Living things take chemicals from their surroundings and later return them. The supply of each chemical on Earth is limited, so materials must be used again and again. This is called the cycling of materials. Materials cycle between the biotic parts of an ecosystem, which are the living things such as plants, animals and microorganisms, and the abiotic parts, which are the non-living things such as air, soil and water.
Many different materials cycle. Carbon is one example. Plants take in carbon dioxide from the air and use the carbon to make glucose. Animals get their carbon by eating plants or other animals, so carbon is passed along by feeding. Carbon returns to the air as carbon dioxide when organisms respire and when dead organisms are broken down.
Nitrogen is another example. Plants take up nitrates from the soil and use the nitrogen to make proteins. Animals get nitrogen compounds by feeding. When organisms die, or produce waste, the nitrogen compounds return to the soil. Water also cycles through ecosystems.