Text 1

This is because of the strong triple bond between its atoms that makes it difficult to break. Something needs to happen to the nitrogen gas to change it into a form that it can be used. At some later stage, these new compounds must be converted back into nitrogen gas so that the amount of nitrogen in the atmosphere stays the same. This process of changing nitrogen into different forms is called the nitrogen cycle.

³ The nitrogen cycle is a biogeochemical cycle that describes how nitrogen and nitrogen-containing compounds are changed in nature. The nitrogen cycle is made up of the following processes:

Nitrogen fixation - The process of converting inert nitrogen gas into more useable nitrogen compounds such as ammonia.

Nitrification - The conversion of ammonia into nitrites and then into nitrates, which can be absorbed and used by plants.

Denitrification - Conversion of nitrates back to atmospheric nitrogen gas.

⁴ Nitrogen fixation is needed to change gaseous nitrogen into forms such as ammonia that are more useful to living organisms. Most fixation is done by different types of bacteria. Some of these bacteria live in the soil while others live in the root nodules of legumes (e.g. soy, peas and beans). In these roots they form a mutualistic relationship with the plant. The bacteria get carbohydrates (food) from the plant and, in exchange, produce ammonia which can be converted into nitrogen compounds that are essential for the survival of the plant. In nutrient-poor soils, planting lots of legumes can help to enrich the soil with nitrogen compounds.

⁵ Another important source of ammonia in the soil is decomposition. When animals and plants die, the nitrogen compounds that were present in them are broken down and converted into ammonia. This process is carried out by decomposition of bacteria and fungi in the soil.

⁶ Some fixation occurs in fertilizer processes and also in lightning strikes. In the Haber-Bosch process, nitrogen (N2) is converted together with hydrogen gas (H2) into ammonia (NH3) fertilizer. This is an artificial process. In the atmosphere, lightning and photons are important in the reaction between nitrogen (N2) and oxygen (O2) to form nitric oxide (NO) and then nitrates. No ammonia is created in this type of fixation.

⁷ Nitrification is an important step in the nitrogen cycle in soil because it converts the ammonia (from the nitrogen fixing part of the cycle) into nitrates, which are easily absorbed by the roots of plants. This absorption of nitrates by plants is called assimilation. Once the nitrates have been assimilated by the plants, they become part of the plants’ proteins. These plant proteins are then available to be eaten by animals. In other words, animals (including humans) obtain their own nitrogen by feeding on plants. Nitrification is performed by bacteria in the soil, called nitrifying bacteria.

⁸ Denitrification is the process of reducing nitrate and nitrite into gaseous nitrogen. The process is carried out by denitrification bacteria. The nitrogen that is produced is returned to the atmosphere to complete the nitrogen cycle.

Human influences on the nitrogen cycle have been significant in a number of ways.

⁹ Both artificial fertilization and the planting of nitrogen fixing crops increase the amount of nitrogen in the soil. In some ways this has positive effects because it increases the fertility of the soil, and means that agricultural productivity is high. However, if there is too much nitrogen in the soil, it can run off into nearby water courses such as rivers, or can become part of the groundwater supply.

Note: The words in bold are assessed See Unit 5 Vocabulary. The words in italics are in the glossary below.

Glossary

Triple bond (n) - A triple bond is a type of chemical bond consisting of three distinct covalent bonds linking two atoms in a molecule.the sharing of three pairs of electrons between two atoms in a molecule, usually represented in structural formulas by three dots, as in C⋮C.

Nitrogen cycle (n) - The nitrogen cycle is the natural circulation of nitrogen by living organisms. Nitrates in the soil, derived from dead organic matter by bacterial action, are absorbed and synthesized into complex organic compounds by plants and reduced to nitrates again when the plants and the animals feeding on them die and decay .

Nitric oxide (n) - Nitric oxide is a colourless slightly soluble gas forming red fumes of nitrogen dioxide in air.Formula: NO .

AnalyZing The Text: Main Ideas

Read the first 8 paragraphs of Text 1. What is the main idea of each paragraph?

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Write a sentence summarizing all the points together in a few words.

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Write a different title for Text 1.

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analyzing the text: making inferences

To infer is to conclude information from some evidence in the text. Answer the following questions by using inferential skills:

1. The text states that nitrogen makes up 78% of the air we breathe in and that “Nitrogen (N2) is essential for many biological processes.” Do we get nitrogen for our bodies directly from breathing in air?

2. Why is nitrogen so unreactive? (Hint: think in terms of bonding.)

3. The text states that, “Nitrogen fixation is needed to change gaseous nitrogen into forms such as ammonia that are more useful to living organisms. Some fixation occurs in lightning strikes....” How do lightning strikes fix nitrogen in the soil? What do you think is the main factor that causes this to happen?

4. When nitric oxide (NO) is formed in the atmosphere by lightning, what is the result of its reaction with water vapor?

5. What is another source of NO in the air?