The Nitrogen Cycle Lecture Notes

Key Concepts

• Finite Earth: Matter is reused and recycled, atoms move between air, soil, water, and living organisms constantly.
• Nutrient Cycles: Different elements have their own cycles; today’s focus is on the Nitrogen Cycle.

Importance of Nitrogen

• Crucial for Life: Necessary for DNA, RNA, and proteins.
• Atmospheric Nitrogen: 78% of the Earth's atmosphere is nitrogen gas (N2).
• Inaccessibility: N2 is not directly usable by most organisms due to strong triple bonds.

The Nitrogen Cycle Process

1. Nitrogen Fixation

• Role of Bacteria: Nitrogen-fixing bacteria convert N2 from the air into ammonia (NH3) or ammonium (NH4+).
  • Found in soil and root nodules of legumes.
• Result: Moves nitrogen from air to soil.

2. Assimilation

• Plant Uptake: Ammonia is absorbed by plants and used to produce biological molecules.
• Preference: Some plants prefer nitrates over ammonia.

3. Nitrification

• Conversion Process: Ammonia is converted to nitrates ( O3-) by nitrifying bacteria.
  • Ammonia → Nitrite (NO2-) → Nitrate (NO3-).
  • Requires oxygen, occurs in well-aerated soils.

4. Movement Through Food Webs

• Transfer of Nitrogen: Nitrogen-containing compounds move through ecosystems when organisms eat plants and each other.

5. Ammonification

• Decomposition Process: Dead organisms decompose, returning nitrogen to the soil as ammonia.

6. Denitrification

• Return to Atmosphere: Denitrifying bacteria convert nitrates back to nitrogen gas.
  • Occurs in low-oxygen environments, e.g., waterlogged soils.

Summary

• Cycle Review:
  1. Nitrogen Fixation → Ammonia.
  2. Ammonia Assimilation or Nitrification → Nitrates.
  3. Nitrates Assimilation or Denitrification → Nitrogen Gas.
  4. Nitrogen moves through food webs.
  5. Organism decomposition returns nitrogen to soil.

Additional Resources

• Interactive Game: Practice the nitrogen cycle at biomanbio.com.

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