Spatial changes of soil solution and mineral composition in the rhizosphere of Norway-spruce seedlings colonized by Piloderma croceum

Ectomycorrhizae (ECM) or the root-fungal association in forest ecosystems provide a unique soil microenvironment where soil properties and processes differ from the bulk soil. In this study, we would like to better understand the role of ECM systems in mineral weathering and its implications to soil formation and nutrient cycling in forest ecosystems. Specifically, we would like to document the spatial variations in the composition of soil solution and mineralogy of the rhizosphere as influenced by the ECM of Norway spruce + Piloderma croceum. Two-month-old seedlings of Norway spruce (control and colonized by P. croceum) were cultivated in special rhizotrons designed to allow spatial collection of soil solution. We used A and C horizons of a Dystric Cambisol collected from Höglwald forest near Munich. Micro suction cups (5 mm x 1 mm) were installed in colonized and control rhizotrons, and soil solution was collected from September to November 2000. Our results show that the concentrations of NH₄⁺, Ca²⁺, and Mg ²⁺ in the soil solution were lower in <1.0 cm than in >3.0 cm distance from the roots of Norway spruce, due to the possible range of influence of Piloderma mycelium reaching about 2-3 cm from the surface of the mycorrhizal root. In the rhizotron with soil from the A horizon, a higher phosphorus content in Piloderma-colonized seedlings was observed. X-ray diffraction data indicate that chlorite and possibly mica are being transformed to 2:1-expanding clay minerals (probably smectite) within <1.0 cm distance from roots. The spatial variations in soil solution composition and mineral transformation are likely to be due to Piloderma colonization and concentrated mycelial growth within <1.0 cm distance from the roots. This is also evident in more intricate growth of mycelia on surfaces of micaceous minerals as compared to quartz. We assume that Piloderma modifies soil solution and mineralogy through acquisition of essential elements for its own survival and/or for the uptake by plant roots. However, the presence of spontaneous infection with wildtype ECM in the control plots may have altered the influence of Piloderma and must be taken into consideration when interpreting our results.