Carbon is a building block of life; it is necessary for the creation of organic molecules like carbohydrates, proteins and fats. Both plants and animals release carbon dioxide as part of the decomposition process; when the carbon dioxide is released into the air, living plants use it in photosynthesis to create energy from sunlight. The carbon from the tissue of decomposing plants and animals also leaches into the soil, creating fossil fuels.
Phosphorous is a vital element in providing energy for organisms. All living things produce adenosine triphosphate, or ATP, which performs many functions in organic cells. ATP is formed by three phosphate molecules; this phosphate enters the ecosystem when plants absorb it from the soil and turn it into phosphorous. Decomposing plants and animals release phosphorous into the soil where it is re-absorbed by plants for continued use in the ecosystem.
Nitrogen appears in all living things. Organisms use nitrogen to build proteins and genetic material (DNA and RNA). Certain types of bacteria, as well as some types of fungi, feed on the nitrogen present in decomposing plants and animals. They convert the nitrogen into ammonium; nitrifying bacteria then converts the ammonium to nitrites and finally to nitrates before returning it back to the ecosystem to be absorbed by plants and continue the life cycle.
Decomposing animals release water into the soil as their bodies dehydrate and break down. This water either releases back into the atmosphere through transpiration or seeps into the groundwater, where it supplies larger bodies of water. It feeds into the water cycle, the process by which the earth's water is recycled through evaporation of water from the surface of the ocean and then returned to the earth through precipitation.