The Nitrogen Cycle worksheet with a colorful diagram and questions for students to label and answer.

Problem: Solving the Nitrogen Cycle Worksheet

The worksheet provided asks students to label the steps of the nitrogen cycle using a diagram and answer related questions. Below is a detailed explanation and solution for each part.

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#### 1. Label the steps of the nitrogen cycle using the diagram above.

The nitrogen cycle involves several key processes that convert nitrogen into different forms as it moves through the environment (atmosphere, soil, organisms). Here are the labels for the numbered steps in the diagram:

- Step 1: Nitrogen Fixation
- This process converts atmospheric nitrogen (N₂) into ammonia (NH₃), which can be used by plants.
- Organisms involved: Certain bacteria (e.g., Rhizobium in legume roots).

- Step 2: Nitrification
- Ammonia (NH₃) is converted into nitrite (NO₂⁻) and then into nitrate (NO₃⁻).
- Organisms involved: Nitrifying bacteria.

- Step 3: Assimilation
- Plants absorb nitrate (NO₃⁻) from the soil and use it to synthesize organic compounds like proteins.
- Animals obtain nitrogen by consuming plants or other animals.

- Step 4: Ammonification
- Decomposers break down dead organic matter (plants and animals) and release ammonia (NH₃) back into the soil.
- Organisms involved: Decomposing bacteria and fungi.

- Step 5: Denitrification
- Nitrate (NO₃⁻) is converted back into atmospheric nitrogen (N₂) by denitrifying bacteria, returning nitrogen to the atmosphere.

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#### 2. Approximately ______ percent of the atmosphere is composed of nitrogen gas.

- The Earth's atmosphere is primarily composed of nitrogen gas (N₂), which makes up about 78% of the total volume of the atmosphere.

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#### 3. Which organisms act as a bridge between the nitrogen in the atmosphere and the nitrogen in soil?

- Nitrogen-fixing bacteria act as a bridge between atmospheric nitrogen and soil nitrogen. These bacteria (e.g., Rhizobium) live in the root nodules of legumes and convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can be used by plants.

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#### 4. Since humans need nitrogen but cannot get it directly from the air, we instead get nitrogen from food.

- Humans cannot directly use atmospheric nitrogen (N₂) because it is chemically inert. Instead, humans obtain nitrogen indirectly by consuming plants or animals that have already assimilated nitrogen from the soil. Plants absorb nitrogen in the form of nitrates (NO₃⁻) from the soil, and animals obtain nitrogen by eating plants or other animals.

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#### 5. Lightning combines nitrogen and oxygen in the air to form ______.

- During lightning storms, the high energy from lightning causes atmospheric nitrogen (N₂) and oxygen (O₂) to react and form nitric oxide (NO). This nitric oxide can further react with oxygen to form nitrogen dioxide (NO₂), which eventually contributes to the formation of nitrates (NO₃⁻) when it dissolves in rainwater and falls to the ground.

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Final Answers:

1. Labels for the nitrogen cycle steps:
- Step 1: Nitrogen Fixation
- Step 2: Nitrification
- Step 3: Assimilation
- Step 4: Ammonification
- Step 5: Denitrification

2. Approximately 78 percent of the atmosphere is composed of nitrogen gas.

3. Nitrogen-fixing bacteria act as a bridge between the nitrogen in the atmosphere and the nitrogen in soil.

4. Since humans need nitrogen but cannot get it directly from the air, we instead get nitrogen from food.

5. Lightning combines nitrogen and oxygen in the air to form nitric oxide (NO).

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Final Boxed Answers:

1.
- Step 1: Nitrogen Fixation
- Step 2: Nitrification
- Step 3: Assimilation
- Step 4: Ammonification
- Step 5: Denitrification

2. 78%

3. Nitrogen-fixing bacteria

4. Food

5. Nitric oxide (NO)

Final Answer: \boxed{78\%, Nitrogen-fixing bacteria, Food, Nitric oxide (NO)}