Decompose and release to the nutrients to the sources.
The micro-organisms decompose the dead and decaying organic matters. The nutrients are absorbed from soil or atmosphere by the plants. During the absorption these nutrients are depleted from the sources.
From the plants, these nutrients are transferred to the animals at different tropic levels. After the death of plants and animals are decomposed by micro-organisms and release the nutrients to the sources which finally maintain the biogeochemical cycle of nitrogen, Sulphur etc.