AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/1/1-5)
Unit 1 · Topic 1.5
1.5 The Nitrogen Cycle
Nitrogen is needed for proteins and DNA, and the atmosphere is mostly nitrogen gas, but most living things can't use N₂ directly. Bacteria convert it through fixation, nitrification, ammonification and denitrification, and because usable nitrogen is often scarce, it frequently limits plant growth.
Key terms
- nitrogen fixation
- assimilation
- ammonification
- nitrification
- denitrification
- limiting nutrient
Why nitrogen matters
Every living thing needs nitrogen to build proteins and nucleic acids like DNA. The atmosphere is about 78% nitrogen gas (N₂), making it the largest nitrogen reservoir. But N₂ has a very strong triple bond, and almost no organisms can break it. Plants take up nitrogen mainly as ammonium (NH₄⁺) or nitrate (NO₃⁻). Turning N₂ into those forms is the job of the nitrogen cycle.
Because usable nitrogen is often in short supply, it is commonly a limiting nutrient, meaning that adding it makes plants grow more. That's why nitrogen is one of the main ingredients in fertilizer.
The steps of the cycle
One way to remember the order: fixation brings nitrogen in from the air, nitrification turns it into nitrate, assimilation moves it into living things, ammonification recycles it from waste and dead matter, and denitrification sends it back to the air.
Many nitrogen-fixing bacteria live in nodules on the roots of legumes like beans, peas, clover and soybeans. This is a mutualism: the plant gives the bacteria sugar, and the bacteria give the plant usable nitrogen. Lightning can also fix a small amount of nitrogen by splitting N₂ so it combines with oxygen, and the nitrogen compounds fall to the ground in rain.
| Process | What changes | Who does it |
|---|---|---|
| Nitrogen fixation | N₂ → ammonia (NH₃), which becomes ammonium (NH₄⁺) in soil | Bacteria in soil and in root nodules of legumes; also lightning and factories |
| Assimilation | NH₄⁺ or NO₃⁻ → nitrogen in plant tissue; animals get it by eating plants | Plants (roots), then consumers |
| Ammonification | Nitrogen in wastes and dead bodies → NH₃ / NH₄⁺ | Decomposer bacteria and fungi |
| Nitrification | NH₄⁺ → nitrite (NO₂⁻) → nitrate (NO₃⁻) | Nitrifying bacteria in soil |
| Denitrification | NO₃⁻ → nitrous oxide (N₂O) → N₂ gas, back to the air | Bacteria in low-oxygen (anaerobic) soils and wetlands |
How fast it cycles
Nitrogen stays in the atmosphere for a very long time, so the atmospheric reservoir changes slowly. But nitrogen in soil and living things cycles relatively fast, over days to years. Nitrate dissolves easily in water and isn't held tightly by soil particles, so it is easily leached (washed down) into groundwater or carried by runoff into streams.
Human impacts
People now fix huge amounts of nitrogen. The Haber-Bosch process, developed in the early 1900s, uses fossil fuel energy to turn N₂ into ammonia for synthetic fertilizer. Together with planting legume crops and burning fossil fuels, human activity has roughly doubled the rate at which usable nitrogen enters land ecosystems.
Excess nitrogen causes problems. Fertilizer runoff feeds algal blooms that can lead to low-oxygen dead zones (eutrophication). Nitrate leaching contaminates drinking water wells. Burning fuels makes nitrogen oxides that contribute to smog and acid rain. Some fertilizer nitrogen becomes nitrous oxide (N₂O), a strong greenhouse gas that also damages the ozone layer.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Naming the steps
Name the process in each case: (a) bacteria in a waterlogged rice paddy turn nitrate into N₂ gas; (b) a dead deer's proteins are broken down into ammonium; (c) soybean root nodules turn N₂ into ammonia; (d) soil bacteria turn ammonium into nitrate; (e) corn roots absorb nitrate.
Show the solutionHide the solution
- Step 1: Match each change to its starting and ending forms.
- Step 2: (a) Nitrate → N₂ in low-oxygen soil: denitrification.
- Step 3: (b) Organic nitrogen in a dead body → ammonium: ammonification.
- Step 4: (c) N₂ → ammonia: nitrogen fixation.
- Step 5: (d) Ammonium → nitrate: nitrification.
- Step 6: (e) Plants taking in nitrate and building it into their tissue: assimilation.
Answer: (a) Denitrification, (b) ammonification, (c) nitrogen fixation, (d) nitrification, (e) assimilation.
- Example 2
Why farmers rotate in legumes
A farmer grows corn on a field for two years, then plants soybeans for a year before returning to corn. Explain how this helps the corn.
Show the solutionHide the solution
- Step 1: Corn takes a lot of nitrogen from the soil, so years of corn deplete it.
- Step 2: Soybeans are legumes. Bacteria in their root nodules fix N₂ from the air into ammonia/ammonium.
- Step 3: When the soybean roots and leftover plants decompose, that nitrogen is released into the soil (ammonification), and it can then be nitrified to nitrate.
- Step 4: The next corn crop has more usable nitrogen, so the farmer needs less synthetic fertilizer.
Answer: Soybeans host nitrogen-fixing bacteria that add usable nitrogen to the soil, so the following corn crop needs less fertilizer.
Common mistakes
- Confusing nitrification with nitrogen fixation. Fixation starts with N₂ gas; nitrification starts with ammonium and makes nitrate.
- Saying plants absorb N₂ from the air. Plants can't use N₂; they take up ammonium or nitrate through their roots.
- Forgetting that denitrification needs low-oxygen conditions, like waterlogged soil or wetland mud.
- Calling soil the largest nitrogen reservoir. The atmosphere is.
On the exam
- Questions often describe a step and ask you to name it, or ask what happens to a waterway when nitrogen fertilizer runs off. Link excess nitrate to algal blooms and low oxygen.
- When you explain nitrogen fixation, mention bacteria and legumes, and note that lightning and the Haber-Bosch process also fix nitrogen.
Connected topics
Videos
Check yourself
4 questions on 1.5 The Nitrogen Cycle. Pick an answer to see if you got it, and why.
A farmer rotates corn with soybeans. Soybean roots have small swellings called nodules that contain Rhizobium bacteria.
After a week of heavy rain, one low part of a soybean field stays waterlogged for two weeks. Soil tests there show nitrate (NO₃⁻) levels falling while nitrous oxide (N₂O) gas given off by the soil rises. In a well-drained part of the same field, nitrate levels stay steady.
Described scenario
The Rhizobium bacteria in the soybean root nodules are most directly responsible for which process?
Which process best explains the changes measured in the waterlogged soil?
The farmer finds that corn planted the year after soybeans needs less nitrogen fertilizer. Which explanation is best supported?
Nitrogen in the proteins of a dead deer eventually returns to the atmosphere as N₂ gas. Which sequence of processes is most likely?
0 of 4 answered