Nitrogen cycle consists of 6 stages:
- 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
a) Anaerobic bacteria
b) Aerobic bacteria
c) Photosynthetic bacteria
d) Chemosynthetic bacteria
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.
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.
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
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
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
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