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Lesson Note
Subject: Biology
Topic: Nutrients Cycling in Nature
Subtopic: Nitrogen Cycle and Decomposition in Nature

Lesson Objective: by the end of the lesson, the learners should be able to:

  1. Define the terms: Nitrogen Cycle and Decomposers;
  2. Mention ways by which Nitrogen is added to and removed from the soil;
  3. State the differences between nitrogen fixation and denitrification;
  4. Mention the two types of decomposers;
  5. Explain the process of decomposition;
  6. State the roles of decomposers in ecosystem.

Lesson Summary / Discussions


Nitrogen cycle involves the complex process by which nitrogen is naturally added and removed from the soil. It is a sequence of reaction indicating the various means by which nitrogen is added to and removed from the
atmosphere and the soil.


  1. Through nitrogen fixation
  2. Through thunder storms and lightening.
  3. By putrefaction/decay of organic matter
  4. By nutrification.
  5. By application of artificial fertilizers.
  6. By application of farmyard manure organic fertilizers/poultry, droppings, animal dungs/green manure.

Nitrogen Fixation: Nitrogen fixation is the process by which free atmospheric nitrogen is converted to nitrate in the soil either by thunderstorm or bacteria in the root noddles of legumes or soil.
Nitrogen fixation process involves soil organisms which add reasonable amount of nitrogen to the soil.

Nitrogen cycle involves the following Processes:
1. Symbiotic nitrogen fixation: Some
bacteria such as rhizobium leguminosarium which live in the root nodules of leguminous plants can fix atmospheric nitrogen directly into the plants. the plant supplies carbohydrate for use by the bacteria while the bacteria supply the plant with combined nitrogen.

2. Electrical discharge: Nitrogen can also be fixed into the soil during lightening and thunderstorm. Nitrogen in the air combines with oxygen to form nitric oxide or nitrogen (ii) oxde which further undergoes oxidation to form nitrogen dioxide or nitrogen (iv) oxide. The nitrogen (iv) oxide formed will dissolve in rain water to form nitrous (HNO2) and nitric acids (HNO3) which later dissociates to form nitrate in the soil.

3. Non-symbiotic nitrogen fixation

Some bacteria such as Azotobacter and Clostridium also live freely in the soil and can fix atmospheric nitrogen into the soil either aerobically or anaerobically.

4. Ammonification and nitrification: The process involving the formation of ammonium compounds from the dead and decaying of plants and animals and their waste products like urine and faeces is called ammonification. A further reaction known as nitrification involves the conversion of ammonium compounds first into nitrite by nitrifying bacteria called nitrosomonas. These nitrites are converted by oxidation to nitrates by another bacteria called nitrobacter. Plants can only absorb nitrates from the soil.

5. Putrefaction: Putrefaction is a process by which agents of decay or saprophytes or putrifying bacteria or fungi decompose or breakdown dead organic remains or waste products of other organisms into ammonium
compounds or nitrates.

Ways by which Nitrogen is lost from the soil

1. Action f denitrifying bacteria: Denitrification is the process
which involves the conversion of nitrate to nitrogen gas by certain bacteria. The nitrogen gas so formed can escape into the air.
Note: Denitrification is the only major stage in which nitrogen can be lost from the soil while other stages involve the fixing of nitrogen into the soil.
2. Absorption by plant roots.
3. By action of leaching.

Differences Between Nitrogen Fixation And Denitrification

Meaning: Decomposers are fungi or bacteria which live saprophytically or feed on dead remains of plants, animals and organisms leading
to or resulting in recycling of nutrients by breaking down organic matters to produce soluble nutrients which are absorbed by plants.
These are organisms, mainly bacteria and saprophytes responsible for the breaking down of dead organic materials which could be of plants or animals origin. In other words,
decomposers cause the remains of plants and animals to decay. These decomposers are grouped into two classes, namely:

1. Micro-decomposers: These are small or microscopic organisms that can cause decay, e.g. certain bacteria and fungi.
2. Macro-decomposers: These are bigger organisms that can cause decay of dead organic materials, e.g. earthworms, termites, snails, mushroom and toad stools.

Process of Decomposition
The decomposers secrete enzymes onto their food source such as a decaying plant. These enzymes break down complex organic compounds (food) like carbohydrates and proteins into simple soluble inorganic
compounds. A lot of the chemical energy in the organic compounds is lost as unstable heat energy. The decomposers only absorb a small
amount of nutrients and energy for their use. The rest is released into the soil, air and water.
When decomposers die, other decomposers feed on them. The nutrient released are used by plants to manufacture their food.
Products released during decomposition are gases such as carbon dioxide, hydrogen sulphide, ammonia and water vapour. Others
are heat energy and nutrients such as nitrates, sulphates and phosphate ions.

Roles of Decomposers in Ecosystem
Decomposers play major roles in the ecosystem in the following ways

  1. They enrich the soil with nutrients required for plant growth.
  2. They contribute to environmental pollution.
  3. Decomposition is useful in the making of cheese and yoghurt.
  4. They allow the ecosystem to function by enabling the recycling of nutrients.
  5. They also prevent an unsightly accumulation of remains and wastes of living organisms on earth surface.

See previous lessons in Biology

Lesson Evaluation / Test

  1. What is Nitrogen Cycle?
  2. Define Decomposers
  3. Mention ways by which Nitrogen is added to and removed from the soil.
  4. What are the differences between nitrogen fixation and denitrification?
  5. Mention the two types of decomposers.
  6. Explain the process of decomposition.
  7. What are the roles played by decomposers in ecosystem?

Questions answered correctly? Bravo!

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