Nitrogen Fixation

Nitrogen Fixation

The conversion of atmospheric nitrogen into useful nitrogenous compounds, by natural or artificial methods is called fixation of nitrogen.Nitrogen present in these nitrogenous compounds is called "Fixed" or combined nitrogen.Nitrogenase is the enzyme finds in nitrogen fixing bacteria.The best known dinitrogen fixing bacteria is Rhizobium which contains metalloenzyme nitrogenase.This enzyme catalyses the conversion of dinitrogen to ammonia.the full reaction catalysed by this enzyme is given below:

$$N_2+8H^++8e^-+16MgATP \rightarrow 2NH_3+H_2+16MgADP+16Pi$$

ATP is required to provide the necessary energy for the process.Nitrogenase is made up of two proteins,thje iron protein and the molybdenum-iron protein and are linked to an electron-transport chain.the iron protein accepts electrons from this chain and transfers them to the molybdenum-iron protein.The MoFe protein is then able to reduce a number of substrates in addition to dinitrogen.

The iron protein is made up of two identical subunits.The Fe protein has two binding sites for MgATP and is generally aacepted to contain four Fe and four S^2- which can be extruted as a single [4Fe-4S] cluster.The role of the [4Fe-4s] is assumed to be that of electron transfer to the MoFe protein.

Natural Methods Of the fixation of nitrogen

i)By lightning discharge

The nitrogen and the oxygen,present in the air,combine together to give nitric oxide,under the influence of the lightning discharge.Nitric oxide is oxidised by excess of oxygen present in the atmosphere to give N₂O₅,which further combines with water to give nitric acid.This acid is washed down by the rain into the soil,where it reacts with limestone and alkalies of the soil to give nitrates and is stored there as plant-food.

$$N_2+O_2 \xrightarrow[]{lightning} 2NO$$

$$4NO+3O_2 \rightarrow 2N_2O_5$$

$$N_2O_5+H_2O \rightarrow 2HNO_3$$

$$2HNO_3+CaCO_3 \rightarrow Ca(NO_3)_2+CO_2+H_2O$$

A few species of bacteria and cyanobacteria can 'fix' atmospheric dinitrogen,that is can convert N₂ gas into combined forms.These bacteria can have a great effect on the fertility of the soil by producing combined nitrogen.the most important nitrogen fixing genus of bacteria is called Rhizobium.It lives symbiotically in the nodules on the roots of the plants in the family Leguminosae,e.g.peas,beans,clover and alder trees.Other bacteria exist in the soil near roots, and are also able to fix dinitrogen,but in the smaller amounts.Though plaants require nitrates,bacteria in soil will readily convert other nitrogenous compound into nitrates.

$$NH_4^+ \xrightarrow[]{Nitrosomonas and nitrobacter} NO_2^- \xrightarrow[]{Nitrobacter} NO_3^-$$

Artificial Methods Used For The Fixation Of Nitrogen

i)Fixation of nitrogen as NH₃(Haber-Bosch process)

$$N_2+3H_2 \rightleftharpoons 2NH_3+Heat$$

ii)Fixation of nitrogen as calcium cyanamide

When nitrogen gas,obtained by the evaporation of liqquid air,is passed over calcium carbide,heated to 1100°C,the two combine to form a mixture of calcium cyanamide and carbon,which is extensively used as a fertilizer under the name of

nitrolim.

$$CaC_2+N_2 \xrightarrow[]{1100°C} [CaCN_2+C]Nitrolim$$

$$CaCN_2+5H_2O \rightarrow CaCO_3+2NH_4OH$$

iii)By using transition metal complexes catalysed system

There is considerable research interest into finding transition metak catalysed system which will absorb dinitrogen and produce ammonia for fertilizers cheaply and without the necessity for high temperatue on pressure.The first dinitrogen complex,the pentaamimne(dinitrogen) ruthenium cation,was made in 1965 by reducing ruthenium trichloride with thw hydrazine.Other methods have now been found,e.g. replacement of a labile ligand ina complex by N₂.Dinitrogen complexs have been made for allmost all the transition elements.

$$[Ru(NH_3)_5H_2O]^2++N_2 \xrightarrow[]{aqueous solution} [Ru(NH_3)_5N_2]^2++$$

The formation of this stable dinitrogen complex led to studies with other metals.Complexes with titanium (II) arre the most promising, and reduction of titanium alkoxides yields either ammonia or hydrazine.A complete cycle of reactions for fixing atmospheric dinitrogen to ammonia or hydrazine is given below:

$$Ti(OR)_4+2e^- \xrightarrow[]{2Na} Ti(OR)_2+2NaOR$$

$$Ti(OR)_2+N_2 \rightarrow [Ti(OR)_2N_2]$$

$$[Ti(OR)_2]+4e^- \xrightarrow[]{reduction} [Ti(OR)_2N_2]^4-$$

$$[Ti(OR)_2N_2]^4-+4H^+ \rightarrow N_2H_4+Ti(OR)_2$$

$$[Ti(OR)_2N_2]^4-+2e^- \rightarrow [Ti(OR)_2N_2]^6-$$

$$[Ti(OR)_2N_2]^6- +6H^+ \rightarrow 2NH_3+Ti(OR)_2$$

Synthetic Fertilizers

Synthetic fertilizers are the chemical compounds obtained by the artificial methods,which are added to the soil to supply to it the essential nutrients that is nitrogen,phosphorus and potassium,so that thebsoil may become more fertile.Plant fertilizers normally contain three main ingredients:Nitrogen,phosphorus and potassium(N,P and K)

i)Nitrogen

Nitrogen in a combined forms commonly as ammonium nitrate,other ammonium salts or nitrates,or as urea,are used in plant fertilizer.Nitrogen is essential for plant growth,particularly of leaves,since it is a constituent of amino acid and proteins,which must be made to make new cells.The nitrogen fertilizer is necessary for the plant for early growth to promote stem,leaves.The most common nitrogen fertilizers are as follows:

• Natural organics
• Animal and human waste
• Mineral nitrogen
• Salt petre(NaNO₃)
• Anhydrous NH₃
• Aqua NH₃
• Ammonium nitrate
• Urea
• Ammonium Sulphate

ii)Phosphorus

Phosphorus is used in plant fertilizers in the form of phosphate such as'superphosphate' or 'triple superphosphate'.Basic slag,which is a by-product from the steel industry,is also used as a phosphate fertilizer.the phosphorus stimulate early root formation and rapid growth of roots.It hepls seed formation.It helps in the formation of high-yielding seeds.The most common phosphorus fertilizers are as follows:

• Natural organic phosphate fertilizer
• Phosphate rocks
• Ordinary superphosphate(OSP)
• Triple superphosphate(TSO)
• Nitro phosphate
• Diammonium phosphate(DAP)
• Monoammonium phosphate(MAP)

iii)Potassium

Potassium is used in the plant fertilizers in the form of potassium sulphate,potassium nitrate and muriate of potash(KCl).Potassium ions are essential for flowering.It retain water in the palnt cell by osmotic pressure.It provides healthy root system to the plant.With the help of this system,the plants become capable of getting a regular supply of nutrients from the soil.It is essential for the development of starch in poattoes and grains,the sugars of fruits and vegetables,fibrous material for plants,help tto prevent disease and minimize the effect of excessive nitrogen application.The most common potash fertilizers are as follows:

• Saltpetre(KNO₃)
• KCl(muriate of potash)
• Sylvinite(mixture of NaCl and KCl)
• Potassium magnesium minerals
• Kainite(KCl.MgSO₄.3H₂O)
• Longnite(K₂SO₄.2MgSO₄)
• Potassium sulphate(Sulphate of potash or SOP)
• Carnilite(KCl.MgCl₂.6H₂O)