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Biogeochemical Cycles Within an Ecosystem

Unlike energy, which flows in one direction from the sun into an ecosystem and is eventually lost as heat, atoms and molecules needed by organisms to survive are recycled. Biogeochemical cycles describe the pathways taken by these materials as they move between organisms and their environment and how they are transformed and stored. The six cycles most important for living organisms are those of water, carbon, oxygen, nitrogen, phosphorus and sulfur.

Water (Hydrological) Cycle
- In its gaseous form as clouds and vapor, water travels long distances and falls back to the earth as rain and snow. The water is either taken in by animals and plants or enters the ground, rivers, lakes or oceans. The water then returns to the atmosphere from the soil or leaves of plants via evapotranspiration, meaning it evaporates directly from bodies of water. All organisms need water to survive, but water also affects local climates, carries nutrients and pollutants and is involved in erosion and weathering.
Carbon Cycle
- Carbon is essential for life. In its inorganic state outside of living organisms, carbon exists as gaseous carbon dioxide in the atmosphere, dissolved ions in water or locked within sedimentary rocks. Carbon enters plants and some bacteria from the environment during photosynthesis, which converts carbon dioxide to other molecules. Carbon is transferred when organisms eat the plants. It eventually returns to the environment during respiration again as carbon dioxide or through the decomposition of dead plants and animals. Carbon plays a role in global warming and is released when fossil fuels, such as oil, coal and gas, are burned.
Oxygen Cycle
- Most oxygen in the atmosphere comes from plants and bacteria that generate energy through photosynthesis. The oxygen and carbon cycles are tied together because plants take in carbon dioxide during photosynthesis and release oxygen. Animals, on the other hand, breathe in oxygen and give off carbon dioxide, which is known as respiration. In addition to being part of respiration, oxygen is important because it forms the ozone layer in the upper atmosphere that blocks harmful ultraviolet radiation from the sun.
Nitrogen Cycle
- Gaseous nitrogen, which makes up 80 percent of the Earth's atmosphere, can only be used by a few organisms known as nitrogen fixers. Once converted to a biologically useful form, nitrogen passes among organisms in an ecosystem and then eventually back to the atmosphere.
Phosphorus and Sulfur Cycles
- Phosphorus, essential for energy storage and DNA formation in organisms, does not exist as a gas. Phosphorus in the environment comes mainly from rocks and is absorbed by plants and then ingested by other organisms. Sulfur in the atmosphere is released during decomposition of dead organisms, volcanic eruptions and burning of fossil fuels. Sulfur is used in small quantities by living organisms but also causes acid rain in its form as sulfuric acid.

Nature