A nutrient cycle refers to the movement and exchange of organic and inorganic matter back into the production of living matter. The process is regulated by the food web pathways previously presented, which decompose organic matter into inorganic nutrients. Nutrient cycles occur within ecosystems. Nutrient cycles that we will examine in this section include water, carbon, oxygen and nitrogen cycles.
A simple video explaining nutrient cycling:
Over two thirds of the Earth's surface is covered by water. It forms an important component of most life forms, with up to of plants and animals being composed of water. Vast quantities of water cycle through Earth's atmosphere, oceans, land and biosphere. This cycling of water is called the water or hydrological cycle. The cycling of water is important in determining our weather and climate, supports plant growth and makes life possible.
Evaporation: Most water evaporates from the oceans, where water is found in highest abundance. However some evaporation also occurs from lakes, rivers, streams and following rain.
Transpiration: Is the water loss from the surface area (particularly the stomata) of plants. Transpiration accounts for a massive of land-based evaporation, and of total evaporation.
Evapotranspiration: The processes of evaporation and transpiration are often collectively referred to as evapotranspiration.
Condensation: The process by which water vapour is converted back into liquid is called condensation. You may have observed a similar process occurring when dew drops form on a blade of grass or on cold glass. Water in the atmosphere condenses to form clouds.
Precipitation: Water returns to Earth through precipitation in the form of rain, sleet, snow or ice (hail). When rain occurs due to precipitation, most of it runs off into lakes and rivers while a significant portion of it sinks into the ground.
Infiltration: The process through which water sinks into the ground is known as infiltration and is determined by the soil or rock type through which water moves. During the process of sinking into the Earth's surface, water is filtered and purified. Depending on the soil type and the depth to which the water has sunk, the ground water becomes increasingly purified: the deeper the water, the cleaner it becomes.
Melting and freezing: Some water freezes and is 'locked up' in ice, such as in glaciers and ice sheets. Similarly, water sometimes melts and is returned to oceans and seas.
The processes involved in the water cycle are shown in Figure 8.21.
Figure 8.21: The water cycle.
TEACHER RESOURCES:
This page is a part of PhysicalGeography.net, an educational website maintained by Dr. Michael Powdery, a member of the Geography Department at Okanagan University in British Columbia. In addition to this excellent introduction with tables and diagrams, the site also includes a glossary of terms, additional readings, and links to outside resources.
The Georgia state office of the US Geological Survey provides a very basic and kid-oriented site to explain various aspects of the water cycle, including following a single drop of water through the cycle's main stages. US Geological Survey: The Water Cycle:
This is an animation of the water cycle
Oxygen is one of the main gases found in the air, along with nitrogen. Oxygen is re-cycled between the air and living organisms in the following ways:
The oxygen cycle is shown in Figure 8.22.
Figure 8.22: Oxygen cycle
Watch a video about the oxygen cycle. Focus on the first part of the video clip and the summary at the end.
Carbon is the basic building block of all organic materials, and therefore, of living organisms. Most of the carbon on earth can be found in the crust. Other reservoirs of carbon include the oceans and atmosphere.
Learn more about the carbon cycle in this video:
Carbon moves from one reservoir to another by these processes:
Photosynthesis and gaseous exchange are the main carbon cycling processes involving living organisms. Figure 8.23 depicts the carbon cycle.
TEACHER RESOURCES:
Nitrogen (N$_{2}$ ) makes up most of the gas in the atmosphere (about ). Nitrogen is important to living organisms and is used in the production of amino acids, proteins and nucleic acids (DNA, RNA).
The nitrogen cycle involves the following steps:
The nitrogen cycle is shown in Figure 8.24.
TEACHER RESOURCES:
Below are some excellent interactive animations of the nitrogen cycle: