Distribution of Prokaryotic Abundance and Microbial Nutrient Cycling Across a High-Alpine Altitudinal Gradient in the Austrian Central Alps is Affected by Vegetation, Temperature, and Soil Nutrients

Studies of the altitudinal distributions of soil microorganisms are rare or have led to contradictory results. Therefore, we studied archaeal and bacterial abundance and microbial-mediated activities across an altitudinal gradient (2700 to 3500 m) on the southwestern slope of Mt. Schrankogel (Central Alps, Austria). Sampling sites distributed over the alpine (2700 to 2900 m), the alpine-nival (3000 to 3100 m), and the nival altitudinal belts (3200 to 3500 m), which are populated by characteristic plant assemblages. Bacterial and archaeal abundances were measured via quantitative real-time PCR (qPCR). Moreover, microbial biomass C, microbial activity (dehydrogenase), and enzymes involved in carbon (CM-cellulase), nitrogen (protease), phosphorus (alkaline phosphatase), and sulfur (arylsulfatase) cycling were determined. Abundances, microbial biomass C, and activities almost linearly decreased along the gradient. Archaeal abundance experienced a sharper decrease, thus pointing to pronounced sensitivity toward environmental harshness. Additionally, abundance and activities were significantly higher in soils of the alpine belt compared with those of the nival belt, whereas the alpine-nival ecotone represented a transitional area with intermediate values, thus highlighting the importance of vegetation. Archaeal abundance along the gradient was significantly related to soil temperature only, whereas bacterial abundance was significantly related to temperature and dissolved organic carbon (DOC). Soil carbon and nitrogen concentrations explained most of the variance in enzyme activities involved in the cycling of C, N, P, and S. Increasing temperature could therefore increase the abundances and activities of microorganisms either directly or indirectly via expansion of alpine vegetation to higher altitudes and increased plant cover.