement of phosphorus.

The diagrammatic representation of phosphorus cycle is as follows:

SULPHUR CYCLE

Sulphur is considered as a secondary element along with calcium and magnesium. The process of sulphur availibility is same as the process of nitrogen availibility. Most of the sulphur present in soil is in the form of organic matter. Sulphur is an essential element being a constituent of many proteins and co-factors.

The sulphur  cycle is the collection of processors by which sulphur moves to and from minerals and living systems. Steps of sulphur cycle includes mineralisation of organic sulphur into hydrogen sulphide, oxidation of hydrogen sulphide to sulphate, reduction of sulphate to sulphide and lastly incorporation of sulphide into organic compounds.