⏳ Estimated Reading/Watching Time: 11 - 13 minutes

Explain the steps and reservoir interactions in the nitrogen cycle.

The Cycling of Nitrogen

a nitrogen atom, showing 7 electrons circling the central nucleus — ***Nitrogen has 7 protons, 7 neutrons, and 7 electrons.***

Nitrogen is an important element of living things.

Nitrogen is the most abundant element in the atmosphere and is about 78% of our atmosphere, but it is in the form of nitrogen gas (N2), which cannot be used by living things.

Nitrogen is converted to biologically useful nitrogen through the nitrogen cycle.

In addition to the conversion of nitrogen to usable forms through the nitrogen cycle, the law of conservation of matter states that matter can change form, but it is never destroyed. Thus, the amount of nitrogen on Earth is the same now as it was millions of years ago.

So, the nitrogen needs to cycle through the environment and living things so it can be recycled for further use. Otherwise, we’d run out of nitrogen eventually.

a DNA molecule shaped into a double-helix, showing the 4 nitrogenous bases and the phosphate backbone — ***Nitrogen is in DNA, RNA, and proteins.***

Even though nitrogen isn’t the basis of living things like carbon, it is still really important to living things.

Essential Knowledge

Biogeochemical cycles are essential for life and each cycle demonstrates the conservation of matter.

The atmosphere is the major reservoir of nitrogen.

The nitrogen cycle is the movement of atoms and molecules containing the element nitrogen between sources and sinks.

Phases of the Nitrogen Cycle

There are 5 processes that occur as atmospheric nitrogen is converted and then used by living things. These processes are nitrogen fixation, assimilation, ammonification, nitrification, and denitrification. Explore the tabs below to learn more about each phase of the nitrogen cycle.

Phase 1
Phase 2
Phase 3
Phase 4
Phase 5

Phase 1: Nitrogen Fixation

the nitrogen cycle, showing nitrogen cycling through plants, animals, decomposers, bacteria, and the soil — ***The nitrogen cycle is responsible for converting atmospheric nitrogen into a form usable by living organisms and recycling it for reuse.***

a pea plant on a black background — ***Nitrogen-fixing bacteria are found in the roots of pea plants.***

Although the atmosphere contains 78% nitrogen, living organisms can’t directly use atmospheric nitrogen (N₂). The nitrogen cycle ensures that nitrogen is converted into forms usable by plants and animals. Bacteria can take nitrogen gas found in the atmosphere and convert it to biologically useful forms of nitrogen. This process is called nitrogen fixation.

$LaTeX: 2N_2+H_2\Longrightarrow2N_2O$

Usually, it can occur in two ways. The nitrogen (N₂) can be combined with hydrogen (H₂) to form ammonia (NH₃):

$LaTeX: N_2+3H_2\Longrightarrow2NH_3$

Bacteria can also combine nitrogen (N2) and oxygen to form nitrous oxide (N₂O):

$LaTeX: 2N_2+H_2\Longrightarrow2N_2O$

These bacteria are called nitrogen-fixing bacteria and are commonly found in the roots of plants known as legumes. Two examples of legumes are beans and peas.

The lightning can convert atmospheric nitrogen into nitrous oxide (N₂O or nitrate (NO₃^-).

lightning on a dark night — ***Lightning can form biologically useful nitrogen compounds.***

Lastly, industrial processes can chemically convert nitrogen gas (N₂) into ammonia (NH₃).

Typically, this is done during the process of making fertilizer. Fertilizers are often rich in nitrogen and phosphorous. When they are applied, they increase the amount of nitrogen in the soil, which can runoff into aquatic ecosystems during hard rains or intense irrigation.

Phase 2: Assimilation

Once nitrogen has been converted to a biologically useful form, living organisms can use this nitrogen to make DNA, RNA, and proteins. This is called assimilation.

Any nitrogen that is not used is excreted as waste. You may have noticed that sometimes urine smells like ammonia. This is because urine often has large amounts of ammonia in it, which is a nitrogen-containing compound.

Once the organism dies, it can be recycled during the process of ammonification, which we will discuss next.

Phase 3: Ammonification

bacteria under a scanning electron microscope — ***Ammonification is largely carried out by bacteria.***

Ammonification plays a vital role in recycling nitrogen from dead organisms back into the ecosystem, allowing it to be available for new life. After living organisms use organic nitrogen, decomposers can break the nitrogen in their bodies or released as waste back into ammonia (NH3) in the process of ammonification.

During ammonification, these decomposers convert the organic nitrogen into inorganic ammonia (NH₃) or ammonium ions (NH₄⁺). This conversion makes nutrients available in the ecosystem.

The ammonia formed during ammonification combines with other compounds in the soil to create ammonium compounds. These compounds can then be taken up by plants. As plants grow, they incorporate this nitrogen into their tissues, passing it along the food chain.

Phase 4: Nitrification

After decomposers convert biological nitrogen back into ammonia, the ammonia is converted into nitrites (NO₂-) and nitrates (NO₃-). This is called nitrification. Once nitrogen has been converted to nitrites and nitrates, plants can use either of these compounds for their cellular processes. If nitrogen is not in the right form, plants cannot use nitrogen. They need nitrogen in the correct form, just like other living things.

a venus fly trap — ***Venus fly traps catch prey in bogs.***

A Note on Carnivorous Plants

Most people believe that carnivorous plants like pitcher plants and Venus fly traps consume insects instead of performing photosynthesis. However, the reality is a bit more complex than that. These unique plants actually thrive in nitrogen-deficient environments known as bogs. In such habitats, nitrogen, a crucial nutrient for plant growth, is scarce.

Pitcher plants, for instance, employ a clever strategy to acquire the necessary nitrogen.

They rely on trapping small insects and animals in their pitcher-shaped structures. By digesting these creatures, the plants extract the nitrogen they need to survive and flourish.

the hanging pitchers of a pitcher plant — ***Pitcher plants trap prey for nitrogen in N-poor soils.***

Contrary to popular belief, this feeding behavior is not a replacement for photosynthesis but rather a smart adaptation to obtain essential nutrients.

In essence, carnivorous plants' carnivorous behavior is not a sign of them being lazy or unable to photosynthesize. Instead, it showcases their remarkable ability to adapt to challenging environments and creatively gather the nutrients required for their growth.

So next time you encounter a Venus flytrap snapping shut, remember it's not just about catching prey for a meal - it's about securing the nitrogen necessary for its survival.

Phase 5: Denitrification

The process of denitrification occurs next in the nitrogen cycle. In this process, organic nitrogen is converted back into atmospheric nitrogen. Again, this process is accomplished by decomposing bacteria.

The burning of fossil fuels through industrial processes can also release nitrogen back into the atmosphere, but this is in the form of NO_x and can contribute to acid rain.

Essential Knowledge

Nitrogen fixation is the process in which atmospheric nitrogen is converted into a form of nitrogen (primarily ammonia) that is available for uptake by plants and that can be synthesized into plant tissue.

Putting it all Together

The nitrogen cycle regulates the movement of the element nitrogen. Nitrogen, integral to amino acids and nucleic acids, undergoes a continuous process involving fixation by soil bacteria, absorption by plants, and transfer through the food chain. Decomposers break down organic matter, releasing nitrogen back into the soil. Human activities, notably the use of nitrogen-based fertilizers, have disrupted this cycle, leading to environmental issues like nutrient runoff. The video from Khan Academy below will provide you with a brief overview of the nitrogen cycle and its importance to ecology:

A transcript of the video is available on this page.

Essential Knowledge

Most of the reservoirs in which nitrogen compounds occur in the nitrogen cycle hold those compounds for relatively short periods of time.

AP Exam Tip

You will need to know which types of nitrogen can be used by living things and which cannot.

You should know the ways human activity affects the nitrogen cycle.

[CC BY-NC-SA 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION - INTENDED ONLY FOR USE WITHIN LESSON.