The nitrogen cycle describes how nitrogen is converted between its various chemical forms and circulates between the atmosphere, soil, water, and living organisms. The majority of the Earth's atmosphere consists of nitrogen gas (N2), which cannot be used by most organisms. Nitrogen-fixing bacteria form symbiotic relationships with legume plants and convert atmospheric nitrogen into ammonium (NH4+) through a process called nitrogen fixation. Nitrifying bacteria further convert ammonium into nitrites (NO2-) and nitrates (NO3-), which can be absorbed by plants as nutrients. Human activities like burning fossil fuels and using nitrogen fertilizers also contribute to nitrogen fixation.

1. From : DobianSwabi
S.M.Owais
ShahRoll No : 490

2. The Nitrogen Cycle

3. • Nitrogen (N) is an
essential component
of DNA, RNA, and
proteins,
The building blocks of
life.
• All organisms require
nitrogen to live and
grow.
Nitrogen

4. The majority (78%) of the Earth’s
atmosphere is N2.

7. Nitrogen Fixation (N2 --> NH3 or NH4
+)
HOW?
Biological or
Environmental or Humans
BIOLOGICAL:
Bacteria (called Nitrogen-
fixing bacteria)
These bacteria form
symbiotic relationships with
host plants.
The bacteria live in nodules
found in the roots of the
legume family of plants
(e.g. beans, peas, and
clover)
Nitrogen Fixation (N2 --> NH3 or NH4
+)
HOW?
Biological or
Environmental or Humans
BIOLOGICAL:
Bacteria (called Nitrogen-
fixing bacteria)
These bacteria form
symbiotic relationships with
host plants.
The bacteria live in nodules
found in the roots of the
legume family of plants
(e.g. beans, peas, and
clover)

8. Nitrogen Fixation (N2 --> NH3 or NH4
+)
ENVIRONMENTAL
High-energy natural events which
break the bond N2
Examples:
lightning
forest fires
hot lava flows

9. Nitrogen Fixation N2 --> NH3 or NH4
+
How?
HUMAN IMPACT
• Burning fossil fuels,
• using synthetic nitrogen
fertilizers,
• and cultivation of legumes
all fix nitrogen.

10. Nitrogen Mineralization
also called Ammonification
Organic N --> NH4
+
• Decay of dead things,
manure, etc.
• Done by decomposers
(bacteria, fungi, etc.)
• During this process, a
significant amount of the
nitrogen contained within the
dead organism is converted
to ammonium (NH4
+).

11. Nitrification
NH3 or NH4
+ --> NO2
- --> NO3
-
(Nitrifying) Bacteria add oxygen to nitrogen.
Two steps:
• STEP 1:
Bacteria take
NH3 or NH4
+ & make NO2
- = nitrite
• Step 2:
• Bacteria take in NO2
-
• & make NO3
- = nitrate

14. • Yellow arrows indicate human sources of nitrogen to the environment. Red
arrows indicate bacterial transformations of nitrogen. Blue arrows indicate
physical forces acting on nitrogen. And green arrows indicate natural, non-
microbial processes affecting the form and fate of nitrogen.

15. • References
Burford, J.R., and J.M. Bremner. 1975. Relationships between the
nitrification capacities of soils and total, water-soluble and readily
decomposable soil organic matter. Soil Biochem. 7:389-394.
• Marschner, H. 1995. Mineral Nutrition in Higher Plants. Academic
Press, London.
• Tisdale, S.L., W.L. Nelson, J.D. Beaton, and J.L. Havlin. 1993. Soil
Fertility and Fertilizers. MacMillan Publishing Co., New York, N.Y.