Nitrogen is the most available gas in the atmosphere. It
accounts roughly for 70% atmosphere. Nitrogen is found as nitrogen gas which is
inert form. So it needs large amount of energy to break the bond between the two
nitrogen atoms. This atmospheric nitrogen fix into nitrate via two methods such
as atmospheric fixation and biological fixation.
In
atmospheric fixation of nitrogen, lighting
releases a considerable amount of energy that is enough for the breakdown of
the bond of nitrogen gas. Then nitrogen atoms react with water in the rain and
convert into nitrate ions. These nitrate ions reach to the soil through the
rain.
Certain
bacteria those inhibit in the soil involve in the biological fixation of nitrogen into ammonium ions. Some nitrogen
fixing bacteria are free living. Ex: Azotobactor,
Bacillus polymyxa. Other nitrogen
fixing bacteria form symbiotic relationship with legume plants. Ex: Rhizobium sp. They fix atmospheric
nitrogen into nodes on the roots of legume plant. Those nitrogen are available
to the plant when roots are dead or they add nitrogen directly when legume
plant are buried or grown as an intercrop. Cyanobacteria are another group that
is capable of fixing atmospheric nitrogen. They also form symbiotic
relationship with roots of certain plant such as Cycad, Azolla and
liverworts. Among them, Anabena-Azolla
relationship is more efficient than legume plants in nitrogen fixation so
farmers use Anabena-Azolla in their
rice paddies in order to increase their yield.
Decomposition of nitrogenous organic matter
releases ammonium ion to the soil. Organic matter contains protein those are
rich in ammonium compounds. Leaf litter,
root decomposition and animal excretions make the soil fertile via release of
ammonium.
Then
ammonium ions in the soil are used as the substrate by ammonium oxidizing
bacteria such as Nitrosomonas in the
presence of oxygen. They oxidize this ammonium into nitrite ions. They do this
oxidation to take electron to run the electron transport chain to ATP
syntheses.
Those
nitrite ions are used as substrate by nitrite oxidizing bacteria such as Nitrobactor
in the presence of oxygen. They oxidize nitrite into nitrate which the
preferable form of plant to be absorbed. They do this oxidation to take
electron to run the
electron transport chain to synthesis ATP.
The oxidation or conversion of
ammonium ions into nitrate ions refer to as the nitrification. Nitrification helps to increase the soil fertility. Aeration
is the only accelerating factor that is responsible for the nitrification.
Tilling helps to aeration of the soil.
In
the absence of oxygen, certain facultative anaerobic microorganisms in the soil
such Thiobacillus
dentrificans use these nitrate ions as terminal electron acceptor in
their anaerobic respiration. They reduce nitrite into nitrogen gas which expel
from the soil into atmosphere. The process that convert nitrate into nitrogen
gas is called as denitrification.
Denitrification makes the soil infertile. But as Thiobacillus dentrificans is facultative anaerobe, they shift into
aerobic respiration when soil is aerated. Therefore aeration of soil helps to
stop the denitrification of soil.
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