Steps of Nitrogen Cycle are Assimilation, Ammonification, Nitrification and Denitrification.

The nitrogen cycle is a biochemical cycle where nitrogen cycles take place in the atmosphere, terrestrial and marine ecosystems, resulting in the conversion of nitrogen from one form to another. This conversion of nitrogen can be done either physically or biologically.

Atmospheric nitrogen cannot be absorbed directly by plants. That is why this nitrogen in the atmosphere must be converted into nitrate and nitrite suitable for plants to be processed. The conversion of nitrogen gas (N2) to nitrate and nitrite by atmospheric, industrial and biological processes is called nitrogen fixation. Most of the nitrogen fixation is caused by symbiotic bacteria. These are known as diazotrophs. These bacteria contain an enzyme called nitrogenesis enzyme, which combines atmospheric nitrogen with hydrogen to form ammonia. An example of a nitrogen fixing bacterium is Rhizobium, Azotobacter etc. Currently, 30% of nitrogen fixation occurs through the Heber-Bosch process in industrial establishments where ammonia is produced by reacting nitrogen gas with hydrogen at high temperatures and pressures.

Steps of Nitrogen Cycle:

The amount of nitrogen in the atmosphere is the highest. This conversion of nitrogen to ammonium (NH4 +), nitrate (NO3-), nitrite (NO2-), nitrous oxide (N2O), nitric oxide (NO) etc. takes place in several stages. The steps are as follows

• Assimilation
• Ammonification
• Nitrification
• Denitrification

Assimilation:

The plant absorbs nitrate or ammonia from the soil through the roots. When nitrate is absorbed, it first decomposes into nitrite ions and then into ammonium ions. Plants then use these ammonium ions in various biological processes. In plants that form symbiosis with symbiotic bacteria, the bacteria produce ammonia and supply it to the plant in exchange for receiving sugars from the plant. Since the bacteria live in legumes, the lower the amount of nitrogen in the soil, the higher the amount of nitrogen in the soil due to symbiosis.

Ammonification:

When a plant or animal dies or an animal excretes waste products, the bacteria and fungi present in the environment or in the soil re-convert the nitrogen of these carcasses and waste materials into ammonium ions, a process called ammonification.

Nitrification:

Nitrification is the conversion of ammonium to nitrate and nitrite by various nitrifying bacteria. Initially, bacteria of the species Nitrosomonas convert ammonium ions (NH4 +) to nitrite (NO2-). Bacteria of the Nitrobacter species subsequently convert nitrite to nitrate (NO3-).

2NH₄⁺ + 3O₂→ 2NO₂^– + 4H⁺ + 2H₂O 

2NO ₂ ^–  + O ₂  → 2NO ₃ ^–

It is important to convert ammonia to nitrate and nitrite because ammonia gas is toxic to plants.

Denitrification:

The conversion of nitrate back to nitrogen gas (N2) is called denitrification. This results in a complete cycle of nitrogen. Bacteria of the species Pseudomonas and Paracoccus carry out the process of denitrification. These denitrifying bacteria use soil nitrates to regulate their respiration and release nitrogen gas into the atmosphere.

Importance of nitrogen cycle

• Helps plants produce chlorophyll from nitrogen compounds.
• Biochemical processes help to convert nitrogen gas into usable form for plants.
• The carcasses and wastes of animals and plants are decomposed by the bacterial ammonification process. This helps to keep the environment clean.
• Nitrogen deficiency increases the amount of nitrogen in the soil and provides nutrients required for cultivation.
• Nitrogenous fertilizers combine with lake and river water to cause eutrophication.
• The percentage (%) of nitrogen in the atmosphere is maintained.

The human effect on the nitrogen cycle:

The increasing use of Heber-Bosch process in industrial plants for the manufacture of chemical fertilizers is doubling the amount of nitrogen compounds in the environment. The amount of Nr (reactive nitrogen) has increased tenfold in the last century due to globalization. The amount of nitrous oxide (N2O) in the environment is increasing day by day which acts as a catalyst for weight loss. Nitrous oxide is currently the third major gas responsible for global warming. Air quality is declining due to increase in ammonia in the air. Excessive ammonia produces nitric acid which is responsible for acid rain. Atmospheric ammonia is also damaging the respiratory system.

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