Soil microbial community structure and function mainly respond to indirect effects in a multifactorial climate manipulation experiment

Future climate change scenarios predict increases in surface temperature as well as atmospheric CO2 concentration. In this study we simultaneously addressed individual and combined effects of these factors on the soil microbial community structure and function. We tested linear as well as non-linear responses in a multifactorial climate manipulation experiment. After two years of climate change manipulations on a pre-Alpine managed grassland, topsoil samples were taken for analysis of functional enzyme activities, as well as microbial community structure. Besides, soil and vegetation parameters were measured to allow evaluation of direct and indirect effects. Pronounced and statistically significant spatial effects were observed on our field site for some variables. It is assumed that the history of site preparation could provide an explanation for the observed differences. Elevation of temperature or atmospheric CO2 did not induce strong shifts of soil fungal or bacterial communities. Only the inclusion of the spatial effects in the response surface regression model allowed the detection of subtle microbial responses to climate change scenarios. Mucor globulifera responded to temperature and CO2 in a pattern similar to soil water content. An increase in the relative abundance of coprophilous white rot fungi was observed upon warming, and this might be attributed to preferences of macrofauna for warmer plots. Specific extracellular enzyme activities were positively correlated with each other, especially within two groups of enzymes, which were involved in C-acquisition and in N-mining. The latter group responded positively to elevated CO2 concentrations. Chitinolytic activity increased with the relative abundance of the nematophagous and entomopathogenic ascomycete Purpureocillium lilacinum. We conclude that the indirect effects of future climate change scenarios prevail over direct effects on soil microbial community composition and function. Soil water content, nutrient pools, atmospheric CO2 and plant root identity were identified as drivers of the observed changes after removal of unintended spatial effects. Application of advanced statistical tools, which take spatial variability into account, was necessary to detect these effects. Minor changes in the fungal community occurred already after a short period of climate manipulation. More pronounced effects of elevated atmospheric CO2 concentration and surface warming on soil microbial community structure and function are expected on the longer-term, but indirect effects will most likely remain the dominant drivers.