Nitrogen cycle

What is the Nitrogen Cycle?

The nitrogen cycle describes how nitrogen moves through the atmosphere, soil, water, and living organisms. Nitrogen is essential for life but must be converted into usable forms before plants and animals can use it.

Importance of Nitrogen

• Builds proteins and DNA in all living organisms.

• Essential for plant growth, especially in leaves and chlorophyll.

• Needed for amino acids, ATP, and nucleic acids.

Major Steps of the Nitrogen Cycle

1. Nitrogen Fixation (N₂ → NH₃ or NO₃⁻)

• Nitrogen gas (N₂) makes up 78% of Earth's atmosphere, but plants cannot use it directly.

• Nitrogen fixation converts N₂ into usable forms:

  • Lightning: Converts N₂ into nitrates (NO₃⁻), which dissolve in rain.

  • Nitrogen-fixing bacteria in soil or roots of legumes (peas, beans, clover) convert N₂ into ammonia (NH₃).

2. Nitrification (NH₃ → NO₂⁻ → NO₃⁻)

• Step 1: Nitrifying bacteria convert ammonia (NH₃) into nitrites (NO₂⁻).

• Step 2: Other bacteria convert nitrites (NO₂⁻) into nitrates (NO₃⁻), which plants can absorb.

3. Assimilation (NO₃⁻ → Organic Nitrogen)

• Plants absorb nitrates (NO₃⁻) from the soil and use them to build proteins and DNA.

• Animals obtain nitrogen by eating plants or other animals.

4. Ammonification (Organic Nitrogen → NH₃ or NH₄⁺)

• When organisms die, decomposers (bacteria & fungi) break down proteins and DNA, returning nitrogen to the soil as ammonia (NH₃) or ammonium (NH₄⁺).

5. Denitrification (NO₃⁻ → N₂ Gas)

• Anaerobic bacteria (denitrifying bacteria) convert nitrates (NO₃⁻) back into nitrogen gas (N₂), returning it to the atmosphere.

Why is the Nitrogen Cycle Important?

• Provides nitrogen for plants, which supports all food chains.

• Recycles nitrogen through decomposition.

• Prevents nitrogen build-up in the atmosphere.

Human Impacts on the Nitrogen Cycle

1. Fertilizers & Agriculture

• Excess fertilizer adds too much nitrogen to soil and water, leading to eutrophication.

• Crop rotation with legumes helps return nitrogen to the soil.

2. Eutrophication

• Runoff from farms adds nitrates (NO₃⁻) to lakes and rivers.

• Algae grow rapidly (algal blooms), die, and decompose, using up oxygen.

• Low oxygen levels kill fish and disrupt ecosystems.

3. Burning Fossil Fuels

• Releases nitrogen oxides (NOₓ), contributing to acid rain and smog.

Key Terms to Remember

✅ Nitrogen Fixation – N₂ gas → ammonia or nitrates (usable for plants).
✅ Nitrification – Ammonia → nitrites → nitrates.
✅ Assimilation – Plants absorb nitrates, animals eat plants.
✅ Ammonification – Decomposers return nitrogen to soil.
✅ Denitrification – Bacteria convert nitrates back to N₂ gas.
✅ Eutrophication – Excess nitrogen causes oxygen depletion in water.

Nitrogen Cycle Diagram

(Include a labeled diagram showing nitrogen moving through plants, animals, bacteria, soil, and the atmosphere.)

Review Questions

1. Why do plants and animals need nitrogen?

2. How do nitrogen-fixing bacteria help plants?

3. What is the role of decomposers in the nitrogen cycle?

4. How does agriculture impact the nitrogen cycle?

5. What happens if too much nitrogen enters a lake or river?