XXIV. Read the following paragraphs on the nitrogen cycle. Some of the sentences have numbers after them. Write that number in the corresponding box in the picture below

XXII. Read and translate the text.

Water travels through out the earth through the hydrological cycle. As the sun radiates energy to the earth, the water on the earth, such as that in lakes, rivers, and oceans, evaporates. Evaporation is the process by which water changes from a liquid to a gas. Trees and plants absorb water from the soil, and this water also evaporates through their leaves. As the gas rises in the atmosphere, it cools and changes back to a liquid form through the process of condensation. As a result of condensation clouds form. Within the clouds, the droplets of water grow and eventually return to the earth as some form of precipitation, either rain or snow. Some of the water returnes to the sea in streams and rivers through runoff.

XXIII.Write each of the following words in the appropriate arrow drawn in the diagram: Evaporation, condensation, absorption, precipitation, runoff.

Almost 80% of the air is nitrogen gas. But most living things cannot use nitrogen in this form. It must first be changed into nitrogen compounds. The process whereby nitrogen gas is changed into ni­trogen compounds is called nitrogen fixation. The nitrogen cycle is the movement of nitrogen through an ecosystem.

There are two ways that nitrogen is fixed (changed from nitrogen gas to nitrogen compounds). The first is through lightning. As lightning passes through the air, it causes nitrogen to combine with oxygen to form nitrates (1). These compounds are washed out of the air to the earth by rain. Then plants and animals can use nitrogen in this form.

The second way that nitrogen can be fixed is by the actions of certain microorganisms called blue-green bacteria (2). After the nitrogen has been fixed, plants use this form of nitrogen (3). Then, animals eat the plants (4). When the animals die, the nitrogen in their bodies returns to the soil (5).

In order to complete the cycle, there has to be some nitrogen gas returned to the air so the process can start again. This is done by denitrifying bacteria (6). These bacteria live in the soil and change nitrogen compounds back into nitrogen gas.

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