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Decomposition is a process that releases nutrient materials in the structure of plant and animal tissues to the soil solution where the nutrients are once again potentially available to plants. Because most nutrient elements cycle from soil solution to plant and back to the soil solution, decomposition processes are the key features of nutrient cycles. The decomposition process is subdivided into two related processes: fragmentation and mineralization. The spatial and temporal linkage between the fragmentation and mineralization processes is tight in mesic systems. In desert ecosystems, fragmentation and mineralization may be separated both temporally and spatially. Both processes of decomposition are affected by biotic and abiotic parameters, and abiotic effects may be both direct and indirect. Roots are the main source of carbon inputs in desert soils. Variations in microclimate, physicochemical soil properties resulting from topographic position, and vegetative cover can produce different patterns of decomposition, mineralization, and soil organic-matter accumulation. Termites are abundant in soil and have a significant effect on nutrient cycling and nutrient availability in arid and semiarid systems. Termites remove a large fraction of certain species of litter. Oil moisture and temperature affect the activity of termites. The best documented spatial patterns of nutrient distributions in arid ecosystems are the “islands of fertility” associated with shrubs and trees. Nitrogen appears to be the most important limiting nutrient in semiarid and arid ecosystems. Mycorrhizal fungi are known to enhance nutrition of the host plant and receive benefit from the host plant in the form of usable energy: carbohydrates.