Immobilization and Movement of Nutrients by Fungi

During the course of decomposition, mineral nutrients are sequestered by decomposer soil organisms by being incorporated into the organism's biomass. The residence time of these elements is usually equivalent to the turnover time (lifespan) of that organism. During this period, the element is not in a soluble form in the soil solution, but is immobilized in microbial tissue. The amount of accumulation within the fungal component varies among ecosystems, depending on the chemical composition of the plant parts available for decomposition and the main fungal groups involved in the process. Shorter-lived, ephemeral molds, utilizing simple carbohydrates, thus have lower investment in biomass than longer-lived basidiomycetes growing on woody resource; the potential accumulation in basidiomycetes is therefore greater. Unlike bacteria, fungi are larger organisms and their rate of turnover is lower, particularly in the long-lived Basidiomycotina. A discussion of the role of basidiomycetes in decomposition is given by Frankland et al. (1982).

Where the C:nutrient ratio of a resource is very high, as in wood, the model of Swift et al. (1979) proposes initial immobilization and the import of free nutrient into organic form (fungal thallus) during the initial stages of decomposition until the fungal resource C:nutrient content is equivalent to that of the fungus. Fungal immobilization of nutrients can be considerable. Stark (1972) showed that hyphae had 193-272% greater N content and 104-223%

Table 2.16 Allocation of Standing Crop Organic Matter in a Second Growth Douglas Fir Forest Ecosystem

Forest component

Total standing

Tree component

Total tree

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