CYCLES OVERVIEW

•Carbon

•Nitrogen

•Water

Carbon Cycles Through the Environment Carbon dioxide makes up only .03% of the air but it is an

important gas. It is used by plants during photosynthesis. The carbon dioxide is taken in through the plant leaves. Green plants use

carbon dioxide to manufacture their own food; they release oxygen as a by-product. Animals then use the plants for food.

For animals, carbon dioxide is a waste product that is given off when they obtain energy from their food.

The process by which carbon dioxide is taken from the air, and then returned is called the carbon cycle.

The amount of carbon dioxide used by plants is approximately equal to the amount used by animals and other natural processes.

This balance is changed by the burning of fossil fuels such as oil and coal. As a result, it appears that the amount of carbon dioxide in the atmosphere

is increasing.

Carbon Cycles through the Environment

Nitrogen Cycles through the Environment Nitrogen is the most plentiful of all gases in the air, making up

about 78% of the volume of the atmosphere. You (and other animals) do not use the nitrogen in the air; although you take it into your lungs with each breath, you exhale it out.

There are plants that need nitrogen to make proteins, but they cannot get it from the air. Some bacteria that live in the soil and the roots of some plants, such as clover, take nitrogen from the air and change it into forms that plants can use.

Animals eat the plants and get the nitrogen they need through the food web. Decomposers eat dead plants and animals and cause nitrogen to be released back into the atmosphere.

This process is called the nitrogen cycle. The cycle returns as much nitrogen to the air as is removed, so that atmospheric nitrogen levels stay approximately constant.

Nitrogen-fixing Rhizobium bacteria live in roots of plants like soybeans, clover and alfalfa.

Nitrogen Cycles through the Environment

How much water is there on (and in) the Earth?

As you know, the Earth is a watery place. About 70 percent of the Earth's surface is water-covered.

Water also exists in the air as water vapor and in the ground as soil moisture and in aquifers.

Thanks to the water cycle our planet's water supply is constantly moving from one place to another and from one form to another. Things would get pretty stale without the water cycle!

How much water is there on (and in) the Earth?

When you take a look at the water around you, you see water in streams, rivers, and lakes. You see water sitting on the surface of the earth. Naturally, this water is known as "surface water." Your view of the water cycle might be that when rain falls it fills up the rivers and lakes. But, how would you account for the flow in rivers after weeks without rain? The answer is that there is more to our water supply than just surface water, there is also plenty of water beneath our feet.

Even though you may only notice water on the Earth's surface, there is much more water stored underground than on the earth’s surface. In fact, some of the water you see flowing in rivers comes from seepage of ground water into river beds. Water from precipitation continually seeps into the ground to recharge the aquifers, while at the same time water from underground aquifers continually recharges rivers through seepage.

Earth's WaterDistribution

The left-side pie chart shows that over 99 percent of all water (oceans, seas, ice, and atmosphere) is not available for our uses.

Even of the remaining 0.3 percent (the small brown slice in the top pie chart), much of that is out of reach.

Considering that most of the water we use in everyday life comes from rivers (the small yellow slice in the right-side pie chart), you'll see we generally only make use of a tiny portion of the available water supplies.

The right-side pie shows that the vast majority of the fresh water available for our uses is stored in the ground (the large brown slice in the second pie chart).

Water on Our Planet