U1.3 Nitrogen and Phosphorus Cycles
Master the nitrogen and phosphorus cycles for AP Environmental Science: fixation, nitrification, denitrification, weathering, and how fertilizer runoff drives eutrophication.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on U1.3 Nitrogen and Phosphorus Cycles, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Every protein in your body and every strand of DNA depends on two elements that plants can't simply grab from the air in usable form: nitrogen and phosphorus. These nutrient cycles decide how fertile an ecosystem is — and how badly humans can throw it off balance. In this lesson you'll trace nitrogen through five transformations driven mostly by bacteria, follow phosphorus through a slower, rock-based cycle with no gas phase, and connect both to the real-world problem AP loves to test: eutrophication from fertilizer runoff. Nail the vocabulary and the direction of each conversion, and these become some of the most reliable points on the exam.
The Nitrogen Cycle: Five Key Steps
Earth's atmosphere is about 78% nitrogen gas (), yet most organisms cannot use it directly because the triple bond in is extremely stable. The nitrogen cycle is really a series of chemical conversions, most carried out by bacteria, that move nitrogen into and out of biologically usable forms.
The five steps you must know in order of function:
A common misconception is that plants pull nitrogen straight from the air — they cannot. They rely on fixation and nitrification to supply nitrate or ammonium in the soil. Another frequent error is confusing nitrification (making nitrate) with denitrification (destroying it). Remember denitrification is the only step that returns nitrogen to the atmosphere, closing the cycle.
The five steps you must know in order of function:
| Step | What happens | Form produced |
|---|---|---|
| Nitrogen fixation | converted to ammonia by bacteria (e.g. Rhizobium in legume roots), lightning, or industrial Haber process | / |
| Ammonification | Decomposers break down dead organisms and waste | (ammonium) |
| Nitrification | Bacteria oxidize ammonium to nitrite then nitrate | , |
| Assimilation | Plants absorb nitrate/ammonium and build proteins and nucleic acids | organic N |
| Denitrification | Bacteria convert nitrate back to gas, returning it to the atmosphere |
The Phosphorus Cycle: A Slow, Rock-Based Cycle
Phosphorus behaves very differently from nitrogen. There is no significant gaseous phase — phosphorus does not enter the atmosphere in meaningful amounts. Instead it moves through rock, soil, water, and organisms, making it a much slower cycle.
The main reservoir is phosphate rock. Weathering of rock releases phosphate ions () into soil and water. Plants take up dissolved phosphate through their roots (plant uptake), and animals get phosphorus by eating plants. When organisms die, decomposition returns phosphate to the soil. Over long timescales, phosphate settles into bodies of water and forms sediment (sedimentation); geologic uplift can eventually expose this sediment as new rock, restarting the cycle over millions of years.
Because phosphorus has no gas phase and cycles slowly, it is often the limiting nutrient in freshwater ecosystems — meaning its scarcity limits how much life the system can support. That detail matters: adding phosphorus (from detergents or fertilizer) can trigger explosive growth where phosphorus was previously the bottleneck.
The main reservoir is phosphate rock. Weathering of rock releases phosphate ions () into soil and water. Plants take up dissolved phosphate through their roots (plant uptake), and animals get phosphorus by eating plants. When organisms die, decomposition returns phosphate to the soil. Over long timescales, phosphate settles into bodies of water and forms sediment (sedimentation); geologic uplift can eventually expose this sediment as new rock, restarting the cycle over millions of years.
| Feature | Nitrogen cycle | Phosphorus cycle |
|---|---|---|
| Atmospheric phase | Yes ( major reservoir) | No |
| Main reservoir | Atmosphere | Rock/sediment |
| Speed | Faster | Slower |
| Key drivers | Bacteria | Weathering, decomposition |
Human Disruptions and Eutrophication
Humans dramatically accelerate both cycles. For nitrogen, the industrial Haber-Bosch process fixes atmospheric into synthetic fertilizer, and combustion of fossil fuels releases nitrogen oxides () that contribute to acid rain and smog. For phosphorus, mining of phosphate rock for fertilizer and detergents removes it far faster than weathering replaces it.
The classic exam scenario is eutrophication. Excess nitrogen and phosphorus from fertilizer runoff, animal waste, and sewage enter waterways. These nutrients fuel rapid algal growth (an algal bloom). The bloom blocks sunlight, and when the algae die, aerobic bacteria decompose them, consuming dissolved oxygen. This produces hypoxic (low-oxygen) dead zones where fish and other aquatic life suffocate.
Walk the causal chain in order for full FRQ credit: nutrient runoff → algal bloom → blocked light and mass die-off → bacterial decomposition → oxygen depletion → death of aquatic organisms. A related term is cultural eutrophication, meaning eutrophication accelerated by human nutrient inputs rather than occurring naturally. Solutions the exam expects include buffer strips of vegetation along waterways, reduced or timed fertilizer application, and treating sewage and animal waste before discharge.
The classic exam scenario is eutrophication. Excess nitrogen and phosphorus from fertilizer runoff, animal waste, and sewage enter waterways. These nutrients fuel rapid algal growth (an algal bloom). The bloom blocks sunlight, and when the algae die, aerobic bacteria decompose them, consuming dissolved oxygen. This produces hypoxic (low-oxygen) dead zones where fish and other aquatic life suffocate.
Walk the causal chain in order for full FRQ credit: nutrient runoff → algal bloom → blocked light and mass die-off → bacterial decomposition → oxygen depletion → death of aquatic organisms. A related term is cultural eutrophication, meaning eutrophication accelerated by human nutrient inputs rather than occurring naturally. Solutions the exam expects include buffer strips of vegetation along waterways, reduced or timed fertilizer application, and treating sewage and animal waste before discharge.
Key terms
- Nitrogen fixation.
- Conversion of atmospheric into ammonia () by bacteria, lightning, or industrial processes, making nitrogen biologically available.
- Nitrification.
- Bacterial oxidation of ammonium () into nitrite () and then nitrate (), the form plants most readily absorb.
- Denitrification.
- Bacterial conversion of nitrate back into gas, the only step that returns nitrogen to the atmosphere.
- Assimilation.
- Uptake of inorganic nitrogen (nitrate or ammonium) or phosphate by plants to build proteins, DNA, and other organic molecules.
- Ammonification.
- Breakdown of dead organisms and waste by decomposers, releasing nitrogen as ammonium ().
- Limiting nutrient.
- The nutrient in shortest supply relative to demand that restricts growth; phosphorus is often limiting in freshwater, nitrogen in marine systems.
- Eutrophication.
- Nutrient enrichment of water that triggers algal blooms, decomposition, and oxygen depletion, forming hypoxic dead zones.
- Weathering.
- Physical and chemical breakdown of phosphate rock that releases phosphate ions into soil and water, the entry point of the phosphorus cycle.
Worked example
A farm applies large amounts of phosphorus- and nitrogen-rich fertilizer to fields beside a freshwater lake. Within weeks the lake surface turns green, and by late summer fish are dying near the bottom. Explain the sequence of events, and identify why phosphorus in particular may have triggered the change.
Start with the source: rain washes excess nitrogen and phosphorus fertilizer off the fields into the lake as runoff. This is nonpoint source pollution because it enters from a broad area rather than one pipe.
Next, connect nutrients to growth. Phosphorus is typically the limiting nutrient in freshwater, so adding it removes the bottleneck on algal growth. The result is a rapid algal bloom that turns the surface green and blocks sunlight from reaching submerged plants, which then die.
Now trace the oxygen crash. The huge mass of algae eventually dies. Aerobic bacteria decompose the dead algae and dead plants, and this decomposition consumes large amounts of dissolved oxygen in the water.
Finally, link to the fish kill. With dissolved oxygen depleted, the water becomes hypoxic, especially in deeper layers. Fish and other aquatic organisms cannot obtain enough oxygen and suffocate — the dead zone described in the problem.
The key reasoning for the phosphorus emphasis: because it was the limiting nutrient, even a modest addition produced a disproportionately large biological response, making it the trigger for the entire eutrophication chain.
Next, connect nutrients to growth. Phosphorus is typically the limiting nutrient in freshwater, so adding it removes the bottleneck on algal growth. The result is a rapid algal bloom that turns the surface green and blocks sunlight from reaching submerged plants, which then die.
Now trace the oxygen crash. The huge mass of algae eventually dies. Aerobic bacteria decompose the dead algae and dead plants, and this decomposition consumes large amounts of dissolved oxygen in the water.
Finally, link to the fish kill. With dissolved oxygen depleted, the water becomes hypoxic, especially in deeper layers. Fish and other aquatic organisms cannot obtain enough oxygen and suffocate — the dead zone described in the problem.
The key reasoning for the phosphorus emphasis: because it was the limiting nutrient, even a modest addition produced a disproportionately large biological response, making it the trigger for the entire eutrophication chain.
Practice questions
Which step of the nitrogen cycle is the only one that returns nitrogen to the atmosphere as gas?
- Nitrification
- Ammonification
- Denitrification
- Assimilation
Answer: Denitrification
Denitrification is carried out by bacteria that convert nitrate () back into nitrogen gas (), releasing it to the atmosphere and closing the cycle. Nitrification produces nitrate, ammonification produces ammonium, and assimilation moves nitrogen into plant tissue — none of these release .
Explain two key differences between the nitrogen and phosphorus cycles, and describe one human activity that disrupts each cycle.
Answer: Nitrogen has a large atmospheric reservoir () and cycles relatively quickly through bacteria, while phosphorus has no significant atmospheric phase, is stored mainly in rock and sediment, and cycles very slowly. Human disruption of nitrogen: fossil fuel combustion releases nitrogen oxides, and the Haber process plus fertilizer add usable nitrogen. Human disruption of phosphorus: mining phosphate rock for fertilizer releases it far faster than weathering.
Full credit requires two genuine contrasts (atmospheric phase and cycling speed, or reservoir type) plus a distinct human activity for each element. Note that fertilizer runoff can be cited for both, but combustion is unique to nitrogen and mining is characteristic of phosphorus, making them cleaner examples.
In a freshwater lake, phosphorus is described as the limiting nutrient. What does this mean, and why does adding phosphorus often trigger eutrophication?
Answer: It means phosphorus is the nutrient in shortest supply relative to what organisms need, so it caps the amount of growth the ecosystem can support. Adding phosphorus removes that cap, allowing algae to grow rapidly into a bloom that eventually dies, is decomposed by oxygen-consuming bacteria, and produces hypoxic conditions.
The concept being tested is that growth is controlled by whatever resource is scarcest. Because phosphorus was the bottleneck, its addition produces a large, disproportionate response, initiating the eutrophication chain of bloom, die-off, decomposition, and oxygen depletion.
FAQ
- What is the main difference between the nitrogen and phosphorus cycles?
- The biggest difference is that nitrogen has a major atmospheric reservoir ( gas) and moves quickly through bacterial conversions, while phosphorus has no significant gaseous phase. Phosphorus is stored mainly in rock and sediment and cycles slowly through weathering, uptake, decomposition, and sedimentation.
- Why can't plants use nitrogen gas directly from the air?
- Nitrogen gas () has a strong triple bond that most organisms cannot break. Plants can only absorb nitrogen after fixation converts it to ammonia/ammonium and nitrification converts it to nitrate. This is why nitrogen-fixing bacteria are essential to the cycle.
- What causes eutrophication and dead zones?
- Excess nitrogen and phosphorus from fertilizer runoff, sewage, or animal waste enter water and fuel algal blooms. When the algae die, bacteria decompose them and use up dissolved oxygen, creating hypoxic dead zones where fish and other aquatic life suffocate.
- How is the phosphorus cycle disrupted by humans?
- Humans mine phosphate rock for fertilizers and detergents, releasing phosphorus much faster than natural weathering would. This phosphorus washes into waterways as runoff, and because phosphorus is often the limiting nutrient in freshwater, it strongly promotes eutrophication.
Learn this with a teacher, not a page
The Crimsora tutor teaches U1.3 Nitrogen and Phosphorus Cycles live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.