Agriculture notes prepared by IAS Topper for all agriculture based competitive exams-Physiology-Nitrogen Cycle

Nitrogen Availability in soil and atmosphere (N2 cycle)

(Nitrogen Cycle)

Sources:

• Volcanic soil contains minerals of nitrogen which degrade and become available in the soil after weathering and soil forming process are over
• Electrification and photochemical fixation of atmospheric nitrogen
• Atmospheric nitrogen fixation by the symbiotic and non-symbiotic organisms
• Decay of dead organic matter and excretion

 Process involved in nitrogen cycle

• Nitrogen fixation by biological organisms and lightning-freely available nitrogen in the atmosphere is converted into ammonia by physical and bio-chemical process

N-N-------N+N----------2NH3

• Ammonificaiton-conversion of organic matter into ammonia/ammonium

Dead organic matter------------ Ammonia

• Nitrification-conversion of ammonia/ammonium into nitrite/nitrate

Ammonia---------------Nitrite (by nitrosomonas)

Nitrite-------------------Nitrate (by nitrobacter)

• Denitrification-conversion of nitrite/nitrate into gaseous nitrogen

Nitrite/Nitrate-------------Ammonia-----------Nitrogen

Biological Nitrogen fixation

Symbiotic fixaton:

It is grouped into two groups

Fast growers:  Rhisobium legunimosarum

                        Rhisobium meloloti

                        Rhisobium trifoli

Slow growers: Rhisobium japoinicum

                        Rhisobium lupinii

Nodule formation

a). Host recognition: It is controlled by a specific protein in the host called lectin, which interact selectively with microbial cell carbohydrate (glycoprotein) present in capsule of bacteria. Calcium ions also plays a crucial role in modifying the ability of root to absorb Rhizobia.

b). Nodulation: 

• Rhizobium does not have cellulose, therefore it enters through tip of root haris which is not having any cellulose. Plant releases the tryptophan into soil, which is absorbed by bacteria and metabolize it to produce Indole Acetic Acid.
• Rhizobium produces root hair culing factor which deform and twist root hair after penetration. Root hairs are subsequently penetrated by large number of Rhizobia, root hair curls and converted into an infection thread lined by cellulose which is provided by the host cell. This thread releases its content (bacteria) into cortical cells. Liberated Rhizobia may be in polyploid single or in small groups enclosed by a membrane.
• Bacteria assumes a pleomorphic shape and are called as baceroids. They induce polyploid cells and diploid cells of cortex and others to undergo multiplication (IAA). Thus nodules are formed.
• Legheamoglobin is a pink pigment produced only in host Rhizobial interaction which is important for providing anaerobic condition.

Nitrogenase enzyme

• Function: Provides surface for attachment of nitrogen and fixes atmospheric nitrogen into ammonia
• Substrates: Atm. N2, N2O and acetylene
• It consists of two protein components
• Mo-Fe protein component contains one Mo for every 2-3 lakh units
• Fe protein component, has molecular weight of 55,000 to 65,000 Dalton 

• Enzyme is very sensitive to oxygen and higher oxygen content in the atmosphere denatures the enzyme. Therefore, plants have evolved Leghaemoglobin and heterocysts. Enzyme also undergoes confirmational changes so that it becomes insensitive to oxygen content or the excess oxygen is utilized for oxidation of other substrates.
• Functions of Leghaemoglobin:
• Removal of oxygen to provide anaerobic condition
• Provide oxygen to bacteroid for ATP production which in turn generate electron for fixation

Mechanism: Atmospheric nitrogen attaches/binds on enzyme surface and is not released until it is completely reduced to ammonia.

N-N ---------- bond broken N+N ------------------- NH3

Reaction: There are three important conditions required for this process

• Availability of ATP through respiration of bacteroids
• Availability of reduced substrate, Glucose-6-phosphate from host and reduced NADP together ferredoxin (electron carrier)
• Enzyme nitrogenase for fixing of N2 on its surface

Steps:

• Glucose-6-phosphate provide hydrogen for the reaction. ATP interacts with non-haeme iron protein (NHI) component of nitrogen and brings about confirmational changes to convert it to powerful reductants so that they are capable of transferring electron to reduce nitrogen NH3

Note: Six electrons are needed to reduce one molecule of nitrogen to produce 2NH3. Therefore tow electron release, 4ATPs required. At least 12 ATPs are required to reduce one molecule of nitrogen into two NH3

Nitorogen cycle Flow diagram………………………………………………