Cycles in Nature

There is a general order to many things in nature.

Each 24 hours here in East Moline, night follows day. The length of each may vary, but night follows day, over and over again.

In August, when we start school, it is always technically Fall or Autumn. The season following that will always be Winter, followed by Spring, and then Summer. Despite variations from the traditional expectations which may occur in each, these things follow cycles: they start at any given point, change in an established order, and eventually come back to the starting point.

Some Major Cycles in Nature

The cycles we’ll be discussing are called “biogeochemical cycles”, “bio” meaning life, “geo” referring to rocks, soils, water and air, and “chemical” referring to the changes that occur as the elements go through these cycles.

There are two main types of these cycles: gaseous and sedimentary. In a gaseous cycle, the element is mainly stored in the earth’s atmosphere where it exists as a gas.  Sedimentary cycles have the earth’s crust as the main storage area for their elements.

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We believe that it's vital to understand how the carbon cycle works in order to see the danger of it not working. Therefore, let's look at a sample carbon cycle and explore how carbon atoms move through our natural world. Plants, animals, and soil interact to make up the basic cycles of nature. In the carbon cycle, plants absorb carbon dioxide from the atmosphere and use it, combined with water they get from the soil, to make the substances they need for growth. The process of photosynthesis incorporates the carbon atoms from carbon dioxide into sugars. Animals, such as the rabbit pictured here, eat the plants and use the carbon to build their own tissues. Other animals, such as the fox, eat the rabbit and then use the carbon for their own needs. These animals return carbon dioxide into the air when they breathe, and when they die, since the carbon is returned to the soil during decomposition. The carbon atoms in soil may then be used in a new plant or small microorganisms. Ultimately, the same carbon atom can move through many organisms and even end in the same place where it began. Herein lies the fascination of the carbon cycle; the same atoms can be recycled for millennia!

The carbon cycle is an important gaseous cycle. Most of the carbon in our world is stored as carbon dioxide, in either the atmosphere or dissolved in the ocean. Plants need carbon dioxide to carry on photosynthesis, a process where the carbon atoms become part of molecules of simple carbohydrates (sugars) and may later be changed to fats, proteins and DNA or more complex carbohydrates. The carbon becomes part of the energy system for ecosystems, since fats and carbohydrates are forms of stored energy. The carbon atoms travel through food chains, with decomposers at any step of the way. To fuel their bodies living things take in oxygen from their environment and use it to oxidize carbohydrates to release energy, the reverse process of photosynthesis. The process in which this energy is made available for growth and other activities is called respiration, and produces carbon dioxide which must be excreted from the body, usually through lungs, gills or leaves.

The process of respiration makes carbon dioxide available for photosynthesis. Many living things build shells of calcium carbonate (a combination of carbon, calcium and oxygen) and when they shed them in growth or when they die these shells may accumulate and build up into rocks. The White Cliffs of Dover is a famous example of this. Carbon in animals might stay in its bones until the bone decays after the animal’s death, or might leave an animal’s body relatively quickly with waste matter. Some carbon in dead bodies of plants or animals becomes trapped and fossilized before decomposure is complete, and can form materials such as peat, coal or oil.

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The nitrogen cycle involves the earth’s atmosphere, which is about 78 per cent nitrogen. Nitrogen is of no use by itself to most living things, and we breathe it in and back out again. However, nitrogen is vital to life when combined with other elements. The nitrogen cycle is largely dependent on bacteria, fungi and blue-green algae which convert the nitrogen from the air into nitrogen compounds. These nitrogen-fixing organisms may live in the soil or on the roots of plants. The nitrogen compounds they generate may be given off into the soil or become available when the nitrogen-fixing organisms die, at which point the nitrogen compounds move through food chains.4 Nitrogen is returned to circulation when the bodies of dead plants and animals disintegrate, and when animals excrete uric acid after breaking down proteins in their food. The excreted matter is eventually converted to nitrates after complex bacterial action.5 Nitrogen compounds can thus cycle from the soil to plants to bacteria and back to the soil again and again without returning to the atmosphere. However, denitrifying bacteria and fungi break down nitrogen compounds and release nitrogen gas into the atmosphere.

The nitrogen cycle also has a component affected by humans. Industrialists have learned how to take nitrogen gas from the air and “fix” it in compounds that are an important part of fertilizers. It is significant to note that the amount of nitrogen treated this way has been doubling every six years. Since large amounts of these nitrogen compounds tend to wash from the lands where they are used into the waters of our streams, lakes and rivers, they are having an effect the size of which is still being studied. When these waters become highly nitrogen enriched, there is too much plant growth and a loss of oxygen which can result in the death of fish and other animals.

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The water cycle is different from the other two cycles we will talk about, because of the fact that water can exist in all three forms of matter: solid, liquid and gas. It can exist naturally at all of the temperatures which occur on earth, and can easily change from one form into another. Interestingly enough, if water did not change form it would all exist today in the oceans or deep in the lithosphere since it responds to the pull of gravity pulling it downward.

When water is in contact with the atmosphere it evaporates, with the rate of evaporation depending on such things as air temperature, water temperature, wind speed and humidity of the air. The amount of water vapor in the air is called the relative humidity and for any given temperature there is a set amount of water that the air can hold. In general, the warmer the air is, the more water it can hold. Relative humidity is an expression of the percentage of water in the air at a given temperature as compared to what it could hold.

As water evaporates it moves upward on currents of warm air until it reaches a temperature where the temperature is low enough that it cannot hold any more water. At this level the water vapor condenses and form clouds. Droplets of the water in the clouds condense on dust particles until they are too large to withstand the force of gravity’s pull and they drop to earth. This precipitation may fall on a place far from the land area where the water initially evaporated, due to wind movement, and so water forms all over the earth are constantly being replenished.

Lakes or oceans are not the only sources from which water enters the air. Water evaporates from everything that is wet. Clothes drying on the line, a swimming pool, puddles after a rainshower, a steamy bathroom after your shower, all of these are sources of water for evaporation. Perhaps the easiest way to observe water evaporation is to boil water in a pan or tea kettle and watch the process. It should be stressed with students that most normal processes of evaporation are not readily observable; there may be evaporation going on in the room around them at the very moment they are talking about it, but normally they will not see anything. They will however see the results: a saucer of water which has dried up after being left out for some time, or the drying of their skin as they calm down to a cooler temperature after running around the track for gym on a hot day.