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When one speaks of a cycle of any type, we are referring to how an element of that cycle is recycled along in many forms, never being used up, but continually changing forms.
The nitrogen cycle involves plants removing nitrogen gas from the atmosphere, fixing it into their plant cell structure. Humans can not absorb nitrogen from the air, so they either eat the plants that pull it from the air, or they eat other animals that have nitrogen as part of their body's chemical makeup.
The phosphorous cycle is similar, plants absorb phosphorous as a nutrient from the soil. Humans eat the plants or eat other animals that have phosphorous as a part of the chemical makeup of their bodies. Elemental phosphorous was first identified in the sediment of human urine, and bone ash was used as an early fertilizer to replace phosphorous in the soil.
Carbon in the carbon cyle is an interactive process between plants and humans. Humans take in oxygen and give off carbon dioxide as a waste product. Plants take in carbon dioxide and give off oxygen as a waste product.
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