Soil Fertility and Nutrient management (1+1)

N- Transformation and availability in soils

N- Transformation in soils
N- Availability in soil.
Dynamics and transformation of nitrogen in soil is very important with respect to plant nutrition. A bulk of total N is present in the organic form (98%) and only about 2% in inorganic form. However there are continuous transformations between these two pools. The crops utilize nitrogen in the inorganic forms only such as NO3-N and NH4-N. The inorganic form of N is also liable to undergo different types of loses like runoff, ammonia volatilization, leaching, denitrification and fixation by clay minerals.

Nitrogen Cycle in soil
• The cycling of N in the soil – plant – atmosphere system involves many transformations of N between inorganic and organic forms.

N transformations in aerobic soils
In aerobic soils, NO3-N is the dominant form of available N. The mineralization/transformation of added or native organic matter in soil proceeds up to nitrification stage and giving predominantly NO3-N and small amount of NH4-N. There is a quick transformation of NH4+ to NO3-N in the aerobic soils occur which will be utilized by plants.
Any fertilizer containing ammonium nitrogen when added to soil gets dissociated to NH4+ which readily gets oxidized to NO3- ion which is either taken up by the crop or leaches down to the lower horizon as it is readily soluble in water. Some amount of NO3-N is also immobilized by soil microbes during the process of mineralization of organic matter.
The organic form of N, particularly hydrolyzable form is slowly mineralized and is transformed to mineral nitrogen through following process
a. Amminization
b. Ammonification.
c. Nitrification
d. Denitrification

Nitrogen mineralization
Mineralization is the conversation of organic N to inorganic forms of N as a result of microbial decomposition. Mineralization increases with a rise in temperature and is enhanced by adequate, soil moisture and a good supply of O2. Mineralization of organic N involves in two reactions.

Aminization

Aminization is the decomposition of proteins and the release of amines, amino acids and urea. A large number of soil microorganisms bring about this change.

Under aerobic condition the major end products are CO2, (NH4)2 SO4 and H2O. Under anaerobic conditions the end products are ammonia, amides, CO2 and H2S.

The organic compounds and proteins are mainly decomposed by various species of Pseudomonas, Bacilli, clostridium, serrotia, Micrococcus.

Conversion of urea
Urea is a product of aminization. The hydrolysis of urea by the action of urease enzyme is effected by Bacilli micrococcus, Pseudomonas, clostridium, Acromobactor and coryne bactor.
Co (NH₂) + H⁺ + 2H₂O--------------> 2NH₄+ + HCO₃^-
NH₄⁺ ----------------->NH₃ + H⁺
2NH₃ + H₂ CO₃ ----------------------->(NH₄)₂ CO₃ -------------------->NH₄⁺ + CO₃^-
The optimum water holding capacity for these reactions is 50 – 75% and optimum temperature is 30 – 50°C.

Ammonification
“Amines and Amino acids produced during aminization of organic N are decomposed by other heterotrophs with release of NH4+ is termed Ammonifcation

Nitrification
Nitrification is the process of biological oxidation by which the ammonical (NH4+) form of N converts to nitrate (NO3-) form of N. There are two steps.
(a) NH4 is converted first to NO2- and then to NO3-.

Nitrosomonas
2NH₄ + 3O₂ ------------------------->2 NO₂^- + 2 H₂O + 4 H⁺

(b) In the second reaction NO₂^- is further oxidized to NO₃^- by nitrobactor

Nitrobactor
2 NO₂^- + O2 -------------------------->2 NO₃^-.

Nitrosomonas are obligate autotropic bacteria that obtain their energy from the oxidations of N and their C from CO₂.

Denitrification
Denitrification is the biochemical reduction of NO₃^-N or NO₂^-N to gaseous N forms, either as molecular Nitrogen or an oxide of Nitrogen. The most probable bio chemical pathway is

NO₃-----------------> NO₂ ---------------------->NO--------------------- >N₂O-----------------------------> N₂

Nitrate Nitrite Nitrogen Nitrous monoxide Nitrous oxide Dinitrogen

This is loss mechanism of nitrogen happening in anaerobic soil conditions.

Nitrogen immobilization
“Immobilisation is the process of conversion of inorganic N (NH4+ or NO3-) to organic N and it is basically the reverse of N mineralization”. By this process plant available N forms are converted to unavailable organic forms. The Microorganisms accumulate NH4- N and NO3- N in the form of protein, nucleic acid and other complexes. If C:N ratio is wider than 30, it favors immobilization and lesser C:N ratio encourage mineralization.

Losses of Nitrogen
The major losses of N from the soil are due to (1) crop removal and leaching, however under certain conditions inorganic N ions can be converted to gases and lost to the atmosphere. The primary pathway of gaseous N losses are
1. Denitrification
2. NH3 volatilization.

Nitrogen fixation
The conversation of atmospheric nitrogen to plant available forms readily usable by biological process mediated by microorganisms. .

Nitrogen Transformation in anaerobic soils
The N-transformations from added or native sources stops at NH₄⁺ stage, since nitrification is not possible due to lack of oxygen. When NH₄⁺-N containing fertilizers are added to soil, then NH₄⁺ is oxidized to NO₃^- in the top layer (aerobic) of flood water or oxidized layer and later which moves down to reduced layer. On the reduced layer NH₄⁺-N remains as NH₄^-N only for plant uptake. If NO₃^-N exists, then it moves down to the reduced layer, where it undergoes denitrification by bacteria (NO₃-NO₂-N₂). The denitrified NO₃^- is lost from the soil to the atmosphere in the form of N₂O or N₂.
If NH₄⁺ ions present in the flood water it is subjected to volatilization to atmosphere as NH₃ (ammonia gas) because of higher partial presence of CO₂ and high pH value developed due to alkalinity. Therefore, there is an accumulation of NH₄⁺ ions in the reduced layer, which is either absorbed by root or gets oxidized in the rhizosphere to NO₃^- ions and are lost due to dentrification. Some of the NH₄⁺ and NO₃^- ions also get immobilized by the soil microbes & some NH₄⁺ may get fixed by the clay lattice.
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