Nitrogen Cycle




About 78 percent of the atmosphere is made up of "free" nitrogen, or nitrogen that is not combined with other elements.
All living things need nitrogen to build proteins and certain other body chemicals.
However, most organisms -- including plants, animals, and fungi -- cannot get the nitrogen they need from the free nitrogen in the air.
They can use only nitrogen that is combined with other elements in compounds.
But how are these nitrogen-containing compounds made?

Nitrogen transferred from the non-living portion of the environment into living things:

Certain kinds of bacteria are able to use the free nitrogen in the air to make nitrogen compounds through a process known as nitrogen fixation.
Most of the nitrogen fixation on Earth occurs as a result of the activity of bacteria.
Some of these bacteria live in the soil.
Others grow inside special structures on the roots of certain plants, including beans, clover, alfalfa, peas, and peanuts.

One family of nitrogen compounds produced by nitrogen-fixing bacteria consists of substances called nitrates.
Nitrates can be taken from the soil by plants.
Inside the plants, the nitrogen in the nitrates is used to make compounds such as proteins.
The compounds made by the plants can be used by animals, fungi, and other organisms that cannot use nitrates directly.

Look at the illustration below and trace the steps of the nitrogen cycle from the free nitrogen in the air to the nitrogen in the bodies of animals.


The following picture illustrates the various paths nitrogen takes as it travels through the environment in a cycle. Arrows indicate the direction of the flow. All organisms, including people, need nitrogen to survive. Nitrogen is an important element in proteins. People get their nitrogen either by eating plants or by eating animals that have eaten plants.

Use the following picture to answer questions 2 through 5.
Where are nitrogen-fixing bacteria located on plants know as legumes?


What are two ways in which nitrogen can get into the soil?


What is the name of the process used by denitrifying bacteria to break down nitrates in the soil?


Describe three different pathways that nitrogen can take through the nitrogen cycle.


The following diagram describes the Nitrogen Cycle. The description is missing one phase or part. The missing phase (or part) would have gone in the blank rectangle.
Write a phrase or sentence that fills in the missing part.


Study the Nitrogen Cycle in the following illustration and explain in your own words what is happening.


Consider all the cycles (Water, Oxygen - Carbon and Nitrogen) you have studied. What chemicals flow in cycles from the nonliving to living things and back again?


In your own words, explain what a natural cycle is.


What would happen if the cycle did not occur?


Most farmers periodically rotate nonleguminous plants, such as corn, with leguminous plants, such as alfalfa. What are the advantages of this practice?


Nitrogen transferred from the nonliving portion of the environment into nonliving things:

Decomposers, such as bacteria, break down the complex nitrogen compounds in dead organisms and animal wastes.
This returns simple nitrogen compounds to the soil.
These simple compounds may be used by bacteria to make nitrates.

Nitrogen can go back and forth between the soil and plants and animals many times.

Eventually, however, certain kinds of bacteria break down nitrogen compounds to produce free nitrogen.

The free nitrogen is released into the air, completing the cycle.

What organism is involved in the most nitrogen fixation on Earth?


In ecology, what is meant by recycling?


What are the different kinds of bacteria in the nitrogen cycle? Give the function of each.


What do plants make that require nitrogen?


What is the element that makes up most of the Earth's atmosphere?

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