The Nitrogen Cycle

Jack Wilson
7 min read
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Study Guide Overview
This study guide covers the nitrogen cycle, including why nitrogen is important as a macronutrient and limiting nutrient. It details the cycle's key steps: nitrogen fixation, nitrification, assimilation, mineralization, and denitrification, emphasizing the role of bacteria. The guide also explains human impacts like fertilizer overuse, eutrophication, and effects on species richness. Finally, it provides practice questions and exam tips.
AP Environmental Science: The Nitrogen Cycle ๐ฟ
Hey there, future AP Environmental Science superstar! Let's dive into the nitrogen cycle โ it's a big deal, but we'll break it down so it's super easy to understand. Remember, this isn't AP Chem, so we're focusing on the big picture and impacts, not memorizing every single reaction. Let's get started!
Why Nitrogen Matters
Nitrogen is a macronutrient, essential for all living things. It's a key component of:
- Proteins
- DNA
- Other vital biomolecules
Basically, no nitrogen = no life! The nitrogen cycle is how this crucial element moves between the atmosphere, land, and water.
Nitrogen is a limiting nutrient, meaning it's essential but often in short supply. This is why it's a common ingredient in fertilizers.
Think of nitrogen as the 'building block' of life. Just like you need bricks to build a house, plants and animals need nitrogen to build their bodies.
The Nitrogen Cycle: Step-by-Step ๐
Here's a simplified journey of nitrogen through the environment:
1. Nitrogen Fixation ๐จโก๏ธ๐ฑ
- What it is: Conversion of atmospheric nitrogen gas (N2), which is unusable by most organisms, into usable forms like ammonia (NH3) or nitrate (NO3).
- How it happens:
- Biotic Fixation: Nitrogen-fixing bacteria (like those in the roots of legumes) convert N2 into ammonia (NH3), which quickly becomes ammonium (NH4+).
- Abiotic Fixation: Lightning, fires, and fossil fuel combustion can convert N2 directly into NO3. Quick Fact
78% of the Earth's atmosphere is nitrogen gas (N2), but it's not usable by most organisms until it's 'fixed'.
2. Nitrification ๐ฑโก๏ธNO2-โก๏ธNO3-
- What it is: Ammonia (NH3) and other nitrogen compounds are converted into nitrite (NO2-) and then nitrate (NO3-).
- How it happens: Bacteria in soil and water do the heavy lifting.
- Why it matters: Keeps nitrate levels in check, preventing pollution and algal blooms.
3. Assimilation ๐ฑโก๏ธ ๐งฌ
- What it is: Plants absorb usable nitrogen (mostly nitrate) through their roots and incorporate it into their tissues.
- How it happens: Plants synthesize nitrogen into proteins, DNA, and other biomolecules.
- What it leads to: Consumers get nitrogen by eating plants.
4. Mineralization (Ammonification) ๐โก๏ธ NH4+
- What it is: Decomposers break down dead organisms and waste, converting organic nitrogen back into inorganic ammonium (NH4+).
- How it happens: Bacteria and fungi are the main decomposers.
- Why it matters: Returns nitrogen to the soil, restarting the cycle.
5. Denitrification NO3-โก๏ธN2Oโก๏ธN2 ๐จ
- What it is: Conversion of nitrate (NO3-) back into nitrogen gas (N2) and nitrous oxide (N2O).
- How it happens: Specialized bacteria perform this step.
- Why it matters: Returns nitrogen to the atmosphere, completing the cycle.
FAN-MAD: Fixation, Assimilation, Nitrification, Mineralization (Ammonification), Assimilation, Denitrification. This is a helpful way to remember the steps of the nitrogen cycle.
Focus on the flow of nitrogen and the roles of different organisms in the cycle, rather than memorizing every chemical formula.
Visualizing the Cycle
Image Courtesy of Wikimedia Commons
Notice how bacteria play a crucial role in almost every step of the nitrogen cycle! They are the unsung heroes of the environment.
Human Impacts on the Nitrogen Cycle โ ๏ธ
Fertilizer Overuse
- Problem: Human use of nitrogen fertilizers now exceeds natural nitrogen fixation.
- Consequences:
- Eutrophication: Excess nitrogen runoff leads to algal blooms in water bodies, which can deplete oxygen and harm aquatic life.
- Reduced Species Richness: Plants that need more nitrogen outcompete others, decreasing the variety of species in an ecosystem.
Don't confuse nitrogen fixation with nitrogen pollution. Nitrogen fixation is a natural process, while human activities like fertilizer use can lead to excess nitrogen in the environment.
Be ready to discuss how human activities (like fertilizer use and fossil fuel combustion) disrupt the nitrogen cycle and their environmental impacts.
Final Exam Focus ๐ฏ
- Key Processes: Nitrogen fixation, nitrification, assimilation, mineralization, and denitrification.
- Human Impacts: Fertilizer use, eutrophication, and altered species richness.
- Question Types: Expect to see multiple-choice questions about the steps of the cycle and free-response questions about human impacts.
Last-Minute Tips:
- Time Management: Quickly read the questions and prioritize those you know well. Don't get stuck on one question for too long.
- Common Pitfalls: Avoid confusing the different forms of nitrogen (N2, NH3, NO2-, NO3-). Focus on the big picture and the flow of nitrogen through the cycle.
- Strategies: Think through the steps of the cycle logically. Use diagrams to help you visualize the processes.
Practice Question
Practice Questions
Multiple Choice Questions:
-
Which of the following processes converts nitrogen gas into a form usable by plants? (a) Denitrification (b) Nitrification (c) Assimilation (d) Nitrogen Fixation
-
The process by which decomposers convert organic nitrogen back into inorganic ammonium is called: (a) Nitrification (b) Denitrification (c) Mineralization (d) Assimilation
-
Excessive use of nitrogen-based fertilizers can lead to which of the following environmental problems? (a) Increased biodiversity (b) Decreased algal blooms (c) Eutrophication (d) Decreased soil fertility
Free Response Question:
Question: Describe the nitrogen cycle, including the major steps and the role of bacteria. Then, discuss two ways in which human activities have altered the nitrogen cycle and the environmental consequences of these alterations. (10 points)
Answer Key:
- (2 points) Describe the nitrogen cycle:
- (1 point) Mention the key steps: nitrogen fixation, nitrification, assimilation, mineralization, and denitrification.
- (1 point) Briefly explain each step (e.g., nitrogen fixation converts N2 to usable forms, nitrification converts ammonia to nitrate, etc.).
- (2 points) Role of bacteria:
- (1 point) Explain that bacteria are involved in nitrogen fixation, nitrification, and denitrification.
- (1 point) Note that decomposers (some of which are bacteria) perform mineralization.
- (4 points) Two ways human activities have altered the nitrogen cycle:
- (2 points) Identify two human activities (e.g., fertilizer use, fossil fuel combustion).
- (2 points) Explain how each activity alters the nitrogen cycle (e.g., fertilizer adds excess nitrogen, fossil fuel combustion leads to atmospheric deposition).
- (2 points) Environmental consequences:
- (1 point) Describe the environmental impact of each activity (e.g., eutrophication from fertilizer runoff, acid rain from fossil fuel combustion).
Alright, that's the nitrogen cycle in a nutshell! You've got this. Remember to stay calm, take deep breaths, and trust your knowledge. You're going to do great!

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Question 1 of 11
Nitrogen is a key component of which of the following biomolecules? ๐ค
Carbohydrates
Lipids
Proteins
Simple sugars