MICA-FUNGI INTERFACE: A MESH BAG STUDY IN SPRUCE FOREST SOILS

Micas are common sources of potassium in soils and experimental studies have demonstrated that short-term weathering processes in root-zones are often represented by microbe-fungi-mineral interactions. It is documented that fungal hyphae directly attach to mineral surfaces and exert both physical and chemical forces to weaken the mineral structure and release the needed cations, simultaneously. The magnitude of alterations caused by fungi in natural soils is difficult to quantify and the laboratory results cannot be realistically scaled to field conditions, so the significance of fungal weathering at the ecosystem or larger scales is still debated. To connect laboratory and field scale studies in the quest of determining the direct contribution of fungal weathering, mica filled mesh bags were buried in three watersheds of the Slavkov Forest in the Czech Republic. The three watersheds are underlined by felsic (Mg limited), mafic (no cation limitations), or ultramafic (K limited) bedrock, and the other soil forming factors are the same. Five replicated mesh-bags containing 20 grams of pure quartz with 3 wt% of biotite or muscovite were buried for five years in the top mineral soil horizon at three slope positions in each watershed. The mesh size allowed fungal hyphae and bacteria to colonize the minerals but excluded plant roots. Various microscopic and spectroscopic techniques were used to document morphological and chemical changes at the mica-fungi interface. Fungal concentration was calculated from the ergosterol content of each mesh bag and mineral alteration was estimated from synchrotron X-ray diffraction analysis. The results show the highest percentage of dissolution features and highest amount of ergosterol concentrations in mesh bags of the potassium-limited watershed, supporting previous laboratory studies. A decrease in major elements (Fe, K, and Mg) and silica ratios of the mica surfaces compared to the initial material were also observed in mesh bags from the potassium and magnesium limited watersheds. However, no clear correlations were found between fungal concentrations and elemental depletion ratios. This data suggests that soil solution mediated weathering overprints the signature of the selective elemental uptake by fungi over the length of the study.