- Define biogeochemical cycles.
- Describe the water cycle and its processes.
- Outline the steps of the nitrogen cycle.
- underground layer of rock that stores water
- biogeochemical cycle
- interconnected pathways through which water or a chemical element such as carbon is continuously recycled through the biotic and abiotic components of the biosphere
- process in which water vapor changes to tiny droplets of liquid water
- process in which liquid water changes to water vapor
- exchange pool
- part of a biogeochemical cycle that holds an element or water for a short period of time
- water that exists in the ground either in the soil or in rock layers below the surface
- nitrogen cycle
- interconnected pathways through which nitrogen is recycled through the biotic and abiotic components of the biosphere
- nitrogen fixation
- process of changing nitrogen gas to nitrates that is carried out by nitrogen-fixing bacteria in the soil or in the roots of legumes
- water that falls from clouds in the atmosphere to Earth’s in the form of rain, snow, sleet, hail, or freezing rain
- part of a biogeochemical cycle that holds an element or water for a long period of time
- precipitation that falls on land and flows over the surface of the ground
- process in which ice and snow change directly to water vapor
- process in which plants give off water vapor from photosynthesis through tiny pores, called stomata, in their leaves
- water cycle
- interconnected pathways through which water is recycled through the biotic and abiotic components of the biosphere
Where does the water come from that is needed by your cells? Or the nitrogen that is needed to make your organic molecules? Unlike energy, matter is not lost as it passes through an ecosystem. Instead, matter is recycled. This recycling involves specific interactions between the biotic and abiotic factors in an ecosystem.
The chemical elements and water that are needed by organisms continuously recycle in ecosystems. They pass through biotic and abiotic components of the biosphere. That’s why their cycles are called biogeochemical cycles. For example, a chemical might move from organisms (bio) to the atmosphere or ocean (geo) and back to organisms again. Elements or water may be held for various periods of time in different parts of a cycle.
- Part of a cycle that holds an element or water for a short period of time is called an exchange pool. For example, the atmosphere is an exchange pool for water. It usually holds water (in the form of water vapor) for just a few days.
- Part of a cycle that holds an element or water for a long period of time is called a reservoir. The ocean is a reservoir for water. The deep ocean may hold water for thousands of years.
The rest of this lesson describes two biogeochemical cycles: the water cycle and nitrogen cycle.
The Water Cycle
Individual water molecules keep moving through the water cycle. The water cycleis a global cycle. It takes place on, above, and below Earth’s surface, as shown in Figure below.
Like other biogeochemical cycles, there is no beginning or end to the water cycle. It just keeps repeating.
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 in the water cycle.
Evaporation, Sublimation, and Transpiration
Water changes to a gas by three different processes:
- Evaporation occurs when water on the surface changes to water vapor. The sun heats the water and gives water molecules enough energy to escape into the atmosphere.
- Sublimation occurs when ice and snow change directly to water vapor. This also happens because of heat from the sun.
- Transpiration occurs when plants release water vapor through leaf pores called stomata (see Figure below). The water is a product of photosynthesis.
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—rain, snow, sleet, hail, or 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 longer.
Groundwater and Runoff
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 may soak into the ground, becoming groundwater. Groundwater may seep out of the ground at a spring or into a body of water such as the ocean. Some groundwater may be taken up by plant roots. Some may flow deeper underground to an aquifer. This is an underground layer of rock that stores water, sometimes for thousands of years.
Untamed Science: The Water Cycle
The Nitrogen Cycle
Nitrogen makes up 78 percent of Earth’s atmosphere. It’s also an important part of living things. Nitrogen is found in proteins, nucleic acids, and chlorophyll. The nitrogen cycle moves nitrogen through the abiotic and biotic parts of ecosystems.Figure below shows how nitrogen cycles through a terrestrial ecosystem. Nitrogen passes through a similar cycle in aquatic ecosystems.
Bear with the silly sounds and such in this next video. It does a good job of explaining the nitrogen cycle!
Nitrogen Cycle by Smart Learning for All:
Plants cannot use nitrogen gas from the air to make organic compounds for themselves and other organisms. The nitrogen gas must be changed to a form called nitrates, which plants can absorb through their roots. The process of changing nitrogen gas to nitrates is called nitrogen fixation. It is carried out by nitrogen-fixing bacteria. The bacteria live in soil and roots of legumes, such as peas.
When plants and other organisms die, decomposers break down their remains. In the process, they release nitrogen in the form of ammonium ions. Nitrifying bacteria change the ammonium ions into nitrates. Some of the nitrates are used by plants. Some are changed back to nitrogen gas by denitrifying bacteria.
- Chemical elements and water are recycled through biogeochemical cycles. The cycles include both biotic and abiotic parts of ecosystems.
- The water cycle takes place on, above, and below Earth’s surface. In the cycle, water occurs as water vapor, liquid water, and ice. Many processes are involved as water changes state in the cycle. The atmosphere is an exchange pool for water. Ice masses, aquifers, and the deep ocean are water reservoirs.
- The nitrogen cycle moves nitrogen back and forth between the atmosphere and organisms. Bacteria change nitrogen gas from the atmosphere to nitrogen compounds that plants can absorb. Other bacteria change nitrogen compounds back to nitrogen gas, which re-enters the atmosphere.
Lesson Review Questions
1. What is a biogeochemical cycle? Name an example.
2. Identify and define two processes by which water naturally changes from a solid or liquid to a gas.
3. Define exchange pool and reservoir, and identify an example of each in the water cycle.
6. Assume you are a molecule of water. Describe one way you could go through the water cycle, starting as water vapor in the atmosphere.
7. Read the following passage, then apply information from the lesson to explain why the farmer plants peas:
A farmer has three fields in which she grows corn for market. Every year, she plants one of the fields with peas, even though she cannot make as much money selling peas as she can selling corn. She rotates the fields she plants with peas so that each field is planted with peas every 3 years.
Explain why bacteria are essential parts of the nitrogen cycle.
Points to Consider
In this lesson, you read how matter is recycled through ecosystems. Ecosystems vary in the amount of matter they can recycle. For example, rainforests can recycle more matter than deserts.
- Consider the abiotic and biotic factors of a rainforest and desert. How might they be different?
- Why do you think a rainforest can recycle more matter than a desert?
Previous: The Science of Ecology
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