Nitrogen Cycle

Lecture Content Introduction Nitrogen is an essential element for proteins, nucleic acids, and chlorophyll. Although nitrogen gas (N₂) makes up about 78% of the atmosphere, most organisms cannot use it directly. The nitrogen cycle describes how nitrogen is converted into usable forms and recycled through ecosystems.

Definition The nitrogen cycle is the biogeochemical cycle that describes the movement of nitrogen among the atmosphere, soil, water, and living organisms.

Major Reservoirs of Nitrogen

• Atmosphere: Nitrogen gas (N₂).

• Soil: Ammonium (NH₄⁺), nitrate (NO₃⁻), nitrite (NO₂⁻).

• Biosphere: Proteins, nucleic acids in plants and animals.

• Hydrosphere: Dissolved nitrogen compounds in water bodies.

Steps in the Nitrogen Cycle

1. Nitrogen Fixation

   • Conversion of N₂ into ammonia (NH₃) or ammonium (NH₄⁺).

   • Carried out by nitrogen‑fixing bacteria (Rhizobium, Azotobacter) or lightning.

2. Nitrification

   • Conversion of ammonium into nitrite (NO₂⁻) and then nitrate (NO₃⁻).

   • Done by nitrifying bacteria (Nitrosomonas, Nitrobacter).

3. Assimilation

   • Plants absorb nitrates and incorporate them into organic molecules.

   • Animals obtain nitrogen by eating plants or other animals.

4. Ammonification

   • Decomposers convert organic nitrogen back into ammonium.

5. Denitrification

   • Conversion of nitrates back into nitrogen gas (N₂).

   • Done by denitrifying bacteria (Pseudomonas, Clostridium).

Ecological Importance

• Provides nitrogen for proteins and nucleic acids.

• Maintains soil fertility and agricultural productivity.

• Balances atmospheric nitrogen levels.

• Human activities (fertilizers, pollution) disrupt the cycle, causing eutrophication and greenhouse gas emissions.

Quick Revision Table

Summary / Key Takeaways

• Nitrogen cycle = movement of nitrogen through atmosphere, soil, water, and organisms.

• Driven by fixation, nitrification, assimilation, ammonification, and denitrification.

• Essential for proteins, nucleic acids, and chlorophyll.

• Human activities disrupt the cycle, leading to pollution and climate issues.