Nitrogen Cycle

Nitrogen Cycle

High School

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Learning Objectives

• Define the nitrogen cycle and its importance for all living things.

• Describe the five key processes of the nitrogen cycle.

• Explain the crucial role of bacteria in converting nitrogen into usable forms.

• Trace the path of a nitrogen atom through the ecosystem.

Key Vocabulary

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Nitrogen Fixation

The conversion of atmospheric nitrogen gas into usable compounds like ammonia by bacteria or lightning.

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Ammonification

The process where decomposers break down dead organic matter and waste products into ammonia.

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Nitrification

The biological oxidation of ammonia to nitrite and then to nitrate by specific bacteria.

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Assimilation

The process by which plants and animals incorporate nitrates and ammonia to form biological tissues.

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Denitrification

The conversion of nitrates back into nitrogen gas, which is then released into the atmosphere.

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What is the Nitrogen Cycle?

• Nitrogen is vital for making proteins and nucleic acids (DNA and RNA).

• The air is 78% nitrogen gas (N₂), which most organisms cannot use.

• The nitrogen cycle moves nitrogen through air, soil, water, and living organisms.

• It converts nitrogen into usable forms like ammonia found in organism waste.

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Solved Example 1
If the atmosphere is 78% nitrogen, how many liters of nitrogen are in a 50-liter air sample?

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Step 1: Analyze and Sketch the Problem

• Goal: Find the volume of nitrogen in the air sample.

• Knowns: Total volume of air = 50 L; Percentage of nitrogen = 78%.

• Unknown: Volume of nitrogen = ?

• Formula: Volume of component = Total volume × Percentage of component

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Solved Example 1
If the atmosphere is 78% nitrogen, how many liters of nitrogen are in a 50-liter air sample?

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Step 2: Solve for the Unknown

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Solved Example 1
If the atmosphere is 78% nitrogen, how many liters of nitrogen are in a 50-liter air sample?

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Step 3: Evaluate the Answer

• The volume of nitrogen (39 L) is 78% of the total volume (50 L), which confirms the calculation is correct.

• The answer is reasonable, as the volume of nitrogen should be less than the total air sample.

Multiple Choice

Why is the nitrogen cycle necessary for life, despite the atmosphere being 78% nitrogen?

• Because nitrogen is only needed by plants, not animals.

• Because atmospheric nitrogen (N₂) is not in a form that most organisms can use directly.

• Because there is not enough nitrogen in the atmosphere to support life.

• Because the cycle converts usable nitrogen into atmospheric nitrogen.

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The First Steps: Fixation and Ammonification

Nitrogen Fixation

• Atmospheric nitrogen gas (N₂) is converted into usable compounds like ammonia (NH₃).

• This is mainly done by nitrogen-fixing bacteria in the soil and on plant roots.

• This process can also occur in the atmosphere as a result of lightning strikes.

Ammonification

• When organisms die or produce waste, decomposers and ammonifying bacteria break them down.

• They convert the organic nitrogen from the waste and remains back into ammonia (NH₃).

• This process is vital because it returns valuable nitrogen back into the soil.

Multiple Choice

What is the key difference between nitrogen fixation and ammonification?

• Nitrogen fixation converts ammonia to nitrates, while ammonification converts nitrates to nitrogen gas.

• Nitrogen fixation converts atmospheric nitrogen to ammonia, while ammonification converts organic matter to ammonia.

• Nitrogen fixation returns nitrogen to the soil, while ammonification removes it.

• Nitrogen fixation is done by decomposers, while ammonification is done by lightning.

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The Next Steps: Nitrification and Assimilation

Nitrification

• ​Nitrifying bacteria in soil convert ammonia (NH₃) into nitrites (NO₂).

• ​​These nitrites (NO₂) are then converted by bacteria into nitrates (NO₃).

• ​This two-part process makes nitrogen available for plants to absorb from the soil.

Assimilation

• ​Plants absorb nitrogen, mainly as nitrates, from the soil through their roots.

• ​​Nitrogen is used by the plant to build its own proteins and nucleic acids.

• ​Animals get the nitrogen they need by eating plants or other animals.

Multiple Choice

How do plants primarily obtain the nitrogen they need for building proteins?

• Through assimilation, by absorbing nitrates from the soil via their roots.

• Directly from the atmosphere in the form of nitrogen gas.

• Through nitrification, by converting ammonia themselves.

• By eating animals that contain nitrogen.

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What is Denitrification?

• Denitrification is the final stage of the nitrogen cycle.

• Bacteria convert nitrates (NO₃) in soil back to nitrogen gas (N₂).

• This nitrogen gas is then released back into the atmosphere.

• Farmers use nitrogen-rich manure on fields to help crops grow better.

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Multiple Choice

What is the primary function of denitrifying bacteria in the nitrogen cycle?

• To break down dead organic matter into ammonia.

• To convert atmospheric nitrogen into ammonia.

• To convert ammonia into nitrates for plants to use.

• To convert nitrates back into atmospheric nitrogen gas.

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Common Misconceptions about the Nitrogen Cycle

Misconception	Correction
Plants absorb nitrogen directly from the air.	Plants absorb nitrates from the soil through their roots.
The nitrogen cycle only occurs in the soil.	The cycle involves the atmosphere, soil, water, and living organisms.
All bacteria in the cycle have the same function.	Different bacteria have specialized roles in the cycle.

Multiple Choice

How do the roles of nitrogen-fixing bacteria and nitrifying bacteria differ in making nitrogen available to plants?

• Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia, while nitrifying bacteria convert ammonia to nitrates, the form plants absorb.

• Both bacteria convert atmospheric nitrogen into different compounds.

• Nitrogen-fixing bacteria create nitrates, while nitrifying bacteria create ammonia.

• Nitrogen-fixing bacteria are decomposers, while nitrifying bacteria are producers.

Multiple Choice

Considering the entire nitrogen cycle, why are decomposers and ammonifying bacteria essential for its continuation?

• They directly provide nitrates to plant roots.

• They recycle nitrogen from dead organic matter back into the soil as ammonia, making it available for nitrification.

• They convert nitrates back into atmospheric nitrogen.

• They are the only organisms that can fix atmospheric nitrogen.

Multiple Choice

A farmer plants a field with legumes, whose roots have nitrogen-fixing bacteria. How would this action impact the nitrogen cycle in that field?

• It would have no impact on the nitrogen cycle, as bacteria are already in the soil.

• It would increase the rate of denitrification, causing a loss of soil nitrogen.

• It would increase the amount of ammonia in the soil through enhanced nitrogen fixation, boosting overall nitrogen availability for future crops.

• It would stop the process of assimilation in the plants.

Multiple Choice

If a powerful chemical was released that completely wiped out all denitrifying bacteria in an ecosystem, what would be the most likely long-term consequence?

• Plants would no longer be able to absorb nitrogen from the soil.

• The conversion of ammonia to nitrates would halt, causing ammonia levels to rise.

• Nitrates would accumulate in the soil, and the amount of nitrogen gas (N₂) being returned to the atmosphere would decrease significantly.

• Nitrogen fixation would stop, preventing any new nitrogen from entering the cycle.

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Summary

• The nitrogen cycle moves essential nitrogen for proteins and DNA through ecosystems.

• Nitrogen fixation converts atmospheric nitrogen gas into usable ammonia.

• Ammonification and nitrification process waste into nitrates for plants to absorb.

• Assimilation builds life with nitrogen; denitrification returns it to the atmosphere.

Poll

On a scale of 1-4, how confident are you about your understanding of the Nitrogen Cycle?

• 1

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• 3

• 4