Nitrogen cycle consists of 6 stages:

1. Nitrogen fixation: Conversion of free nitrogen into nitrogenous salts to make it available for absorption by plants.

Nitrogen fixation is 2 types:-

a) Physical nitrogen fixation

b) Biological fixation

a) Physical nitrogen fixation

N₂ + O₂  —–> electric discharge and Thunder—> 2NO (Nitric oxide)

2NO + O₂ →      2NO₂ (Nitrogen peroxide)

2NO₂ + rain water → HNO₂ + HNO₃

On the ground, the alkali radicles of the soil react with nitric acid to produce nitrides & nitrates which are soluble in water & are absorbed by roots of the plants.

2HNO₃ + CaCO₃→Ca(NO₃)₂+ CO₂ + H₂O

b) Biological nitrogen fixation (Symbiotic and Asymbiotic)

# Symbiotic nitrogen fixation (Nodule producing bacteria):

It is carried out by bacteria frequently present in the root nodules of leguminous plants e.g. Rhizobium spp. Non-nodulating nitrogen fixing symbiotic association are also developed between cyanobacteria & various plants & some bacteria & plants. The Cyanobacterium, Anabaena azollae produces symbiotic association with Azolla while Azotobacter paspali develops colonies on the roots of Paspallum notatum etc. Frankia also fix nitrogen in the roots of Casuarina & Alnus.

# Asymbiotic Nitrogen fixation

Many free living bacteria, fungi & blue green algae are able to fix atmospheric nitrogen.

Bacteria:                     Aerobic bacteria

                                    Anaerobic bacteria

                                    Chemosynthetic bacteria

                                    Photosynthetic bacteria

a) Anaerobic bacteria

Bacillus polymyxa

Clostridium pasteurianum

Azospirillum brassilens

b) Aerobic bacteria

Azotobacter chrococcum

Azotobacter gilis

Azomonas

c) Photosynthetic bacteria

Chlorobium

Rhodopseudomonas

Chloropseudomonas

d) Chemosynthetic bacteria

Thiobacillus

Desulphovibrio

Fungi: Pullularia & yeasts

Blue green Algae (BGA)

Unicellular forms: Gloeotheca & Synecococcus

Filamentous non-heterocystous form: Oscillatoria erythreae and Lyngbya

Filamentous heterocystous form: Nostoc, Anabaena, Cylindrospermum & Scytonema

2. Nitrogen assimilation

Plants absorb nitrogen in the form of nitrates from the soil and absorbed nitrates are changed into NH₃ in the presence of enzymes.

Reactions:

Nitrates   –> Reductase enzymes in Neurospora—->      Ammonia

α-Ketoglutaric acid + Ammonia   —–>    α-imino glutaric acid

α-iminoglutaric acid   —-> NADPH₂   —–>   glutaric acid + glutamic acid dehydrogenase

 Formed amino acids are used in the synthesis of proteins, chlorophyll enzymes & nucleic acid.

3. Ammonification

The proteins & nitrogenous excretory products of living organisms are changed into amino acids, pyruvic acids & ammonia in the presence of microbes.

Proteins —-> enzymes—–> Amino acids

Amino acids + O₂→Pyruvic acid + NH₃

4. Nitrification

Conversion of ammonia & nitrous acid into nitrates is called as nitrification in the presence of microbes e.g. Nitrosomonas, Nitrococcus, and Nocardia

2NH₃ + 3O₂→ 2HNO₂ + 2H₂O

HNO₂ + ½ O₂→HNO₃

HNO₂ + 3H₂    clostridium    NH₃ + 2H₂O

5. Denitrification

Through a series of biochemical reactions, microbes like-Micrococcus, Thiobacillus, Pseudomonas & Agrobacterium convert nitrates & nitrite into gaseous nitrogen. From nitrate, N₂ & NH₃ are produced by the denitrification process in the presence of microbes.

6KNO₃ + 5S + 2H₂O Thiobacillus  Potassium sulphate (K₂SO₄) + potassium hydrogen sulphate + 3NO₂ + energy

HNO₃ + 4H₂     Clostridium           NH₃ + 3N₂O

6. Sedimentation

Nitrates of soil are being lost through the process of i) erosion loss of topsoil & washing it down into sea. ii) Leaching deep into earth along with rain water and thus nitrates lost

A- Ammonification                 S- Sedimentation

C- Denitrification                    SNF- Symbiotic nitrogen fixation

DD- Death & decomposition