COMPLETE NITROGEN CYCLE: MICROBIAL TRANSFORMATIONS OF NITROGEN

MICROBIAL TRANSFORMATIONS OF NITROGEN

 

Biological availability of N, P and K is of considerable economic importance, since they are the major plant nutrients derived from the soil. Of the three, N stands out as the most susceptible one to microbial transformations. This element is the key building block of the protein molecule upon which all life is based on, it is an indispensable component of the protoplasm of plants, animals and micro organism.

 

Molecular N2 constitutes about 78% of the earth’s atmosphere but it is chemically inert and cannot be utilized by more living organism, plant animals and micro organism therefore depend on a source of combined N such as ammonia, nitrate or organic N compounds for their growth.

 

Nitrogen undergoes a number of transformations that involve organic, inorganic and volatile forms of nitrogen. A small part of the large reservoir of N2 in the atmosphere is converted to organic compounds by certain free living micro organism or by plant microbe association that makes the element available to plant growth. The nitrogen present in the proteins or nucleic acids of plant tissue is used by animals. In the animal body, the N is converted to other simple and complex compounds. Upon the death, plants and animals undergo microbial decay and organic N is released as ammonium, which is then utilized by vegetation or is oxidized to nitrate by microorganisms. The nitrate from of N is mostly used by the plants or may be lost by bacteria reduced to gaseous N2, which escapes to atmosphere, there by completing the cycle. The Nitrogen cycle mainly includes transformations such as:-

1. Nitrogen Mineralization:- In which N containing organic complexes are decomposed and converted into inorganic compounds for use by plants.

 

2. N Immobilization:- In which N containing inorganic compounds are assimilated.

 

N2 is acted on by certain micro organism sometimes in symbiosis with a higher plant, which can use it is as a N source for growth. The process of nitrogen fixation, results in the accumulation of new organic compounds in the cells of responsible micro organisms. The N2 thus fixed reenters general circulation when the newly formed cells are in turn mineralized.

 

Nitrogen Cycle:-

I. Nitrogen Mineralization:- The conversion of organic N to the more mobile, inorganic state is known as nitrogen mineralization. As a consequence of mineralization, ammonium and nitrate are generated and organic N disappears. This takes place in two distinct microbiological steps:-

1. Ammonification:- It is the process of mineralization in which proteins, nucleic acids and other organic components are degraded by micro organism with the eventual liberation of ammonia. This is called ammonification. A part of the liberated ammonia is assimilated by the micro organism themselves. The first step in ammonication process is the hydrolysis of proteins, nucleic acids and other organic nitrogenous compounds into amino acids (proteolysis). The amino compounds are then deaminated to yield ammonia.

Ammonification usually occurs under aerobic conditions while under anerobic conditions protein decomposition leads to conversion of ammonia into amines and related compounds (eg) clostridium. The anaerobic decomposition of protein called as putrefaction. These amines are subsequently oxidized in the presence of O2 to release ammonia. Break down of nitrogenous substance is brought about by the activity of a multitude of microbial species. Almost all bacteria, actinomycetes and fungi can bring about proteolysis and the amino acids produced are utilized for the growth of these organisms.

 

(2) Nitrification:- The biological oxidation of ammonium salts (in soil) to nitrites and the subsequent oxidation of nitrites to nitrates is called as nitrification. i.e. the biological convention of N in soil from a reduced to a more oxidized state, called nitrification.

Nitrification occurs in two steps;

 

First ammonia is oxidized to nitrite.

 

             2 NH3 + 1½ H2O2 → NO2 - + 2H+H2O – Nitrosofication

 

This change is brought about by chemoautotrophic bacteria of the genera Nitrosomonas, Nitrosolobus, Nitrosococus, Nitrosospira. These bacteria obtain their energy requirement by the oxidation of NH⁴⁺ to NO^2- Among the nitrifiers Nitrosomonas are most important in soils.

 

Organisms:- Nitrobacter, Aspergillus, Penicillium, Cephalosporium.

 

3. Denitrification:- The convention of nitrate and nitrite into molecular N2 or nitrous oxide through microbial processes is known as denitrification. Certain bacteria are capable of using nitrate as the terminal electron acceptor under anaerobic conditions. This is called nitrate respiration. As a consequence of nitrate respiration, NO3 is reduced to N2 gas or nitrous oxide. Denitirifcation leads to the loss of N from the soil. It depletes N, and therefore it is not a desirable reaction.

 

The escape of molecular N into the atmosphere is also known as volatalization.

Denitirfication occur mostly in waterlogged anaerobic soils with a high organic matter contents. Denitrification of bound nitrogen to gaseous N is mediated by numerous species of bacteria, which normally use O2 as hydrogen acceptor (aerobically) and, also use nitrates and nitrites (anerobically).

Anaerbic convertion of nitrate into molecular nitrogen is known as nitrate respiration. Bacterial genera which bring about denitirfication Pseudomonas, Achromobacter, Bacillus, Micrococcus.

 

                                2NO₃ +10 H → N₂ + 4H₂O+ 2OH^-

                              (or) 2NO₂ +6 H → N₂ +2H₂O +2OH^-

                                    (or) N₂O + 2H → N₂ + H₂O

 

Many soil bacteria like. Thiobacillus denitrificans Oxidize S (chemoautotrophically) and also reduce nitrate to nitrogen

 

                  5S + 6 KNO₃ + 2 H₂O → 3N₂ + K₂SO₄ + 4KHSO₄

             (or) 5 K₂S₂O₃ + 8 KNO₃ + H₂O → 4N₂ + 9 K₂SO₄ + H₂SO₄

 

General pathway of denitrification Nitrate is first reduced to nitrite, which is then transformed to nitrous oxide (NO). The nitrous oxide is converted to N2 with N2O as an intermediate.

 

                         HNO3 → 2HNO2 → 2 NO → N2O → N2

 

The enzymes involved in this process are :-

1. Nitrate reductase

2. Nitric oxide reductase

3. Nitrite reductase

4. Nitrous oxide reductase

 

Fallow soils flooded with water are more congenial for denitrification than well drained and continuously cropped soils. Though it is a undesirable reaction in point of view of plant nutrition, but have ecological importance. Because with out denitrification the supply of N on the earth world have got depleted and NO₃ would have accumulated. High concentration of NO₃ are toxic, denitrification is a mechanism by which some of the N is released back to the atmosphere.

 

5. Nitrate Reduction:- The reverse of nitrification process. That is the reduction of nitrate to nitrite and then ammonia. Since organisms are able to obtain cellular Nth ammonia assimilation, the process is called as assimilatory nitrate reduction.

 

                               HNO₃ + 4H₂ → NH₃ + 3H₂O

 

II. Nitrogen Immobilization:- The process of microbial assimilation of inorganic nitrogen is referred as immobilization. In contrast to mineralization, microbial immobilization leads to the biosynthesis of the complex molecules of microbial protoplasm from ammonium and nitrate. Immobilization results in a marked depression of nitrogen uptake by the plant. The mineralization of organic N and the microbial assimilation of inorganic ions proceeds simultaneously.

Both mineralization and immobilization take place regardless of the % of N in the organic N in organic matter. On the death of micro organism, the immobilized N is however released through mineralization. It is also a loss of nitrogen. NO3 when accumulated in microbial protoplasm it is referred as assimilatory NO3 reduction.

Thank You

Vikas Kashyap:)