Nitrogen Cycle Archives

Nitrogen Fixation and Nitrogen Cycle

Nitrogen fixation is the process by which free atmospheric nitrogen is converted into nitrogen compounds. In nature, nitrogen is fixed during lightning or by nitrogen-fixing bacteria and blue-green algae.

Leguminous plants like pea and soya bean have the nitrogen fixing bacteria Rhizobium in their root nodules. The nitrogen fixed by these bacteria is taken up by the plants and stored in their tissues as plant proteins. This stored nitrogen passes from these plants to herbivores, and then to carnivores.

What is Nitrogen Fixation and Nitrogen Cycle 1

Root nodules of leguminous plant

When these plants and animals die, the nitrogen compounds present in their bodies are decomposed and returned to the soil. Some of these compounds are again absorbed by plants, while some are converted to nitrogen and returned to the atmosphere by special bacteria present in the soil. This cycling of nitrogen, between the atmosphere, soil, and organisms, is called nitrogen cycle.

Nitrogen cycle is a natural cyclic process in which atmospheric nitrogen enters the soil and becomes a part of living organisms, before returning to the atmosphere. A simplified representation of this process is shown in figure.

Nitrogen Cycle

What is the Biogeochemical Cycle

Biogenic elements (macro-, micro- & other elements) flow from the environment into and out of the plant in a cyclic manner.
This flow of nutrients from abiotic to biotic components of the ecosystem and vice-versa constitute the biogeochemical cycles.

Hydrological or Water Cycle :
What is the Biogeochemical Cycle 1

Water on earth is cycled by two processes, evaporation and precipitation.
The atmospheric precipitation occurs in the form of snow, hail or sleet etc. The run off water is finally collected in ocean through rivers.
Some water remains solid in the form of snow which gradually melts and reaches the sea.
Soil water is used by plants and most of it again reaches the atmosphere through transpiration.
Animals consume water directly from water bodies & also the gravitational water.
By evaporation, the water returns to atmosphere and cycle is repeated.

Carbon Cycle :

CO₂ is 0.03% in atmosphere, which is utilized by producers in photosynthesis for making food.
From producers, it goes to consumers and then through decomposers into atmosphere.
The producers, consumers & decomposers may be converted into fossil fuel (petrol, coal etc.) or form carbonate rock after death.
By way of respiration the biotic component returns CO₂ to atmosphere.
CO₂ may get dissolved in water. The lime rocks also contribute to CO₂ in water. The aquatic producer use this CO₂ for photosynthesis and return it by respiration.
By combustion of fossil fuel & also by volcanic activity, CO₂ is returned to the atmosphere.

Nitrogen Cycle :
What is the Biogeochemical Cycle 3

The atmosphere is the source of N₂ where it is about 79%. Plant cannot use N₂ directly.
In living organisms nitrogen is important constituent of protein and nucleic acid.
The N₂ cycle has five important steps –
1. Nitrogen Fixation :
  Conversion of N₂ gas into its compounds like nitrates & nitrites is called N₂ fixation. It is done either non-biologically by lightening or biologically by symbiotic or free- living bacteria. O₂ is harmful for N₂fixing bacteria.
2. Assimilation of Nitrogen :
  N₂ cannot be used by plants directly. They absorb it in the form of nitrate. Nitrate later on reduced to ammonia which provide amino (–NH₂) group. It is important part of proteins.
3. Ammonification :
  Dead plant & animal protein and their waste like urea & uric acid converted to ammonia by some ammonifying bacteria in soil. e.g. Bacillus mycoides, B. vulgaris & B. ramosus etc.
4. Nitrification :
  Ammonia is converted into nitrite by Nitrosomonas bacteria, and Nitrobacter convert nitrite into nitrate. This nitrate again can be absorbed by plant & thus cycled back.
5. Denitrification :
  Some denitrifying bacteria like Pseudomonas reduce nitrate into nitrogen gas in soil. This gas is again back to environment.