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Nitrogen Cycle — Reading Comprehension

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MS-LS2-3

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This comprehensive passage explains the nitrogen cycle for grades 6-8, aligned with NGSS standard MS-LS2-3. Students will explore how nitrogen moves through the atmosphere, soil, plants, and animals, and why nitrogen is crucial for living things. The passage details key processes like nitrogen fixation, nitrification, assimilation, and denitrification, with a focus on underlying mechanisms and cause-and-effect relationships. Real-world examples illustrate the impact of fertilizers and human activities on ecosystems, including eutrophication and algal blooms. The resource also features a glossary, Spanish translation, simplified version for differentiated instruction, multiple-choice quiz, writing prompts, and two graphic organizers. All content is designed for audio integration and supports science literacy and systems thinking in middle school students.

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Nitrogen is a vital element that makes up about 78% of Earth's atmosphere, yet most organisms cannot use it directly from the air. The nitrogen cycle is a complex system that transforms nitrogen into forms usable by living things and returns it to the atmosphere. This cycle connects the atmosphere, soil, plants, animals, and bacteria, playing a key role in supporting life and maintaining healthy ecosystems.

How Nitrogen Moves Through Ecosystems

Although nitrogen gas (N₂) is abundant in the air, most plants and animals cannot use it in this form. The process starts with nitrogen fixation, where special bacteria in the soil or in the roots of legume plants convert nitrogen gas into ammonia (NH₃). This step makes nitrogen available to other living things. Next, other bacteria perform nitrification, turning ammonia into nitrates (NO₃^-), which plants can absorb through their roots. This process is essential because plants need nitrogen to build proteins and DNA, and animals obtain nitrogen by eating plants or other animals.

Connecting Processes: Assimilation, Decomposition, and Denitrification

Once plants absorb nitrates, they use them to grow and develop. When animals eat these plants, they assimilate nitrogen into their own bodies. After plants and animals die, decomposers like bacteria and fungi break down their remains, releasing nitrogen back into the soil as ammonia. Some bacteria carry out denitrification, converting nitrates back into nitrogen gas and releasing it into the atmosphere. This completes the cycle and maintains the balance of nitrogen in ecosystems.

Human Impacts and the Importance of Nitrogen Balance

Human activities, such as using synthetic fertilizers in agriculture, add extra nitrogen to the soil. While this can increase crop growth, too much nitrogen can wash into lakes and rivers, causing eutrophication. Eutrophication leads to rapid algal growth, or algal blooms, which block sunlight and reduce oxygen in water, harming aquatic life. Because nitrogen is often a limiting nutrient, even small changes can have big effects on ecosystems. Scientists study the nitrogen cycle using experiments and measurements of nitrogen compounds in air, soil, and water, helping us understand how natural and human factors affect this crucial process.

In summary, the nitrogen cycle is an example of how matter cycles through living and nonliving parts of the environment, supporting life and connecting different Earth systems. Understanding this cycle helps people make better decisions about land use, farming, and protecting ecosystems.

Interesting Fact: Some lightning strikes can naturally fix nitrogen gas in the atmosphere, turning it into compounds that plants can use!

What percentage of Earth's atmosphere is made up of nitrogen gas?

78%50%21%1%

Why can't most plants and animals use nitrogen gas (N2) directly from the air?

It is not in a usable chemical form for most organisms.It is poisonous to plants and animals.There is not enough nitrogen in the air.Nitrogen gas is too heavy to be absorbed.

Which process turns nitrogen gas into ammonia in the soil?

Nitrogen fixationNitrificationDenitrificationEutrophication

What is the role of decomposers in the nitrogen cycle?

They break down dead matter and return nitrogen to the soil.They absorb nitrogen from the air.They produce fertilizers.They remove all nitrogen from ecosystems.

In the passage, what does the word 'limiting nutrient' mean?

A nutrient that restricts the growth of organisms if it is scarce.A nutrient that is found in unlimited amounts.A nutrient that is not needed by any living things.A nutrient that speeds up decomposition.

What is eutrophication, as described in the passage?

A process where water becomes rich in nutrients, causing too much algae to grow.A step where bacteria fix nitrogen gas in the soil.The process of absorbing nitrogen from the air.A type of fertilizer used on farms.

How might using too much fertilizer affect a lake ecosystem?

It can cause algal blooms and reduce oxygen for aquatic life.It will reduce plant growth in the lake.It will remove all nitrogen from the water.It will have no effect on the ecosystem.

Which of the following happens LAST in the nitrogen cycle?

Denitrification returns nitrogen to the atmosphere.Nitrogen fixation by bacteria.Nitrification in the soil.Assimilation by plants.

True or False: Animals can absorb nitrates directly from the soil.

FalseTrue

True or False: Lightning can help change nitrogen in the air into a form plants can use.

TrueFalse

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Nitrogen Cycle — Reading Comprehension

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