LWE: Lesson - The Nitrogen Cycle (Topic 1.5) 📖

⏳ Estimated Reading/Watching Time: 11 - 13 minutes

Learning Objective

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: 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_2O2N2+H22N2O

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_3N2+3H22NH3

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

LaTeX: 2N_2+H_2\Longrightarrow2N_2O2N2+H22N2O

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.

 

 

 

 

 

 

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 Links to an external site..

 

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.

 

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