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Artificial soils were used in this study to analyse the importance of different mineral compositions for the diversity of soil microorganisms. Variants containing montmorillonite (MT), illite (IL) and illite + ferrihydrite (IL+FH) were compared to each other. Bulk material and their particle size fractions, as obtained by ultracentrifugation and wet-sieving, were characterised for abundance and diversity of Bacteria, Archaea and Fungi. Samples were analysed 6 and 18 months after inoculation with sterilised manure and a soil-extracted microbial community. Generally, IL, and even more pronouncedly IL+FH, supported the growth of more Bacteria, Archaea and Fungi, than MT. This trend was most pronounced in the finest fraction (< 20 μm). The structural diversity of Fungi responded more strongly to the different mineral compositions than the Bacteria, for which particle size fractions were more important. Archaea established a specific community in the finest fraction and showed no response to the different mineral compositions. Overall, this study demonstrates that the mineral composition and the particle size fractions have specific and different selective effects on the three domains and, thus, suggests that these factors strongly contribute to niche separation and the high diversity of microbial communities in natural soils with complex mineral compositions., (© 2014 Federation of European Microbiological Societies. Published by John Wiley & Sons Ltd. All rights reserved.)

Climate change causes temperature increase and alteration of precipitation patterns with frequent droughts. These are known to influence soil microorganisms leading to community shifts and physiological adaptations, with consequences for biogeochemical cycles. However, whether soil microbial communities evolved at different average temperature differ in their response to drought is not well understood. Therefore, we collected ten soil samples per site (0–30 cm soil depth) from a walnut-fruit forest at 1000, 1300 and 1600 m above sea level with similar vegetation which represent average temperature differences of 1.3 °C between sites, mimicking potential climate change. We incubated these for 70 days at 22 °C either at (i) constant moisture of 50 % soil water holding capacity, or subjected them to (ii) two or (iii) three drying-rewetting (DRW) cycles. Respiration was measured during the incubation; microbial and chemical properties were determined at the end. No elevation specific or interactive effects with DRW were detected, except for fungal gene abundance, where values were highest at the intermediate elevation level. This reveals that soil microbial communities evolved at different average temperature regimes do not differ in their response to drought. Therefore, data were pooled across all sites and analyzed for the main effects of DRW. Microbial activity increased with DRW as reflected by enhanced net‑nitrogen mineralization and basal respiration. However, microbial biomass carbon and ergosterol were reduced by 20 and 25 % and bacterial gene abundance between 20 and 40 %. This reflects the strong osmotic pressure of DRW causing death of microbial cells. The higher maintenance requirements for cell adjustment to osmotic pressure of surviving microorganisms was revealed by an increase of the metabolic quotient q CO 2 by 60 % and accumulation of potassium in the microbial biomass. Fungi cope better with DRW as shown by higher fungal gene abundance as well as their ratio to ergosterol after DRW, reflecting shifts in cell volume due to community shifts or morphological adaptations. Our findings highlight that soil microbial communities evolved under different average temperature regimes respond similarly to DRW, but overall shift towards fungi as this taxon can potentially physiologically better adapt to osmotic pressure. Consequently, DRW may cause higher organic matter turnover and nutrient release due to higher microbial maintenance costs for osmotic cell adjustments. • Drying-rewetting reduced microbial, in particular bacterial biomass. • Surviving microorganisms adjusted the cell volume and accumulated potassium. • Osmotic adjustments increased metabolic quotient and net N-mineralization. • Microbes adapted to higher temperature did not cope better with drought. [ABSTRACT FROM AUTHOR]