MINERAL MESH BAG EXPERIMENTS IN THREE CATCHMENTS OF THE SLAVKOV FOREST, CZECH REPUBLIC

Mineral dissolution in the rhizosphere is well documented. There is growing evidence that direct attachment to mineral surfaces is a key condition during cation nutrient acquisition processes by plants and their associated fungi, especially under cation limiting conditions. Laboratory experiments demonstrated that fungi are able to exert both physical and chemical force to obtain the needed cations. However, the magnitude of these fungal alterations are difficult to quantify, and resulted in a wide range of weathering rates. Results from field experiments added further complexities to the understanding of the processes at the plant-microbe-mineral interface. The main focus of this study was to investigate the direct contribution of fungi to weathering of biotite under naturally occurring K and Mg limitations.

Two mesh bag experiments were completed in three spruce covered catchments underlain by felsic (Mg limited, LY), mafic (no cation limitations, NZ), and ultramafic (K limited, PB) rocks. In each catchment, 5 replicated mesh bags containing 20 grams of quartz sand and 3 wt% biotite were buried for 16 months, and for 5 years, at 3 slope positions. The mesh size was 50 µm, which allowed fungal hyphae and bacteria to colonize the bags, but excluded direct plant root contact with the minerals. The study used scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and atomic force microscopy (AFM) to document and quantify mineral grain morphology changes, fungal attachment, and chemical alterations of the biotite particles.

Biotite surfaces exhibited irregular, variable and channel like dissolution features. The highest percentage of etched channels are found under K limitations for both sampling time. Fungal hyphal coverage is found to be highly variable in all treatments. The EDS values show a decrease in the Fe/Si and K/Si ratios indicating Fe and K depletion of biotite in all treatments compared to the initial material at 16 months. However, both ratios increase to the initial or higher values after five years. The Mg/Si ratios show a slight decrease at the Mg limited site and no change, or increase at the other two sites. These data suggest two different weathering mechanisms, a selective elemental uptake earlier on, and a “global” weathering of the biotite grains after a longer incubation time.