Carbon, nitrogen and decay cycles · Year 12
Living things are made of materials that cycle through the living world and the non-living world. Carbon, nitrogen and water are used by organisms, passed along food chains and returned to the air and soil, so they can be used again. Decomposers such as bacteria and fungi are what keep the cycles going.
Key facts
- Photosynthesis takes carbon dioxide out of the air. Plants use the carbon to make glucose and other carbon compounds.
- Carbon passes to animals when they eat plants (feeding). Respiration by plants, animals and decomposers returns carbon dioxide to the air. Combustion of fossil fuels and wood releases carbon dioxide too.
- When plants and animals die, decomposers break them down. They respire, releasing carbon dioxide, and return mineral ions to the soil.
- Decay is fastest when it is warm, moist and there is plenty of oxygen. Gardeners make compost this way; anaerobic decay makes methane, which can be used as biogas.
- Nitrogen gas makes up most of the air, but plants cannot use it directly. Nitrogen-fixing bacteria (some in the root nodules of legumes such as peas and clover) and lightning turn it into nitrogen compounds.
- Decomposers turn the proteins and urea in dead matter and waste into ammonium compounds. Nitrifying bacteria turn ammonium compounds into nitrites and then nitrates. Plants take in nitrates through their roots and use them to make proteins.
- Denitrifying bacteria turn nitrates back into nitrogen gas. They live in waterlogged soils with little oxygen.