Dead plants and animals, and waste such as faeces, contain carbon, nitrogen and other useful materials. If these stayed locked in dead material, living things would run short of them. Microorganisms, mainly bacteria and fungi, solve this problem. They act as decomposers and carry out decomposition, which is also called decay.
A decomposer secretes enzymes onto dead material. The enzymes break large molecules into small soluble molecules, which the microorganism absorbs. It uses some of them for growth and some for respiration. When decomposers respire they release carbon dioxide into the air, which returns carbon to the abiotic part of the ecosystem.
Other nutrients in the dead material, such as nitrogen compounds, are released into the soil. Plants absorb them through their roots and use them to grow, so the materials enter the living part of the ecosystem again. Without decomposers, dead material would pile up and plants would run out of nutrients.