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BIOGEOCHEMICAL CYCLES The chemical elements and water that are needed by living things keep recycling over and over on Earth. These cycles are called biogeochemical cycles. They pass back and forth through biotic and abiotic components of ecosystems. In these cycles, a chemical element or water might move from organisms (bio) to the atmosphere or ocean (geo) and back to organisms again. The process never stops, which is why it is called a cycle.

Elements or water may be held for various periods of time in different parts of a biogeochemical cycle.

• An exchange pool is part of a cycle that holds a substance for a short period of time. For example, the atmosphere is an exchange pool for water. It usually holds water (as water vapor) for just a few days.

• A reservoir is part of a cycle that holds a substance for a long period of time. For example, the ocean is a reservoir for water. It may hold water for thousands of years.

The rest of this article describes four biogeochemical cycles: the water cycle, carbon cycle, nitrogen cycle, and phosphorous cycle.

1. WATER -the series of processes by which water circulates between the

earth's oceans, atmosphere, and land.

2. CARBON -the series of processes by which carbon compounds (specifically

carbon dioxide- CO2) are converted in the environment.

3. NITROGEN -the series of processes by which nitrogen and its compounds

are recycled through the air, soil, and organisms.

4. PHOSPHOROUS -the series of processes by which phosphorus compounds

in the earth are recycled through soil, plants, and decomposition

All of these cycles have been occurring naturally since the dawn of time. Without them, the earth and all of its ecosystems would not be able to survive. In more recent years, human impact on these cycles has had many negative effects on ecosystems. As you read, stop to answer the questions about every cycle.

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WATER CYCLE

Water is an extremely important aspect of every ecosystem. Life can’t exist without water. Most organisms contain a large amount of water, and many live in water. Therefore, the water cycle is essential to life on Earth.

Water on Earth is billions of years old. However, individual water molecules keep moving through the water cycle. The water cycle is a global cycle. It takes place on, above, and below Earth’s surface, as shown in the diagram. During the water cycle, water occurs in three different states: gas (water vapor), liquid (water), and solid (ice). Many processes are involved as water changes state to move through the cycle.

Evaporation, Sublimation, and Transpiration

Water changes to a gas by three different processes called evaporation, sublimation, and transpiration.

• Evaporation takes place when water on Earth’s surface changes to water vapor. The sun heats the water and gives water molecules enough energy to escape into the atmosphere. Most evaporation occurs from the surface of the ocean.

• Sublimation takes place when snow and ice on Earth’s surface change directly to water vapor without first melting to form liquid water. This also happens because of heat from the sun.

• Transpiration takes place when plants release water vapor through pores in their leaves called stomata.

Condensation and Precipitation

Rising air currents carry water vapor into the atmosphere. As the water vapor rises in the atmosphere, it cools and condenses. Condensation is the process in which water vapor changes to tiny droplets of liquid water. The water droplets may form clouds. If the droplets get big enough, they fall as precipitation.

Precipitation is any form of water that falls from the atmosphere. It includes rain, snow, sleet, hail, and freezing rain. Most precipitation falls into the ocean.

Eventually, this water evaporates again and repeats the water cycle. Some frozen precipitation becomes part of ice caps and glaciers. These masses of ice can store frozen water for hundreds of years or even longer.

Condensation may also form fog or dew. Some living things, like the lizard in picture, depend directly on these sources of liquid water.

Runoff and Ground Water

Precipitation that falls on land may flow over the surface of the ground. This water is called runoff. It may eventually flow into a body of water.

Some precipitation that falls on land soaks into the ground. This water becomes groundwater. Groundwater may seep out of the ground at a spring or into a body of water such as the ocean. Some groundwater is taken up by plant roots. Some may flow deeper underground to an aquifer. An aquifer is an underground layer of rock that stores water. Water may be stored in an aquifer for thousands of years.

The thorny devil lizard lives in such a dry environment in Australia that it has a unique specialization for obtaining water. The scales on its body collect dew and channel it to the corners of the mouth, so the lizard can drink it.

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CARBON CYCLE

The element carbon is the basis of all life on Earth. Biochemical compounds consist of chains of carbon atoms and just a few other elements. Just like water, carbon is constantly recycled through the biotic and abiotic factors of ecosystems.

The carbon cycle includes carbon in sedimentary rocks and fossil fuels under the ground, the ocean, the atmosphere, and living things. The diagram represents the carbon cycle. It shows some of the ways that carbon moves between the different parts of the cycle.

Carbon Reservoirs

Major reservoirs of carbon include sedimentary rocks, fossil fuels, and the ocean. Sediments from dead organisms may form carbon-containing sedimentary rocks. Alternatively, the sediments may form carbon-rich fossil fuels, which include oil, natural gas, and coal. Carbon can be stored in these reservoirs for millions of years.

Water erosion by runoff, rivers, and streams dissolves carbon in rocks and carries it to the ocean. Ocean water near the surface dissolves carbon dioxide from the atmosphere. Dissolved carbon may be stored in the deep ocean for thousands of years.

Carbon Exchange Pools

Major exchange pools of carbon include organisms and the atmosphere. Carbon cycles more quickly between these components of the carbon cycle.

• Photosynthesis by plants and other producers removes carbon dioxide from the atmosphere to make organic compounds for living things.

• Cellular respiration by living things releases carbon into the atmosphere or ocean as carbon dioxide.

• Combustion of fossil fuels releases carbon back into the air. Burning coal, oil, natural gas, and other fossil fuels emits it as CO2 into the atmosphere.

• Decomposition of dead organisms and organic wastes releases carbon back to the atmosphere, soil, or ocean.

Human Impact on the Carbon Cycle

Humans have a major effect on the carbon cycle. The two greatest impacts humans have on the carbon cycle are deforestation and the burning of fossil fuels.

Deforestation Deforestation is the permanent removal of trees from forests. Permanent removal of the trees means new trees will not be replanted. Remember, plants are necessary in the removal of carbon dioxide from the atmosphere. They take in carbon dioxide and give off oxygen that other organisms need to survive. When thousands of trees are removed from a forest, it results in increased levels of carbon dioxide in the atmosphere because trees are no longer absorbing carbon dioxide for photosynthesis. As a result, the carbon cycle is affected. According to National Geographic, agriculture is the primary cause of deforestation. Farmers remove trees on a large-scale basis to increase land for crops and livestock.

Burning of Fossil Fuels When oil or coal is burned, carbon is released into the atmosphere at a faster rate than it is removed. As a result, the concentration of carbon dioxide in the atmosphere increases. Natural gas, oil and coal are fossil fuels that are commonly burned to generate electricity in power plants, for transportation, in homes and in other industrial complexes. The primary industrial activities that emit carbon dioxide and affect the carbon cycle are petroleum refining, paper, food and mineral production, mining and the production of chemicals.

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NITROGEN CYCLE

Nitrogen is another common element found in living things. It is needed to form both proteins and nucleic acids such as DNA. Nitrogen gas makes up 78% of Earth’s atmosphere, but it is in a form of Nitrogen that plants, animals, and humans cannot use directly. The nitrogen in the air must first be “fixed” so we can use it. In the nitrogen cycle, nitrogen flows back and forth between the atmosphere and living things. Several different prokaryotes (another name for bacteria) play an important role in this cycle.

How Does Nitrogen Go from the Air to Us?

We get nitrogen by eating plants or by eating other animals that eat plants. Then where do plants get their nitrogen? They can’t use nitrogen gas in the air. The only form of nitrogen that plants can use is in chemical compounds called nitrates. Plants absorb nitrates through their roots. This is called assimilation. Most of the nitrates are produced by bacteria that live in soil or in the roots of bean plants, which are plants called legumes. Here is how nitrogen is “fixed”:

• Nitrogen from the atmosphere enters the soil.

• Nitrogen-fixing bacteria found on certain legume plants changes nitrogen gas from the atmosphere to nitrates in soil.

• Plants absorb the nitrates, and when animals or humans eat the plant, they obtain the nitrates.

How Does Nitrogen Return to the Atmosphere?

So nitrogen has gone from the atmosphere to the soil, the soil to the plant, the plant was eaten by the cow, and now you are getting that nitrogen from eating the cow. Because nitrogen is a form of matter, it must be recycled back into the atmosphere at some point. Here is how nitrogen returns to the atmosphere:

• When organisms die and decompose, their nitrogen is returned to the soil as ammonium ions (ammonium is another form of nitrogen).

• While some ammonium ions are “fixed” back into nitrates, others are “denitrified.” The ammonium ions that are “denitrified” are done so by denitrifying bacteria.

• Denitrifying bacteria works opposite nitrogen-fixing bacteria. Denitrifying bacteria turn nitrogen in the soil back into nitrogen in the atmosphere.

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PHOSPHOROUS CYCLE

Phosphorus is a chemical element found on Earth. Unlike nitrogen, the majority of phosphorous is not found in the atmosphere. Instead, it is found in rocks.

While phosphorous is found on Earth’s land instead of the atmosphere, phosphorous and nitrogen do have some things in common. Phosphorous is an essential nutrient for animals and plants. It plays a critical role in cell development and is a key component of molecules such as DNA. Also like the nitrogen cycle, the phosphorus found in rocks can’t be used directly, and must be broken down first before plants and animals can use it.

How Does Phosphorous Get to Us? Phosphorus moves in a cycle through rocks, water, soil, sediment and organisms. Here are the key steps of the phosphorus cycle

• Over time, rain and weathering cause rocks to release phosphate ions and other minerals. This phosphate is then distributed in soils and water.

• Within the soil, phosphate can be made available to plants by bacteria that break down organic matter to inorganic forms of phosphorus. This process is known as mineralization.

• Plants are now able to take up this phosphate from the soil. The plants may then be consumed by animals. Once in the plant or animal, the phosphate is incorporated into molecules such as DNA.

How is Phosphorous Returned to the Earth?

• When the plant or animal dies, it decays, and the phosphate is returned to the soil.

• Phosphate in soil can end up in waterways and eventually oceans. Once there, it can be incorporated into sediments over time.

Human Impact the Phosphorous and Nitrogen Cycle

Many human activities impact the nitrogen and phosphorous cycle. Nitrogen in our atmosphere and phosphorous in the earth are naturally occurring. Recently, farmers have started using nitrogen and phosphorous based fertilizers in order to grow more crops. Fertilizers made from nitrates and phosphates are used to increase plant production, but unused nitrates and phosphates from the fertilizer can leach out of the soil, enter streams and rivers, and ultimately make its way into our drinking water. In fact, much of the nitrogen and phosphorous applied to crops and fields enters rivers and streams by water runoff. In these ecosystems, increases in nitrates and phosphates in the water result in excessive algae and plant production. When algae cover the water surface, the organisms living in the water do not receive the oxygen they need in order to survive. Also, these algae could be toxic and harmful. Some algae that grows as a result of fertilizer runoff is so poisonous that it kills all of the organisms living in the water.

A lake in France covered in algae from years of fertilizer runoff.