Biogeochemical cycles is the collective name for the cyclical processing of certain substances through living and non-living matter in Earth's biological, geological, and chemical systems. Relatively rare, biogeochemical cycles occur only in atoms which can be found in both organic and inorganic substances. The most common biogeochemical cycles are the nitrogen and the carbon cycle, followed by the oxygen and the hydrologic, or water, cycle.
Nitrogen not only makes up 78.8 percent of Earth's atmosphere, it is also found in proteins in most organic molecules. The nitrogen cycle begins when certain bacteria in the soil near roots of plants combine nitrogen from the air with oxygen or hydrogen atoms to form molecules of NO3, nitrate, or NH3, ammonia, which plants then absorb and process. Herbivorous organisms feed on the plants to gain the nitrogen, and omnivorous animals feed on the herbivores, moving the nitrogen into their bodies. Used nitrogen gets release in animal wastes, which certain bacteria eat, allowing the nitrogen to be released back into the air.
Although carbon is only the fourth most abundant element in the world, it is the element that serves as a base for all living things. The carbon cycle consists of carbon being converted into organic matter and then back into inorganic molecules. Plants use carbon, in the form of carbon dioxide in their respiration process during photosynthesis and to build tissue. Animals gain carbon through carbohydrates, releasing the carbon molecules into the air when they breath. When plants and animals die, carbon is released into the air when organisms oxidize the matter.
The oxygen cycle is linked to carbon cycle through the respiration of animals and plants. The second most abundant element in the Earth's atmosphere, oxygen is also essential to most organic material. Oxygen is released into the atmosphere by green plants, which produce it from water and carbon dioxide as a byproduct of photosynthesis. Then, all animals inhale it to allow their bodies to process the sugars they use for energy. Additionally, oxygen in water is cycled through aquatic plants and animals much the same way the oxygen in the air moves through the terrestrial life.
The last of the biogeochemical cycles, the hydrologic cycle, refers to the movement of water. Over a billion tons of water cycles constantly through the Earth. This cycle helps to maintain energy balances. Water enters the air through evaporation at high energy points and is moved by wind to low energy points where it is released via precipitation. On its return to the Earth, water not only replenishes water supplies, which animals drink, it soaks into the soil to be used by plants.