Cycles within Ecosystems

The carbon cycle describes how carbon molecules move through the environment.
Photosynthesis is the process where plants use sunlight to convert carbon dioxide and water into glucose and oxygen.
With respiration, animals as well as plants convert glucose and oxygen back into carbon dioxide and water, releasing energy.
The process of combustion (burning) also releases carbon dioxide into the atmosphere, for example through the burning of fossil fuels or wood.
Carbon is stored in carbon sinks, such as trees and the ocean, which absorb more carbon dioxide than they release.
When plants and all living organisms die, decomposers break them down, a process known as decomposition, releasing carbon back to the atmosphere as carbon dioxide.
The earth’s crust stores a large quantity of carbon in the form of sedimentary rocks and fossil fuels.

The water cycle describes how water evaporates from the surface of the earth, rises into the atmosphere, cools and condenses into clouds, and falls back to the surface as precipitation.
Evaporation of water can happen over the seas, reservoirs, lakes and rivers, but also from plants through a process called transpiration.
Once water is in the atmosphere, it can condense to form clouds in a process known as condensation.
Precipitation is how water is returned to the earth’s surface — it can be in various forms such as rain, sleet or snow.
Water on the earth’s surface can either evaporate, be taken up by plants, or return to the seas via rivers in a process called run-off.

The nitrogen cycle describes how nitrogen in the atmosphere is converted into different forms that are usable by plants and animals.
Bacteria in soils and the roots of some plants are capable of nitrogen fixation, a process by which atmospheric nitrogen is converted to ammonium ions which plants can use.
In soil, ammonium is converted into nitrites and then nitrates by bacteria in a process known as nitrification. Plants can absorb nitrates and use them to make proteins.
Animals obtain their nitrogen by eating plants or other animals.
When plants and animals excrete waste or die, decomposers break down the organic matter, returning the nitrogen back to the soil in a process called ammonification.
Some of the nitrates in the soil are transformed into nitrogen gas by denitrifying bacteria in a process called denitrification, releasing nitrogen back to the atmosphere and completing the cycle.