Nutrient Cycling Links In Soil Systems

In addition to the translocation abilities of saprophytic fungi mentioned previously, there is a rapidly growing literature on the roles of ectomycorrhizal and other symbiotic fungi in mobilizing nitrogen and phosphorus from organic pools (Chalot and Brun, 1998; Bending and Read, 1996; Northup et al., 1995). This work has extended to the clearly demonstrated roles of ectomycorrhizal fungi in mineral weathering, that is, mobilizing inorganic nitrogen as ammonium from the interstices of feldspar minerals and solubilizing P from volcanic rocks (Landeweert et al., 2001). As noted in Chapter 4 about the nutrition of oribatid mites, the fungal matlike structures formed by ectomycorrhizal fungi (e.g., Hysterangium, Hydnellum, and Gautieria spp.) at the interface of the surface humus layer and upper A horizon may cover several square meters of forest floor. The mineral soil within this concentrated mass of mycorrhizal hyphae is more strongly weathered than the surrounding soil as a result of the excretion of oxalic acid by the fungus. Within the mat, calcium oxalate crystals are abundant and decomposition rates and nutrient availability are increased relative to the nearby soil (Entry et al., 1992). The calcium oxalate crystals are a readily available source of calcium ions for the mites, as was demonstrated elegantly by

Cromack et al. (1988). The fact that this inorganic "hot spot" serves as a possible source of both inorganic and organic nutrients for the microbiv-orous fauna is further proof of the impressive nutrient feedback loops operating in soils. For more extensive coverage of nutrient dynamics in soil profiles over centuries and millennia, see the extensive synthesis of Richter and Markewitz (2001).

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