Nitrogen Cycle

Nitrogen Cycle

Middle School

​

Learning Objectives

• Explain why nitrogen is a crucial element for all living things.

• Describe how bacteria help make nitrogen usable through nitrogen fixation.

• Trace the path of nitrogen as it cycles through an ecosystem.

• Analyze how fertilizers impact the nitrogen cycle and harm our waterways.

Key Vocabulary

​

Nitrogen Fixation

The process where bacteria convert atmospheric nitrogen into forms that plants are able to use.

​

Denitrification

The process where bacteria in the soil convert nitrates back into nitrogen gas, releasing it.

​

Fertilizer Runoff

Water with nitrates from fertilizers drains from agricultural fields into nearby waterways like rivers.

​

Algal Bloom

A rapid increase in the algae population in an aquatic system, often caused by excess nutrients.

​

Assimilation

The process where plants absorb nitrates from the soil to build proteins and their DNA.

​

​

The Importance of Nitrogen

• Nitrogen is an essential element for all living organisms.

• ​It is a key building block for DNA and protein molecules.

• The air we breathe is almost 80% nitrogen gas (N₂).

• Most living things cannot use nitrogen directly from the air.

​

Multiple Choice

According to the slide, what are the two primary reasons living organisms need nitrogen?

• To build proteins & DNA

• To breathe and to obtain water

• To regulate temperature and grow

• To process sunlight and create energy

​

​

What is Nitrogen Fixation?

• Plants cannot use nitrogen gas (N₂) directly from the atmosphere.

• Nitrogen fixation is the process that converts nitrogen gas for plant use.

• Soil bacteria change nitrogen into usable forms like nitrates (NO₃).

• A small amount of nitrogen is also fixed by lightning strikes.

​

Multiple Choice

What is the main process that converts unusable atmospheric nitrogen into forms that are usable by plants?

• Nitrogen fixation

• Photosynthesis

• Ammonification

• Denitrification

How Nitrogen Moves Through the Ecosystem

• Plants take in nitrogen from the soil through their roots.

• Animals get nitrogen by eating plants or other animals.

• Denitrification releases nitrogen gas (N₂) back into the atmosphere.

Multiple Choice

How do animals obtain the nitrogen they need to live?

• By eating plants or other animals

• By breathing it in from the atmosphere

• By absorbing it from sunlight

• Through the process of denitrification

​

​

Nitrogen and Agriculture

• Fertilizers from manure or commercial sources are rich in nitrates.

• These are added to soil to help important food crops grow.

• When it rains, excess nitrates can be washed into rivers.

• This process is called fertilizer runoff and it pollutes the water.

​

Multiple Choice

What is fertilizer runoff?

• When excess nitrates from farms are washed into waterways

• When plants use up all the fertilizer in the soil

• The process of making commercial fertilizers

• When bacteria convert fertilizer into nitrogen gas

​

How Runoff Causes Algal Blooms

• Fertilizer runoff adds excess nutrients like nitrates to bodies of water.

• This causes a rapid overgrowth of algae, creating an algal bloom.

• The bloom blocks sunlight from reaching other plants, causing them to die.

• Bacteria use oxygen to decompose dead algae, harming fish and other animals.

Multiple Choice

What is a primary negative impact of an algal bloom on an aquatic ecosystem?

• It depletes the oxygen in the water, harming fish.

• It provides more food for all aquatic animals.

• It cleans the water by absorbing all the nitrates.

• It increases the amount of sunlight reaching underwater plants.

​

Common Misconceptions

Misconception	Correction
Living things can use nitrogen directly from the air they breathe.	Most airborne nitrogen (N2) is unusable; it must be 'fixed' by bacteria or lightning.
Nitrogen is rare and hard to find in the environment.	Nitrogen is the most abundant gas, making up 78% of Earth's atmosphere.
All nitrates are bad for the environment.	Nitrates are essential for plants; excess amounts from runoff cause pollution.

Multiple Choice

A farmer notices a massive algal bloom in a pond downstream from their fields. What is the most likely cause-and-effect relationship that explains this?

• Use of nitrogen-rich fertilizers led to runoff, which fueled the algae growth.

• The crops used up all the nitrogen, causing the algae to die.

• Bacteria in the soil fixed too much nitrogen, which evaporated into the pond.

• Lightning strikes near the pond created excess nitrates for the algae.

Multiple Choice

Why are bacteria considered the most important part of the nitrogen cycle?

• Because they perform nitrogen fixation.

• Because they are the only organisms that can use nitrogen.

• Because they prevent release of the pollutants.

• Because they help animals digest plants containing nitrogen.

Multiple Choice

A new chemical is discovered that kills all nitrogen-fixing bacteria in the soil. What would be the long-term consequence for the local ecosystem?

• Plants would be unable to get the nitrogen they need, and the food web would collapse.

• Nitrogen gas in the atmosphere would decrease.

• The ecosystem would thrive as plants find new sources of nitrogen.

• Animals would get their nitrogen by breathing, and plants would die.

Multiple Choice

Imagine you are an ecologist trying to solve the problem of recurring algal blooms in a lake surrounded by farms. Based on your knowledge of the nitrogen cycle, which of the following solutions would be most effective?

• Work with farmers to reduce fertilizer use and create buffer zones around the lake.

• Introduce more fish to the lake to eat the algae.

• Add more nitrogen-fixing bacteria to the lake's water.

• Attempt to block sunlight from reaching the lake to stop algae growth.

​

Summary

• Nitrogen is essential for building DNA and proteins in all living things.

• Bacteria fix unusable atmospheric nitrogen into nitrates that plants can absorb.

• Animals get nitrogen by eating plants or other animals.

• Fertilizer runoff pollutes water, causing algal blooms that harm aquatic life.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

• 2

• 3

• 4