Fungi play a central role in nutrient recycling within ecosystems by decomposing dead organic matter and returning its constituent elements to the soil, air, and surrounding environment. This process contributes to the continuity of natural cycles involving elements essential for the growth of living organisms, particularly carbon, nitrogen, phosphorus, and sulfur. In the carbon cycle, fungi decompose plant residues, leaves, and wood, using part of the resulting carbon to build their biomass, while another portion is released as carbon dioxide through respiration. Some carbon may also be converted into more stable organic compounds in the soil, depending on decomposition conditions and soil properties. Therefore, fungi influence the balance between carbon released into the atmosphere and carbon retained in the soil. In the nitrogen cycle, fungi contribute to the decomposition of proteins, nucleic acids, and other nitrogen-containing organic compounds. During decomposition and mineralization, nitrogen may be released in the form of ammonium or temporarily incorporated into fungal biomass. A portion of this nitrogen subsequently returns to the soil following fungal death and the decomposition of their cells. It is important to distinguish between the release of nitrogen from organic matter and atmospheric nitrogen fixation. Decomposer fungi generally do not fix atmospheric nitrogen; this process is carried out by specialized organisms, including certain bacteria and archaea. In the phosphorus cycle, fungal decomposition helps release phosphorus contained in organic compounds. Some fungi can also secrete organic acids that contribute to dissolving certain phosphate compounds, making phosphorus more accessible to plants. Of particular importance are symbiotic mycorrhizal fungi (Mycorrhizal Fungi), which associate with plant roots and extend their hyphae into areas of soil that roots may not easily reach. This relationship helps improve the absorption of phosphorus and other essential nutrients. Fungi also participate in the sulfur cycle by decomposing certain sulfur-containing organic compounds and releasing sulfur in different chemical forms. Other microorganisms subsequently carry out additional transformations within the ecological cycle. Sulfur is an essential element in the formation of certain amino acids and proteins, highlighting the importance of its recycling in maintaining vital biological functions. The role of fungi extends beyond releasing nutrients to include their absorption and incorporation into fungal cells, followed by their return to the environment when fungal biomass decomposes. The nature of these processes varies according to fungal species, environmental conditions, organic matter composition, and the availability of other nutrients required for growth. Thus, fungi constitute an essential component of natural nutrient recycling systems. By connecting organic matter decomposition with nutrient availability and the continued growth of plants and other living organisms, they contribute to maintaining the chemical and biological balance of ecosystems. Al-Mustaqbal University – The First University in Iraq