1. Aminization Ammonification Nitrification
Anaerobic
Mediated by heterotrophic bacteria
Aerobic
Mediated by autotrophic bacteria
Mineralisation of Organic Nitrogen :
Mineralisation – conversion of an element from an immobilized form to
an available form as a result of microbial decomposition.
2. 1. Aminization :
• Hydrolytic decomposition of protein and release of amines and amino acids by
heterotrophs (bacteria like Bacillus, Pseudomonas) in absence of O2 .
O.M.---------------> Polypeptides
Polypeptides/Proteins --------- > R-NH2 +CO2 + Energy + other compounds.
hydrolysis
3. 2. Ammonification :
• The amines and amino acids so released are further utilised by other hetrotrophs by
release of ammonical compounds in absence of O2 .
R-NH2 + HOH --------> NH3 + R-OH + Energy
2NH3 + H2CO3 --------> (NH4)2CO3 2NH+
4 + CO3
2-
NH3 so released are subjected to several fates—
(a) may be converted to Nitrites and Nitrates by nitrification.
(b) may be absorbed directly by higher plants.
(c) may be utilized by heterotrophs organisms in the further decom
position of O.M.
4. (d) may be fixed in a biologically unavailable form (in subsoil about 40-50% and in top
soils 6 %) in the lattice of certain expanding type of clay-minerals like
momtmorillonite , illite and vermiculite.
3. Nitrification :
• By autotrophs in presence of O2 i.e. biological oxidation. According to Winogradsky
(1889), NH3 or NH+
4 salt in the soil is oxidised into NO-
3 in two steps.
(i) NH3 / NH4
+ ------------------------> NO2
- [Nitrite]
Nitrosomonas is Obligate parasite.
(ii) NO2
- ---------------------> NO3
- [Nitrate]
[O]
Nitrosomonas
Nitrobacter
[O]
5. Opt. temperature for Nitrifying bacteria is 30-35° C and Opt. pH 6 .5-7.5. The
activity of bacteria ceases below pH 5.0.
Nitrobacteria = Nitrosomonas + Nitrobacter both.
Reactions:
2 NH 4
+ + 3O2 ---------------> 2NO2
-+ 2H2O + 4H+ + Energy .
2 NO 2
-+ O2 —---------> 2 NO3
- + energy.
When NH 4
+ - Fertilizers are added to soil, three important inferences are drawn by
these nitrification equations –
(i) Reactions require molecular oxygen means nitrification takes place most readily in
well aerated soil.
(ii) Reactions release H+ which results in acidification of soil.
(iii) Microbial activity hence their rapidity and extent of transformation will be greatly
influenced by soil environmental conditions such as moisture supply, temp etc.
6. Factors affecting Nitrification :
(i) Supply of NH 4
+
(ii) Population of nitrifying organisms
(iii) Soil reactions
(iv) Soil aeration
(v) Soil moisture
(vi) Soil temp. (30-35° C)
(vii) C : N ratio -
<20 favours mineralization (release of NH3 & NO3
- )
>30 immobilization
15-30 or 20-30 - both
(viii) Fertility.
