A cycle is a regularly repeating sequence of events. The word comes from the Greek word for circle, kyklos. In nature, cycles occur in weather, climate, seasons, plant and animal life cycles, and many other areas. They are a key part of the Earth’s systems.
The water cycle is the process that moves water around our planet. It starts with evaporation from oceans, lakes and rivers. This water vapor rises into the atmosphere where it cools and condenses into rain or snow. The precipitation falls back to Earth’s surface where it flows into streams and rivers which eventually return the water to the oceans. The water cycle is powered by energy from the Sun.
The carbon cycle is the process that moves carbon through different parts of Earth’s system. Carbon is found in fossil fuels like coal and oil, in living things, and in the air as carbon dioxide gas (CO2). Burning fossil fuels releases CO2 into the atmosphere where it contributes to global warming. Plants use sunlight to convert CO2 into food during photosynthesis. When animals eat plants they get some of this stored energy which they use for their own metabolism or movement; when animals breathe they release CO2 back into the atmosphere. Decaying organic matter also returns its stored carbon back to atmospheric CO2 over time scales ranging from weeks to millennia
The nitrogen cycle is responsible for making sure there is enough nitrogen available for all living things on Earth Nitrogen makes up about 78% of our atmosphere but it isn’t available for use by plants or animals unless it goes through a series of steps first The first step in nitrogen fixation which can be done either by bacteria that live in nodules on leguminous plants’ roots (such as soybeans clover alfalfa) or free-living bacteria found near sources of running water These bacteria change atmospheric nitrogen N₂into ammonium NH₄ which can be used by plants Next decomposers such as fungi break down dead organisms releasing their nitrogen content back into soil as nitrates NO₃and nitrites NO₂ These nitrates can be used directly by plants but must be converted to ammonia NH₃ before being used by most other organisms including animals Finally denitrifying bacteria convert nitrates back into atmospheric nitrogen gas thus completing the nitrogen cycle
The phosphorus cycle starts with rocks on land phosphorus compounds are slowly released into soil solution When phosphate PO43- -containing minerals dissolve they increase soil fertility However once these same minerals become part of sedimentary rock layers they are no longer available for plant uptake Erosion eventually wears away at rock freeing phosphorus compounds which enter aquatic ecosystems There algae and other aquatic plants take up phosphorus promoting growth Eventually these same organisms die fall to bottom sediments where anaerobic conditions allow decomposition without oxygen This lack of oxygen creates ideal conditions for phosphates mineralization meaning phosphorus becomes tied up in insoluble form Once again it becomes unavailable for uptake Sooner or later however erosion will carry these newly minted phosphate minerals away restarting whole process anew