Nitrogen Cycle 🌱
Nitrogen is a macro-nutrient, and the nitrogen cycle is the process by which it is exchanged between the atmosphere, land, and water. Nitrogen is an important component of proteins, DNA, and other biomolecules and is necessary for the growth and development of all living organisms.
The nitrogen cycle has a lot of chemical transformations, so while technically the exam can ask you about the nitrogen cycle, it tends to focus on later chapters and global trends, so don’t focus too much on memorizing all the chemical processes (this isn’t AP Chem!). Instead, focus on why the nitrogen cycle is important and what it can affect/how this change occurs and why.
Nitrogen gas (N2) makes up 78% of the Earth's atmosphere, but it is not readily available to most living organisms. Nitrogen fixation allows nitrogen gas to be converted into a form that can be used by plants and animals, such as ammonia (NH3) or nitrate (NO3). Nitrogen fixation can happen in two different ways: biotic and abiotic. In the biotic pathway, nitrogen-fixing bacteria such as bacteria in the roots of certain plants convert nitrogen gas into ammonia (NH3), which then quickly bonds with hydrogen ions to become ammonium (NH4).
Later, during nitrification, ammonia and other compounds become nitrite (NO2) and later nitrate (NO3). Bacteria in soil and water allow this step of the nitrogen cycle to happen. It is important for bacteria to keep the amount of nitrate at bay to prevent pollution or overabundance of algal blooms in bodies of water. This can also occur in the atmosphere when N2 is exposed to lightning, fires, or fossil fuel combustion, which converts it directly to NO3. The nitrate then enters the soil through precipitation.
In assimilation, nitrogen is in its usable form, and gets absorbed by plant tissues. This nitrogen can be synthesized by consumers, or it can run into the ocean, providing aquatic ecosystems with enough nitrogen. When these organisms die, decomposition occurs and organic nitrogen present in these organisms through the nitrogen cycle reverts back into ammonium. This is called mineralization, and is the last step before nitrification can restart.
At the end of the nitrogen cycle, certain species of bacteria take nitrates and convert them into nitrous oxide (N2O) and eventually back into N2. This last step is appropriately called denitrification, and permits the cycle to once again restart.
- Nitrification: Nitrification is the process by which ammonia and other nitrogen compounds are converted into nitrite (NO2) and nitrate (NO3). Nitrification is carried out by bacteria in soil and water.
- Assimilation: Nitrate is taken up by plants through their roots and is incorporated into proteins, DNA, and other biomolecules.
- Denitrification: Denitrification is the process by which nitrate is converted back into nitrogen gas. Denitrification is carried out by bacteria in soil and water.
Once nitrogen is in usable form, plants can assimilate it, or incorporate it into their tissues. Consumers who eat the plants will also synthesize some of the nitrogen into their tissues. Some nitrogen also leaches into the ocean, either through runoff or precipitation. This is where aquatic organisms can obtain the nitrogen they need.
When organisms die, decomposers break down their tissues and convert the organic nitrogen (the nitrogen in their tissues) back into inorganic ammonium. This process is called mineralization or ammonification. After ammonification, the nitrification process can begin again.
The final step of the nitrogen cycle is denitrification, which returns nitrogen to the atmosphere. During denitrification, specialized bacteria convert nitrate into nitrous oxide (N2O) and then back into nitrogen gas (N2).
Human Impacts on the Nitrogen Cycle
Nitrogen is often classified as a limiting nutrient, meaning that its essentiality is coupled with the fact that it is scarce and hard to find. Since Earth is a closed system, there isn't a way for us to create more of it, but plants still require nitrogen to grow. As a result of this, nitrogen is common in fertilizers. While human usage of fertilizer now exceeds nature's fixation of it, its overuse can have negative consequences.
Even given nitrogen's ability to increase plant growth rate, excess nitrogen can quickly and severely disrupt balances in ecosystems of any size. In specific, an ecosystem's species richness, or the amount of species present, will die down due to the fact that plants with greater need for nitrogen will use it all and deplete it from plants with less need for it.
.
Frequently Asked Questions
What is the nitrogen cycle and why is it important?
The nitrogen cycle is how nitrogen atoms move between the atmosphere, soil, living things, and water—with most nitrogen held briefly in soil and organisms while atmospheric N2 is the major reservoir. Key steps: nitrogen fixation (N2 → NH3/NH4+ by nitrogenase in bacteria like Rhizobium, cyanobacteria, lightning, or the Haber–Bosch process), nitrification (NH4+ → NO2− → NO3− by soil bacteria), ammonification (organic N → NH4+), and denitrification (NO3− → N2 back to the atmosphere). It’s important because nitrogen limits plant growth; fertilizer or runoff increases nitrate, causing leaching and eutrophication in aquatic systems. For the AP exam, you should be able to name these steps, identify reservoirs, and explain human impacts (Haber–Bosch fertilizer, eutrophication). Want a quick study guide and practice questions that map to the CED? Check the Topic 1.5 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and more unit review (https://library.fiveable.me/ap-environmental-science/unit-1).
How does nitrogen fixation actually work in simple terms?
Nitrogen fixation is the step that turns inert atmospheric N2 into a form plants can use (mostly ammonia, NH3). In simple terms: special microbes (like Rhizobium in legume root nodules or free-living cyanobacteria) use the enzyme nitrogenase to break the very strong N≡N bond in N2 and combine N with hydrogen to make ammonia. Plants take up ammonium (NH4+) or nitrate (NO3–) after other microbes convert ammonia (nitrification). Nonbiological fixation also happens—lightning provides energy to form usable nitrogen compounds, and humans do it industrially with the Haber–Bosch process to make fertilizer. For the AP exam, know the key players (nitrogenase, Rhizobium, cyanobacteria), the main product (ammonia), and how fixation connects reservoirs (atmosphere → soil → plants). For a quick review, see the Topic 1.5 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and more practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between nitrogen fixation and nitrification?
Nitrogen fixation and nitrification are different steps in the nitrogen cycle. Fixation converts unusable atmospheric N2 into a biologically available reduced form (primarily ammonia NH3 or ammonium NH4+)—done by the nitrogenase enzyme in free-living bacteria, symbiotic bacteria like Rhizobium in legumes, cyanobacteria, by lightning, or industrially via the Haber–Bosch process (EK ERT-1.E.3). Nitrification is an oxidative two-step microbial process that turns ammonia/ammonium into nitrite (NO2−) and then into nitrate (NO3−)—typically performed by nitrifying bacteria (e.g., Nitrosomonas then Nitrobacter). Plants more readily take up nitrate, so fixation supplies reduced N while nitrification converts that reduced N into the nitrate form plants commonly use. This distinction is tested in the CED’s nitrogen-cycle objectives (ERT-1.E); review the Topic 1.5 study guide on Fiveable for examples (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and try practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why can't plants just use nitrogen directly from the atmosphere?
Because atmospheric N2 is very stable—its two nitrogen atoms are held by a strong triple bond—most plants can’t break that bond to use nitrogen directly. Plants take up nitrogen as ammonia/ammonium (NH3/NH4+) or nitrate (NO3–), not N2. Nitrogen fixation (done by nitrogenase enzymes in free-living bacteria, cyanobacteria, or symbionts like Rhizobium in legumes, plus abiotic routes like lightning or the Haber–Bosch process) converts N2 into ammonia that plants can use. After fixation, nitrification, ammonification, and denitrification move N through soil and organisms (CED keywords). For the AP exam, know that the atmosphere is the major N reservoir and that fixation is required to make N biologically available (ERT-1.E, EK ERT-1.E.3). For a quick topic review, check the Fiveable nitrogen-cycle study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE).
I'm confused about all the different forms of nitrogen - can someone explain N2, NH3, NO3, etc.?
Think of nitrogen as the same element in different chemical “forms” that move through ecosystems. - N2 (dinitrogen): inert gas in the atmosphere (major reservoir). Plants can’t use it directly. - NH3 (ammonia) and NH4+ (ammonium): products of nitrogen fixation (biological via nitrogenase in Rhizobium/cyanobacteria or industrial Haber–Bosch) and ammonification (decomposition). Ammonium (NH4+) is the protonated form in soil water and is plant-available. - NO2− (nitrite) and NO3− (nitrate): produced by nitrification (NH3/NH4+ → NO2− → NO3−) by bacteria. Nitrate (NO3−) is very mobile in soil and easily taken up by plants but also leaches into waterways causing eutrophication. - Organic N: nitrogen bound in proteins, DNA, and soil organic matter; becomes NH4+ via ammonification. - Denitrification converts NO3− back to N2 (returns N to atmosphere). These steps and reservoirs (fixation, nitrification, ammonification, denitrification, leaching, eutrophication) are AP CED keywords—review the Topic 1.5 study guide on Fiveable for practice (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE).
What are the main steps of the nitrogen cycle in order?
Start with the atmospheric reservoir (N2—the major reservoir). Then the main steps in order: 1. Nitrogen fixation—N2 → NH3 (ammonia)/NH4+. Carried out biologically by nitrogenase in bacteria (Rhizobium in legume symbiosis, cyanobacteria), by lightning, and industrially via the Haber–Bosch process. 2. Assimilation—Plants take up NH4+ or NO3− and build organic nitrogen (proteins, nucleic acids); animals get N by eating plants. 3. Ammonification (mineralization)—Decomposers break down organic N from dead organisms/waste to NH3/NH4+. 4. Nitrification—NH4+ → NO2− (Nitrosomonas) → NO3− (Nitrobacter); converts ammonium to nitrate. 5. Denitrification—NO3− → N2 (or N2O) by anaerobic bacteria, returning gaseous N to the atmosphere. 6. Additional processes/impacts—Leaching and runoff of NO3− can cause eutrophication. For the APES exam you should be able to explain these steps, the reservoir interactions, and name key players (nitrogenase, Rhizobium, legumes)—see the Topic 1.5 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE). For broader review, check Unit 1 (https://library.fiveable.me/ap-environmental-science/unit-1) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
How do bacteria help with nitrogen fixation and where are they found?
Bacteria convert inert atmospheric N2 into ammonia (NH3) through the enzyme nitrogenase—that process is nitrogen fixation (EK ERT-1.E.3). Fixed ammonia is quickly turned into ammonium (NH4+) plants can take up or into organic nitrogen in tissues. Key bacteria: Rhizobium (and related genera) form symbiotic root nodules on legumes—they live inside nodule cells and get carbohydrates while supplying fixed N. Free-living soil bacteria (e.g., Azotobacter) and filamentous cyanobacteria (blue-greens) also fix N in soils, freshwater, and some marine/peat habitats. Nitrogenase is O2-sensitive, so many fixers live in low-O2 microenvironments (root nodules, heterocysts in cyanobacteria, soil aggregates). For AP exam framing, link this to reservoirs and fluxes in the nitrogen cycle (EKs and keywords: nitrogenase, Rhizobium, cyanobacteria). Review this topic on Fiveable’s study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and try practice questions to nail the concept.
Why is the atmosphere considered the major reservoir for nitrogen?
Because most of Earth's nitrogen is in the form of N2 gas in the atmosphere, the atmosphere holds far more nitrogen than soils, living things, or water—so it’s the major reservoir (CED EK ERT-1.E.4). Atmospheric N2 is very stable (triple bond) and biologically unavailable until converted by nitrogen fixation (by nitrogenase in bacteria like Rhizobium or cyanobacteria, by lightning, or industrially via the Haber–Bosch process) into ammonia/ammonium that plants can use (EK ERT-1.E.3). Other reservoirs (soil organic N, NH4+, NO3–, biomass) cycle nitrogen more quickly and hold it for shorter times (EK ERT-1.E.2). For the AP exam, be ready to explain that the atmosphere is the largest pool of N, why fixation is required to move N into ecosystems, and how processes like nitrification, ammonification, and denitrification connect reservoirs. For a quick topic review, see the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
What happens during denitrification and why does it matter?
Denitrification is a microbially driven step in the nitrogen cycle where facultative anaerobic bacteria (e.g., Pseudomonas, Paracoccus) convert nitrate (NO3–) into gaseous forms—nitrous oxide (N2O) and dinitrogen (N2)—which return to the atmosphere. It happens in low-oxygen soils and wetlands when microbes use NO3– as an alternative electron acceptor. Why it matters: denitrification closes the biological nitrogen cycle by removing bioavailable nitrogen and replenishing the atmospheric N2 reservoir (CED EK ERT-1.E.4). That’s good because it can reduce excess nitrate that causes leaching and eutrophication, but it’s also a source of N2O, a potent greenhouse gas. On the AP exam know the steps (fixation, nitrification, ammonification, denitrification), common organisms/enzymes, and environmental consequences. For a quick review, see the Topic 1.5 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and more unit material (https://library.fiveable.me/ap-environmental-science/unit-1). Practice questions: (https://library.fiveable.me/practice/ap-environmental-science).
How do fertilizers affect the natural nitrogen cycle?
Fertilizers add large amounts of reactive nitrogen (ammonia NH3/NH4+ and nitrate NO3–) into the soil, speeding up the nitrogen cycle and changing reservoir interactions. Instead of atmospheric N2 being fixed slowly by nitrogenase, the Haber–Bosch process produces lots of plant-available N, so more nitrification (NH4+ → NO2– → NO3–) and ammonification occur. Excess nitrate is highly mobile—it leaches into groundwater and runs off into streams and lakes, causing eutrophication and downstream hypoxic “dead zones.” Microbial denitrification can increase, releasing N2O, a potent greenhouse gas. On the APES exam, expect questions linking fertilizers to leaching, eutrophication, altered nitrification/denitrification rates, and increased N2O emissions (Topic 1.5 keywords: Haber–Bosch, nitrification, denitrification, leaching, eutrophication). For a focused review see the Topic 1.5 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE). More practice problems are at (https://library.fiveable.me/practice/ap-environmental-science).
I don't understand why nitrogen compounds only stay in reservoirs for short periods of time - what does that mean?
“Hold for relatively short periods” means nitrogen compounds don’t stay put long—they move (turn over) quickly between pools like soil ammonium/nitrate, plants, animals, and water. The atmosphere (N2) is the big long-term reservoir, but once N2 is fixed (by bacteria, lightning, or Haber–Bosch) into ammonia/ammonium or converted to nitrate, those forms are rapidly: taken up by plants, mineralized back to ammonium (ammonification), oxidized (nitrification), lost by denitrification to N2/N2O, or leached into waterways. So “short” is relative: these biological/soil pools cycle on time scales of days–years, not the geologic times that keep atmospheric N2. For the AP exam, be able to name the processes (fixation, nitrification, ammonification, denitrification) and explain reservoir interactions per EK ERT-1.E.2–E.4. Review the Topic 1.5 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and practice questions (https://library.fiveable.me/practice/ap-environmental-science) to quiz yourself on residence times and flows.
What's the role of decomposers in the nitrogen cycle?
Decomposers are key to the nitrogen cycle because they convert organic nitrogen in dead plants, animals, and waste back into inorganic forms that ecosystems can reuse—primarily by ammonification: microbes (fungi and bacteria) break down proteins and nucleic acids into ammonia (NH3) or ammonium (NH4+). That released NH4+ can be taken up by plants or transformed by nitrifying bacteria into nitrite (NO2–) and then nitrate (NO3–), which is prone to leaching and can drive eutrophication. Some decomposer bacteria in anaerobic soils perform denitrification, returning N2 to the atmosphere and closing the loop. In short, decomposers perform mineralization (ammonification) and influence reservoir interactions and short residence times of nitrogen—exactly what the CED’s ERT-1.E expects you to explain. For a focused review, see the Topic 1.5 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and try practice questions (https://library.fiveable.me/practice/ap-environmental-science).
How does the nitrogen cycle connect to other biogeochemical cycles like carbon and phosphorus?
Nitrogen links tightly to the carbon and phosphorus cycles because they all control productivity, nutrient availability, and ecosystem responses. Nitrogen availability (from fixation, ammonification, nitrification, denitrification) often limits plant growth; when N increases, plants fix more carbon via photosynthesis, changing carbon storage and CO2 fluxes. Decomposition/ammonification releases both CO2 (carbon cycle) and inorganic N back to soil. Human N inputs (Haber–Bosch fertilizer) boost crop yields but also increase N2O (a potent greenhouse gas) and drive eutrophication when runoff carries N and P into water, causing algal blooms that alter carbon cycling and oxygen budgets. Phosphorus behaves similarly in promoting productivity and eutrophication but has no large atmospheric reservoir. For AP review, focus on processes (fixation, nitrification, denitrification, ammonification) and how N availability controls primary productivity and links to CO2 and P-driven eutrophication (see the Topic 1.5 study guide: https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE). More practice problems: https://library.fiveable.me/practice/ap-environmental-science.
Why do farmers need to add nitrogen fertilizers if nitrogen is so abundant in the atmosphere?
Because most atmospheric nitrogen is N2, which is very stable, plants can’t use it directly. Nitrogen must be “fixed” into reactive forms (ammonia NH3/ammonium NH4+ or nitrates NO3–) by nitrogenase in bacteria (Rhizobium, cyanobacteria), by lightning, or industrially via the Haber–Bosch process. Natural fixation rates and soil pools can’t match the huge N demand of high-yield crops; plants take up fixed N, harvest removes it, and losses (leaching, denitrification) shrink soil N further. So farmers add synthetic or organic fertilizers to replace lost/used nitrogen and keep yields high. This fits EK ERT-1.E.3 and connects to nitrification/ammonification/denitrification steps you should know for the exam. For a quick review, see the Topic 1.5 study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and more practice at the Unit 1 overview (https://library.fiveable.me/ap-environmental-science/unit-1) or practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Can you explain what happens to nitrogen when plants and animals die?
When plants and animals die, decomposers (bacteria and fungi) break their organic nitrogen-containing tissues down in a process called ammonification: organic N → ammonia (NH3) or ammonium (NH4+). That ammonium can be taken up directly by plants or oxidized by nitrifying bacteria through nitrification: NH4+ → NO2- → NO3-. Nitrate (NO3-) is highly mobile in soil, so it can be leached into waterways and contribute to eutrophication. In oxygen-poor soils, denitrifying bacteria can convert nitrate back to N2 or N2O gas (denitrification), returning nitrogen to the atmosphere. Most of these reservoirs hold nitrogen for short periods (soil organic N, ammonium, nitrate). This sequence—ammonification → nitrification → uptake/leaching → denitrification—is exactly what APES expects you to know (Topic 1.5; ERT-1.E). For a concise review, see the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-1/nitrogen-cycle/study-guide/2pAcyAUnoJkC6dEVNgxE) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).