The nitrogen cycle describes the conversion of nitrogen between its various chemical forms and as it circulates between terrestrial and marine ecosystems. Nitrogen exists in the atmosphere as N2 gas and is converted to ammonia or nitrates through nitrogen fixation, which occurs through lightning, bacteria, and industrial fertilizer production. Plants and animals assimilate fixed nitrogen and it is returned to the soil through decomposition, where it is converted to ammonium through ammonification and then to nitrites and nitrates through nitrification. Some nitrates are taken up by plants while others enter oceans, and nitrates are ultimately converted back to N2 gas through de-nitrification, completing the cycle.

1. NITROGEN CYCLE
Submitted by: Syed Usman Abdulah
Roll No: 2712 (148077)
Department: BS-Zoology
Semester: 6th (Evening)

2. Nitrogen Cycle
The nitrogen cycle is the biogeochemical
cycle by which nitrogen is converted into
multiple chemical forms as it circulates
among atmosphere, terrestrial and marine
ecosystem.

3. Nitrogen Cycle Fig.

4. Importance of Nitrogen
 Nitrogen is an essential nutrient for the
production of amino acids, proteins, nucleic
acids, etc., and stone fruit trees require an
adequate annual supply for proper growth
and productivity.
 Nitrogen is so vital because it is a major
component of chlorophyll, the compound
by which plants use sunlight energy to
produce sugars from water and carbon
dioxide (i.e., photosynthesis).

5. Different forms of Nitrogen
 In the atmosphere, nitrogen exists as a gas
(N2),
 In the soils it exists as nitrogen oxide, NO,
and nitrogen dioxide, NO2,
 When nitrogen is used as a fertilizer, can
be found in other forms, such as ammonia,
NH3,

6. Reservoir of Nitrogen
 The major reservoir of nitrogen is the
atmosphere, where the nitrogen exists.
 Atmosphere contains 78.9% nitrogen by
volume.
 Nitrogen is also stored in: watershed in
soil, groundwater, ocean water, sediment
and plant matter (dead and living).

7. Stages of Nitrogen Cycle
Process of Nitrogen Cycle consists of the
following steps –
1. Nitrogen fixation (N2 to NH3/ NH4+ or
NO3)
2. Assimilation (Incorporation of NH3 and
NO3 into biological tissues)
3. Ammonification (Organic nitrogen
compound to NH3)
4. Nitrification (NH3 to NO3)
5. De-nitrification (NO3- to N2)

8. 1.Nitrogen Fixation
 The conversion of free or gaseous nitrogen
into nitrate compounds or ammonia is
called Nitrogen Fixation.
 These are three principal ways in which
nitrogen fixation can occur:
A. Atmospheric fixation
B. Biological fixation
C. Industrial fixation

9. A. Atmospheric fixation:
o A natural phenomenon where the energy of
lightning breaks the nitrogen into nitrates or
nitric acid.
o These are dissolved in rain and carried to
earth.
o Plants acquire these when they absorb
water and other minerals through their
roots.
o It has been estimated that about 10% of
nitrates are added to sail in this manner.

10. B. Biological fixation:
The second method, which contributes 90% of fixed
nitrogen to earth, is biological. It is accomplished by
either,
i. Free living bacteria in the soil: Among the
free living bacteria responsible for nitrogen
fixation are azobacter, Clostridium, Derxia and
so on. These bacteria take up nitrogen from
atmosphere and form ammonia.
ii. Free living blue green algae: In aquatic
systems Cynobacteria like, Nostoc, Anabena
are important nitrogen fixer.
iii. Symbiotic bacteria: Symbiotic bacteria fix far
greater amount of atmosheric nitrogen than do
any other organsim. E.g Rhizobium

11. C. Industrial fixation:
Today large quantities of fixed nitrogen are
artificially added to the agriculture soil in the
form of fertilizers. The production of nitrogen
fertilizers is achieved in large factories by
combining nitrogen and hydrogen in the
presence of heat (about 600° 𝑐 ) and high
pressure. This process has increased the
fertility of soil but requires large amounts of
energy.

12. 2.Assimilation
 Plants can absorb nitrate or ammonium
from the soil by their root hairs. If nitrate is
absorbed, it is first reduced to nitrite ions
and then ammonium ions for incorporation
into amino acids, nucleic acids, and
chlorophyll.
 The plants may be eaten by animals that
turn use the amino acids from the plants
proteins in synthesizing their own
protoplasm.

13. 3.Ammonification
 When plants or animal die, the nitrogen present in
the organic matter is released back into the soil.
The decomposers, namely bacteria or fungi
present in the soil, convert the organic matter
back into ammonium. This process is known as
Ammonification.
 The bacteria responsible for ammonification are
mostly Actinomycetes and species of Bacillus.
 The ammonia so formed is either released to
atmosphere or retained in the soil to be absorbed
by plants or converted into nitrate compounds.

14. 4.Nitification
 Most of the ammonia is converted into nitrate
compounds by a process called Nitrification.
 The Nitrification is completed into two steps,
I. Firstly the ammonia is converted into nitrites (NO2).
The first phase is completed by bacteria which
collectively known as Nitrosofying bacteria.
II. In the second phase the nitrites are converted into
nitrates (NO3). Second phase is completed by
nitrifying bacteria. E.g Nitrocystis,
 Some nitrates are reabsorbed by plants while
others are carried in streams and rivers to the
oceans, where they become part of the dead sea
sediments.

15. 5.De-Nitrification
 Denitrification is the reduction of nitrates
back into nitrogen gas (N2), completing the
nitrogen cycle.
 This process is performed by bacterial
species such as Pseudomonas, under
aerobic condition.
 They use the nitrate as an electron acceptor
in the place of oxygen during respiration.

16. Thank You