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Easy to understand

Nitrogen cycle: Definition?

Nitrogen cycle is the cyclic movement of nitrogen (N2) between atmosphere, organisms and soil.

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N2

5 Steps in Nitrogen cycle

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Step 2

Step 3

Step 4

Step 5Step 1

Why N2 is important?

Nitrogen is an essential element required for the synthesis of bio-molecules like proteins, DNA, vitamins, chlorophyll, alkaloids etc.

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Protein

Amino acids

DNA

Nitrogenous bases Chlorophyll-a

Pigments

Why N2 is the most limiting nutrient?

•Atmospheric N2 is extremely stable, unreactive, inert with triple covalent bond between nitrogen atoms. •Plants and animals cannot use atmospheric N2. So it should be converted to some other usable forms such as nitrates (NO3

-)

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N2 gas is inert941 kj/mol

Atmospheric Nitrogen (79%)

Step 1

Step 2

Step 3

Step 4

Step 5

Step 1: Nitrogen Fixation

Nitrogen fixation: atmospheric N2 → NH3 (Ammonia)

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Step 1: Nitrogen Fixation3 Methods of Nitrogen Fixation

2.Non-biological N2 fixationby lightning, volcaniceruptions etc.

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3. Industrial Nitrogen fixationHaber Bosch process

1. Biological Nitrogen Fixation (BNF)

Step 1: Biological Nitrogen fixation ( BNF)

1. Biological nitrogen fixation (BNF): conversion of atmospheric N2 to ammonia (NH3)2. carried out by prokaryotes called as nitrogen fixers or diazotrophs. 70% of natural nitrogen

fixation.3. Rhizobium in root nodules of leguminous plants, blue green algae like Anabaena and lichens

like Collema.4. high energy requiring process & N2 fixers uses 16 moles of ATP to fix each molecule of

Nitrogen (N2)

N2 + 8 H+ + 8 e−+ 16 ATP → 2 NH3 + H2 +16 ADP +16 Pi

BNF(70% of

NF)

Symbiotic prokaryotes like Rhizobium

Free living like Azotobacter

16 moles of ATP to fix each molecule of Nitrogen (N2)

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Step 2: Nitrification

Nitrosomonas Nitrobacter

Nitrite bacteriaNitrosomonasNitrosospira

Nitrosococcus

Nitrate bacteriaNitrobacter

Nitrifying bacteria

NH3 Ammonia

NO2-

Nitrite

NO3-

Nitrate

Step 2

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Step 3: Nitrate Assimilation by Plants

Now soil nitrates (NO3-) formed by nitrification can be taken up by the

plants for the synthesis of amino acids, DNA, pigments etc. This is known as nitrate assimilation.

NO3-

Soil NO3

-

nitrate

NO3-

NO3-

NO3-

Biomolecules(DNA, protein ,

pigments)

Food chain

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Step 4: Ammonification

organic nitrogen (amino acids, DNA etc) in plant and animal tissues is converted to ammonia (NH3).

Death of organismOrganic Nitrogen (DNA, protein etc) in tissues

NH3 ammonia

decomposers

excretion

Biomolecules(DNA, protein ,

pigments)

soilammonification

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Step 5: Denitrification

It is the conversion of soil nitrate (NO3-) to N2

NO3- NO2

- NON2

Anammox (anaerobic ammonia oxidation)It is the conversion of ammonia (NH3) to N2 using nitrite as the electron acceptor under anoxic condition.NH4

+ + NO2-N2 + 2H2O

Denitrifying bacteriaBacillus, Paracoccus

Soil NO3-

Nitrate

NO2-

Nitrite

NO N2

AtmosphereN2

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