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UPSC MainsZOOLOGY-PAPER-I201115 Marks150 Words
Q12.

Draw a flow diagram of nitrogen cycle, and give accounts of nitrogen fixation, nitrification and denitrification.

How to Approach

This question requires a combination of diagrammatic representation and explanatory notes. The approach should begin with a clear, labelled flow diagram of the nitrogen cycle. Following this, detailed accounts of nitrogen fixation, nitrification, and denitrification should be provided, explaining the processes, involved microorganisms, and environmental conditions. Focus on biological, chemical, and physical aspects of each process. The answer should demonstrate understanding of the interconnectedness of these processes within the broader ecosystem.

Model Answer

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Introduction

Nitrogen is an essential macronutrient for all living organisms, crucial for the synthesis of proteins, nucleic acids, and other vital biomolecules. However, atmospheric nitrogen (N2) is largely inert and unavailable to most organisms. The nitrogen cycle describes the complex series of transformations through which nitrogen moves between the atmosphere, soil, water, and living organisms. This cycle involves several key processes, including nitrogen fixation, nitrification, and denitrification, each mediated by specific microorganisms. Understanding these processes is vital for comprehending ecosystem functioning and agricultural productivity.

Nitrogen Cycle Flow Diagram

Nitrogen Cycle Diagram

(Note: As I am a text-based AI, I cannot directly display images. The above link points to a standard nitrogen cycle diagram on Wikimedia Commons. In an exam setting, a hand-drawn, clearly labelled diagram would be expected.)

Nitrogen Fixation

Nitrogen fixation is the conversion of atmospheric nitrogen (N2) into ammonia (NH3) or ammonium (NH4+), forms usable by plants. This process is primarily carried out by:

  • Biological Nitrogen Fixation: Performed by prokaryotic microorganisms called diazotrophs. These include free-living bacteria (e.g., Azotobacter, Clostridium) and symbiotic bacteria (e.g., Rhizobium in root nodules of leguminous plants). The enzyme nitrogenase catalyzes this reaction, requiring anaerobic conditions and significant energy (ATP).
  • Atmospheric Fixation: Lightning provides energy to convert N2 into nitrogen oxides (NOx), which are then carried to the earth's surface by precipitation.
  • Industrial Fixation: The Haber-Bosch process industrially fixes nitrogen using high pressure and temperature with a catalyst to produce ammonia for fertilizers.

Nitrification

Nitrification is a two-step process carried out by specific groups of chemoautotrophic bacteria. It involves the oxidation of ammonia (NH3) to nitrite (NO2-) and then nitrite to nitrate (NO3-).

  • Step 1: Nitrosomonas and related genera oxidize ammonia to nitrite. NH3 → NO2- + H+ + 2e-
  • Step 2: Nitrobacter and related genera oxidize nitrite to nitrate. NO2- → NO3- + e-
Nitrate is the primary form of nitrogen absorbed by plants. Nitrification requires aerobic conditions and is sensitive to pH.

Denitrification

Denitrification is the reduction of nitrate (NO3-) to gaseous forms of nitrogen, such as nitrogen gas (N2) or nitrous oxide (N2O), and is carried out by facultative anaerobic bacteria (e.g., Pseudomonas, Bacillus). This process occurs in anaerobic conditions, such as waterlogged soils or sediments.

The sequence of reduction is typically: NO3- → NO2- → NO → N2O → N2.

Denitrification returns nitrogen to the atmosphere, completing the cycle. It is often undesirable in agriculture as it leads to loss of available nitrogen from the soil. However, it plays a crucial role in removing excess nitrate from aquatic ecosystems, preventing eutrophication.

Conclusion

The nitrogen cycle is a complex biogeochemical process essential for life on Earth. Nitrogen fixation, nitrification, and denitrification are interconnected processes driven by microbial activity, converting nitrogen between various forms and ensuring its availability to living organisms. Disruptions to this cycle, through factors like excessive fertilizer use or deforestation, can have significant environmental consequences, including water pollution and greenhouse gas emissions. Sustainable agricultural practices and ecosystem management are crucial for maintaining the integrity of the nitrogen cycle.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Diazotrophs
Microorganisms capable of fixing atmospheric nitrogen into ammonia. They can be free-living or symbiotic.
Eutrophication
The enrichment of a water body with nutrients, typically nitrogen and phosphorus, leading to excessive plant growth and depletion of oxygen.

Key Statistics

Approximately 78% of the Earth's atmosphere is composed of nitrogen gas (N2).

Source: National Geographic (as of knowledge cutoff 2023)

Globally, agricultural fertilizers contribute approximately 80% of the total nitrogen input into ecosystems.

Source: Sutton et al., 2011, Nutrient Management and the Nitrogen Cycle

Examples

Legume-Rhizobium Symbiosis

The symbiotic relationship between leguminous plants (e.g., soybeans, peas) and <em>Rhizobium</em> bacteria is a classic example of biological nitrogen fixation. The bacteria reside in root nodules, providing the plant with ammonia in exchange for carbohydrates.

Frequently Asked Questions

▶What is the role of nitrous oxide (N2O) in the nitrogen cycle?

Nitrous oxide is an intermediate product of denitrification and a potent greenhouse gas. It contributes to global warming and ozone depletion.

Topics Covered

EcologyBiochemistryBiogeochemical CyclesNutrient CyclingEnvironmental Science
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