Response of microbial growth and enzyme activity to climate change in European mountain grasslands: A translocation study

Soil microorganisms are the major players controlling the soil carbon (C) and nutrient cycling, however the extent to which the functional properties of soil microorganisms will alter with climate change in mountain grasslands is still unclear. To simulate future climatic conditions (higher temperature and less precipitation), intact plant-soil mesocosms of three mountain grassland sites, Esterberg (1260 m a.s.l.), Graswang (860 m a.s.l.), and Fendt 550 m a.s.l.), were translocated to the lowest elevation Bayreuth (350 m a.s.l.). After two years of translocation to warmer and drier environments, specific growth rates of soil microorganisms were 25 % slower than those at higher elevations. This indicated a shift towards to slow-growing K-strategists with increasing temperature and decreasing soil moisture, presumably attributed to the depletion of available C for microorganisms. Simultaneously, higher activity and faster substrate turnover time of C-acquiring enzymes in the warmer and drier soils implied a stronger C rather nutrient limitation. This was supported by the increased vector length, which was induced by decreased dissolved organic compounds at lower elevation. In contrast, microorganisms were limited by nitrogen (N) at higher elevation because of the stronger competition for N between plants and microorganisms. In short, translocated microorganisms increased their active fraction, enzymatic activity and altered enzyme systems but decreased specific growth rates. This indicated that soil microbial community adjusted to the changing climate resulting in faster substrate turnover, and in turn caused 15 % reduction in soil C content from Esterberg to Bayreuth. We therefore conclude that climate warming in combination with decreased precipitation increases microbial activity, leads to shifts in microbial life strategies and induces alterations in microbial nutrient limitation, i.e. shift from N- to C and P co-limitation with translocation to lower elevation sites. This in turn, is likely to result in C losses of mountain grassland soils under future climate change, which will amplify negative impacts on global warming.