Seasonal influence of climate manipulation on microbial community structure and function in mountain soils

Microbial communities drive soil organic matter (SOM) decomposition through the production of a variety of extracellular enzymes. Climate change impact on soil microbial communities and soil enzymatic activities can therefore strongly affect SOM turnover, and thereby determine the fate of ecosystems and their role as carbon sinks or sources. To simulate projected impacts of climate change on Swiss Jura subalpine grassland soils, an altitudinal soil transplantation experiment was set up in October 2009. On the fourth year of this experiment, we measured microbial biomass (MB), microbial community structure (MCS), and soil extracellular enzymatic activities (EEA) of nine hydrolytic and oxidative extracellular enzymes in the transplanted soils on a seasonal basis. We found a strong sampling date effect and a smaller but significant effect of the climate manipulation (soil transplantation) on EEA. Overall EEA was higher in winter and spring but enzymes linked to N and P cycles showed higher potential activities in autumn, suggesting that other factors than soil microclimate controlled their pool size, such as substrate availability. The climate warming manipulation decreased EEA in most cases, with oxidative enzymes more concerned than hydrolytic enzymes. In contrast to EEA, soil MB was more affected by the climate manipulation than by the seasons. Transplanting soils to lower altitudes caused a significant decrease in soil MB, but did not affect soil MCS. Conversely, a clear shift in soil MCS was observed between winter and summer. Mass-specific soil EEA (EEA normalized by MB) showed a systematic seasonal trend, with a higher ratio in winter than in summer, suggesting that the seasonal shift in MCS is accompanied by a change in their activities. Surprisingly, we observed a significant decrease in soil organic carbon (SOC) concentration after four years of soil transplantation, as compared to the control site, which could not be linked to any microbial data. We conclude that medium term (four years) warming and decreased precipitation strongly affected MB and EEA but not MCS in subalpine grassland soils, and that those shifts cannot be readily linked to the dynamics of soil carbon concentration under climate change. (C) 2014 Elsevier Ltd. All rights reserved.