Weathering and soil production under trees growing on sandstones - The role of tree roots in soil formation.

Rock weathering drives both landform formation and soil production/evolution. The less studied biological component of weathering and soil production caused by tree root systems is the main focus of the present study. Weathering by trees, which likely has been important in soil formation since the first trees emerged in the middle and late Devonian, is accomplished through both physical and biological means, like acids excreted by plants and exudates from associated bacterial communities. However, these processes are relatively poorly known. We assessed the impact of tree roots and associated microbiota on the potential level of biological weathering. Three research plots were selected in two sandstone regions in Poland. Two plots were in the Stolowe Mountains (Zlotno, Batorów), a tableland built of Cretaceous sandstones. The third plot (Zegiestów) was in the Sacz Beskidy Mountains, the Carpathians. Soil samples were taken from tree root zones of Norway spruces from predefined sampling positions. Soils from non-tree control positions were also sampled. Soil samples were a subject of laboratory analyses which included the content of Fe and Al (amorphous and labile forms), carbon (C), nitrogen (N), and soil pH. The microbial functional diversity of soil microorganisms was determined using the Biolog (EcoPlate) system. Rock fragments were collected for mineralogical and a subject of optical microscopy and cathodoluminescence analyses in order to examine their mineralogical composition. Significant differences (pHolm-corrected < 0.05) between sample locations were found mostly for the Zegiestów plot: Soils at control positions differed from the crack and bulk soil sample positions in terms of C, N, C/N, and pH. Tree roots were able to develop a great variety of sizes and forms by following the existing net of bedrock discontinuities and hillslope microrelief. They developed along the most accessible surfaces, and caused rockcliff retreat and scree slope formation. These two features can be considered as initial stages of soil production. Trees add to the complexity of the soil system and allow formation of rhizospheric soils, and horizons rich in organic matter which are zones of a high microbial activity. However, as our study shows, rock cracks with roots cannot be considered as zones of microbial weathering. In addition, C content and microbial activity decreases with depth but can stay on a high level along living and dead roots. When entering rock fractures, they change the intensity of biomechanical weathering and soil properties. The highest biological activity of microorganisms was found in the control samples. Overall, tree roots do change the pattern of soil formation and explain the existing pattern of soil chemical properties, microbial activity, and potentially biological weathering intensity, and the intensity of those processes in correlation with root presence varies in space.

Collision with Gondwana or with Baltica? Ordovician magmatic arc volcanism in the Marmarosh Massif (Eastern Carpathians, Ukraine).

The pre-Alpine Marmarosh Massif is a tectonically complex unit of the crystalline basement within the Eastern Outer Carpathians. In the eastern (Ukrainian) segment of this massif, two units have been identified-the Bilyi Potok Nappe and the Dilove Nappe. Petrological investigations coupled with zircon U-Pb dating were performed on metavolcanic rocks (porphyroids) and their phyllite host rocks, sampled from three locations within the Dilove Nappe. The geochemical characteristics of the meta-rhyodacite porphyroids revealed a volcanic arc affinity of the protolith, with U-Pb zircon ages of 452.8 ± 1.5 Ma and 461.5 ± 1.6 Ma and zircon saturation temperatures in the range of 823-892 °C. The phyllite host rocks (meta-tuff) yield a U-Pb zircon maximum estimate for the eruption age at 584.7 ± 2.9 Ma. Peak amphibolite-facies metamorphism (M1) was estimated at the pressure of 600-900 MPa with a temperature range of 560-600 °C. Retrogression (M2), possibly related to Alpine nappe stacking and shearing, is assumed to have taken place at 200-250 MPa and 384-222 °C. The volcanic arc is interpreted as an early Caledonian arc that was subsequently accreted to the margin of Baltica during the closure of the easternmost Tornquist Ocean rather than Cenerian (early Paleozoic) orogenic events on the margin of East Gondwana. Supplementary Information: The online version contains supplementary material available at 10.1007/s00531-022-02228-8.