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Environmental Science
Grade 7-12
Nitrogen Cycle Visual Cheat Sheet
A printable reference covering nitrogen fixation, nitrification, assimilation, ammonification, denitrification, and human impacts for grades 7-12.
Related Worksheets
The nitrogen cycle shows how nitrogen moves through the atmosphere, soil, water, plants, animals, and decomposers. Students need this cheat sheet because nitrogen is essential for proteins, DNA, and plant growth, but most organisms cannot use nitrogen gas directly. A visual layout helps connect each process to the organisms and chemical forms involved. This reference supports ecology, agriculture, and environmental impact topics.
Key Facts
- About 78 percent of Earth's atmosphere is nitrogen gas, N2, but most plants and animals cannot use N2 directly.
- Nitrogen fixation converts N2 into ammonia or ammonium, shown as N2 -> NH3 or N2 -> NH4+, through bacteria, lightning, or industrial fertilizer production.
- Nitrification is a two-step bacterial process: NH4+ -> NO2- and then NO2- -> NO3-.
- Assimilation occurs when plants absorb nitrate, NO3-, or ammonium, NH4+, and build proteins, DNA, and chlorophyll.
- Animals get usable nitrogen by eating plants or by eating other animals that contain nitrogen-rich molecules.
- Ammonification is decomposition that converts organic nitrogen in dead organisms and waste into ammonium, shown as organic N -> NH4+.
- Denitrification returns nitrogen to the atmosphere when bacteria convert nitrate into nitrogen gas, shown as NO3- -> N2.
- Excess fertilizer can cause nitrate runoff, algal blooms, oxygen loss, and dead zones in lakes, rivers, and coastal waters.
Vocabulary
- Nitrogen fixation
- The process that changes atmospheric nitrogen gas, N2, into ammonia or ammonium that living things can use.
- Nitrification
- The bacterial process that changes ammonium into nitrite and then nitrate in soil or water.
- Assimilation
- The process in which plants take in nitrate or ammonium and build nitrogen into living tissue.
- Ammonification
- The decomposition process that changes nitrogen in dead organisms and waste into ammonium.
- Denitrification
- The bacterial process that converts nitrate back into nitrogen gas and returns it to the atmosphere.
- Runoff
- Water that flows over land and can carry fertilizers, soil, and pollutants into nearby waterways.
Common Mistakes to Avoid
- Thinking plants use N2 gas directly is wrong because most plants absorb nitrogen mainly as NO3- or NH4+ from soil.
- Confusing nitrogen fixation with nitrification is wrong because fixation changes N2 into NH3 or NH4+, while nitrification changes NH4+ into NO2- and NO3-.
- Leaving decomposers out of the cycle is wrong because ammonification by decomposers returns nitrogen from dead matter and waste to the soil.
- Assuming fertilizer always helps ecosystems is wrong because excess nitrate can wash into water, feed algal blooms, and lower dissolved oxygen.
- Drawing arrows in only one direction toward plants is wrong because nitrogen also moves through animals, waste, decomposers, soil bacteria, water, and the atmosphere.
Practice Questions
- 1 Atmospheric nitrogen makes up about 78 percent of air. If a 100-liter sample of air is collected, about how many liters are nitrogen gas?
- 2 A farmer applies 60 kilograms of nitrogen fertilizer, and 25 percent runs off after heavy rain. How many kilograms of nitrogen enter nearby waterways?
- 3 Put these nitrogen cycle steps in a logical order starting with atmospheric nitrogen: assimilation, nitrogen fixation, denitrification, nitrification.
- 4 Explain why reducing excess fertilizer use can protect aquatic ecosystems, even though nitrogen is an essential plant nutrient.