Nitrogen Cycle

1. Nitrogen Cycle
Introduction
Nitrogen is also key in the existence of ecosystems and food chains. Nitrogen forms about 78% of the air
on earth. But plants do not use nitrogen directly from the air. This is because nitrogen itself is unreactive,
and cannot be used by green plants to make protein. Nitrogen gas therefore, needs to be converted into
nitrate compound in the soil by nitrogen-fixing bacteria in soil, root nodules or lightning.
Cycle
Four processes participate in the cycling of nitrogen through the biosphere:
 Nitrogen fixation
 Ammonification/mineralization
 Nitrification
 Denitrification
Nitrogen Fixation
The process of converting N2 into biologically available nitrogen is called nitrogen fixation. N2 gas is a
very stable compound due to the strength of the triple bond between the nitrogen atoms, and it requires a
large amount of energy to break this bond. The whole process requires eight electrons and at least sixteen
ATP molecules (Figure 2). As a result, only a select group of prokaryotes are able to carry out this
energetically demanding process. Although most nitrogen fixation is carried out by prokaryotes, some
nitrogen can be fixed abiotically by lightning or certain industrial processes, including the combustion of
fossil fuels.
Three processes are responsible for most of the nitrogen fixation in the biosphere:
 Atmospheric fixation by lightning
 industrial fixation
 Biological fixation by certain microbes — alone or in a symbiotic relationship with some plants
and animals
Atmospheric Fixation:
The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine
with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to
the earth. Atmospheric nitrogen fixation probably contributes some 5– 8% of the total nitrogen fixed.
Industrial Fixation:
Under great pressure, at a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen
and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH3).
Ammonia can be used directly as fertilizer, but most of its is further processed to urea and ammonium
nitrate (NH4NO3).

2. Biological Fixation:
The ability to fix nitrogen is found only in certain bacteria and archaea.
 Some live in a symbiotic relationship with plants of the legume family (e.g., soybeans, alfalfa).
 Some establish symbiotic relationships with plants other than legumes (e.g., alders).
 Some establish symbiotic relationships with animals, e.g., termites and "shipworms" (wood-
eating bivalves).
 Some nitrogen-fixing bacteria live free in the soil.
 Nitrogen-fixing cyanobacteria are essential to maintaining the fertility of semi-aquatic
environments like rice paddies.
 Biological nitrogen fixation requires a complex set of enzymes and a huge expenditure of ATP.
Ammonification / mineralization:
After nitrogen is incorporated into organic matter, it is often converted back into inorganic nitrogen by
a process called nitrogen mineralization, otherwise known as decay. When organisms die, decomposers
(such as bacteria and fungi) consume the organic matter and lead to the process of decomposition. During
this process, a significant amount of the nitrogen contained within the dead organism is converted to
ammonium. Once in the form of ammonium, nitrogen is available for use by plants or for further
transformation into nitrate (NO3
-
) through the process called nitrification.
Organic N → NH4
+
Nitrification
Some of the ammonium produced by decomposition is converted to nitrate (NO3
-
) via a process
called nitrification. The bacteria that carry out this reaction gain energy from it. Nitrification requires the
presence of oxygen, so nitrification can happen only in oxygen-rich environments like circulating or
flowing waters and the surface layers of soils and sediments. The process of nitrification has some
important consequences. Ammonium ions (NH4
+
) are positively charged and therefore stick (are sorbed)
to negatively charged clay particles and soil organic matter. The positive charge prevents ammonium
nitrogen from being washed out of the soil (or leached) by rainfall. In contrast, the negatively charged
nitrate ion is not held by soil particles and so can be washed out of the soil, leading to decreased soil
fertility and nitrate enrichment of downstream surface and groundwater.
NH4
+
→ NO3
-
Denitrification
Through denitrification, oxidized forms of nitrogen such as nitrate (NO3
-
) and nitrite (NO2
-
) are converted
to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas (NO2). Denitrification is an
anaerobic process that is carried out by denitrifying bacteria, which convert nitrate to dinitrogen in the
following sequence:
NO3
-
→ N2+ N2O
NO3
-
→ NO2
-
→ NO → N2O → N2.

3. Nitric oxide and nitrous oxide are gases that have environmental impacts. Nitric oxide (NO) contributes
to smog, and nitrous oxide (N2O) is an important greenhouse gas, thereby contributing to
global climate change.
Once converted to dinitrogen, nitrogen is unlikely to be reconverted to a biologically available form
because it is a gas and is rapidly lost to the atmosphere. Denitrification is the only nitrogen transformation
that removes nitrogen from ecosystems (essentially irreversibly), and it roughly balances the amount of
nitrogen fixed by the nitrogen fixers described above.