Nitrogen fixation, nitrogen cycle, role of Rhizobium

1. Introduction

1. Nitrogen (N) is an essential macronutrient required for plant growth.
2. It is a key component of proteins, nucleic acids (DNA & RNA), chlorophyll, and ATP.
3. Although nitrogen makes up 78% of the Earth's atmosphere, plants cannot absorb it in gaseous form.
4. The nitrogen cycle helps convert atmospheric nitrogen into usable forms.
5. Biological nitrogen fixation is performed by certain bacteria like Rhizobium.

2. Nitrogen Fixation

1. Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃).
2. It can be carried out by biological, industrial, and atmospheric processes.

(a) Biological Nitrogen Fixation

1. Carried out by nitrogen-fixing bacteria.
2. Some bacteria live freely in the soil (e.g., Azotobacter, Clostridium).
3. Some form a symbiotic relationship with leguminous plants (e.g., Rhizobium in root nodules of peas and beans).
4. Rhizobium bacteria convert atmospheric nitrogen into ammonia (NH₃), which is further converted into plant-usable forms.

(b) Industrial Nitrogen Fixation

1. Involves the Haber-Bosch process to produce ammonia artificially.
2. Used in fertilizer production (e.g., ammonium nitrate, urea).

(c) Atmospheric Nitrogen Fixation

1. Occurs naturally due to lightning and UV radiation.
2. High energy from lightning breaks N₂ bonds, forming nitrogen oxides (NO, NO₂) which dissolve in rainwater to form nitrates (NO₃⁻).

3. Nitrogen Cycle

1. The nitrogen cycle is the process by which nitrogen is converted into different chemical forms.
2. It consists of five main steps: Nitrogen fixation, Nitrification, Assimilation, Ammonification, and Denitrification.

(a) Nitrogen Fixation

1. Conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) by bacteria.

(b) Nitrification

1. Conversion of ammonia (NH₃) into nitrates (NO₃⁻) by soil bacteria.
2. Step 1: Ammonia is converted into nitrites (NO₂⁻) by Nitrosomonas.
3. Step 2: Nitrites are converted into nitrates (NO₃⁻) by Nitrobacter.

(c) Assimilation

1. Plants absorb nitrates (NO₃⁻) and ammonium ions (NH₄⁺) from the soil.
2. These compounds are used to synthesize proteins, enzymes, and chlorophyll.

(d) Ammonification

1. Decomposers break down organic matter (dead plants and animals) into ammonia (NH₃) and ammonium (NH₄⁺).
2. This process is carried out by bacteria like Bacillus and Pseudomonas.

(e) Denitrification

1. Conversion of nitrates (NO₃⁻) back into atmospheric nitrogen (N₂) by denitrifying bacteria (e.g., Pseudomonas, Clostridium).
2. Occurs in anaerobic conditions like waterlogged soils.

4. Role of Rhizobium in Nitrogen Fixation

1. Rhizobium is a symbiotic nitrogen-fixing bacterium.
2. Forms a mutualistic association with leguminous plants (e.g., peas, beans, soybeans).
3. Infects the roots and forms root nodules where nitrogen fixation occurs.
4. Contains the enzyme nitrogenase, which converts N₂ into NH₃.
5. Leghemoglobin (a pigment in nodules) protects nitrogenase by maintaining a low oxygen environment.

5. Importance of Nitrogen Fixation

1. Provides plants with a usable form of nitrogen for growth.
2. Reduces dependence on chemical fertilizers.
3. Improves soil fertility naturally.
4. Essential for the synthesis of proteins, chlorophyll, and nucleic acids.

6. Conclusion

1. The nitrogen cycle plays a crucial role in maintaining ecosystem balance.
2. Biological nitrogen fixation by bacteria like Rhizobium helps plants obtain nitrogen naturally.
3. Understanding nitrogen fixation can help improve agricultural productivity and reduce environmental pollution.