Ferrihydrite Formation and Lepidocrocite Crystallization within a Protective Fungal Mycelium of Pakistani Soil.

Phosphorus (P) is an essential plant nutrient and it is generally deficient in Pakistani soils. The P availability in soil is limited by the effect of iron oxides particularly ferrihydrite (Rhoton and Bigham, 2005). Mycelia were discovered in the Tirnaul surface soil by transmission electron microscopy. The hypothesis of this study is that ferrihydrite forms in fungal mycelia as spherical aggregates of high surface area where P is adsorbed. Objectives: 1. Investigate changes in mycelia with age; 2. Examine Fe mineral formation in fungal mycelia; and 3. Rationalize the changes in P behavior as a plant nutrient during iron mineral formation. Tirnaul surface soil (coarse loamy mixed hyperthermic Udic Haplustept) was sampled and clay fraction was separated. The clay was examined with a transmission electron microscope, energy dispersion spectra and electron diffraction for iron oxides, phosphorus and mycelia. The mycelia have spherical ferrihydrite aggregates protected by thick walls of fungal mycelia. Fungal mycelia are complex structures with multiple layers of different composition (Ruiz-Herrera, 1991). Lepidocrocite laths form in a ladder-like arrangement. Mycelia shrink with age and walls thin suggesting the reason the laths form transverse to the mycelia where iron is most likely to be oxidized and crystal growth occurs. Lepidocrocite crystals may influence P adsorption in soil. Also, lepidocrocite crystals form on seasonal ped coatings in rice paddy soils (Dixon, 1994).   Prior investigations have revealed lepidocrocite aggregates of tubular shape in soils suspected of forming in mycelia (J.B. Dixon, observations). Golden et al (1997) suggested that on reduction and dissolution Fe oxide coatings release Fe, Si, and P to solution and influence the nutrient dynamics of rice plants. Fungal mycelia and the Fe oxides formed in them provide unique opportunity to monitor chemical and mineralogical changes in soils and provide new insight to the chemistry of P in soils.

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