FUNGI AND BIOTITE INTERACTIONS IN THE RHIZOSPHERE OF NORWAY SPRUCE

Biotite is one of the main natural sources of potassium and magnesium in soils. Many studies have investigated the roles of microbes and mycorrhizal fungi in nutrient acquisition of plants. Generally, it has been shown that mycorrhizal fungi and bacteria enhance chemical weathering of biotite and provide nutrients for their hosts under limiting conditions. However, there are contradictory results from field studies that indicate no significant effect and/or retardation of biotite weathering when mycorrhizal fungi are present.

The goal of this study was to investigate the effect of bedrock geology on biotite and fungal interactions. Three catchments were selected in the Slavkov Forest, Czech Republic, one with natural K limitation (serpentinite), one with natural Mg limitation (leucogranite) and a control site with no limitations (amphibolite). Mesh bags containing 3 wt% of biotite, and quartz sand, were buried in spruce forest soils for five years at three sites per catchment. The mesh size was 60 µm, which allowed fungal hyphae and bacteria to colonize the bags, but excluded plant roots. This design allowed us to test the direct contribution of fungi and bacteria to weathering of biotite under various bedrock geology.

Biotite surfaces were analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Macroscopically, mesh bags from the Mg limited site have lower fungal biomass than those from the other two sites. AFM provided nanoscale morphology of both mineral and fungal surfaces from all sites. SEM showed extensively weathered biotite surfaces, abundant etch pits and channels, rounded edges and secondary layer formation from all sites. The area of fungal attachment on basal surfaces of biotite is between 3 to 20% and the estimated channel area is 15%. There is larger variation within than between treatments and the extensive surface alterations of all grains cannot be explained by the observed fungal hyphal attachments to biotite surfaces. Abiotic dissolution processes might dominate the alteration patterns after 5 years of incubation in the soils. Further studies are needed to quantify the amount of organic matter, microbial and fungal community, and alteration of the mineral surfaces at each site to determine the effect of bedrock geology on K and Mg uptake by mycorrhizal fungi from biotite.