Land use intensity has a stronger effect on the temperature sensitivity of soil microbial carbon cycling processes than long-term climate change.

Microbes are responsible for the cycling of carbon (C) in soils, and predicted changes in soil C stocks under climate change are highly sensitive to shifts in the mechanisms thought to control microbial physiological response to warming. Two mechanisms have been proposed to explain the long-term effects of warming on microbial physiology: microbial thermal acclimation and changes in the quantity and quality of substrates available for microbial metabolism. However, studies disentangling these two mechanisms and assessing how land use affects them are lacking. To disentangle the drivers of changes in microbial physiology in response to long-term climate change, we sampled soils from a 10-year old global change and land use intensity experiment at the Pre-Alpine Terrestrial Environmental Observatories (TERENO project). The global change treatment includes a warming of 2oC and a reduction in precipitation of about 450 mm. We took soil samples at different time-points during the spring season and depths. We performed short-term laboratory incubations over a range of temperatures to measure the relationships between temperature sensitivity of physiology (growth, respiration, carbon use efficiency with the 18O-H2O method) and we characterized the quantity and quality of soil organic matter with the ramped thermal rock-eval pyrolysis at different depths. In this ongoing project, we did not observe thermal acclimation of microbial respiration, growth or CUE to climate change. However, fertilization had the strongest effect on the temperature sensitivity of microbial respiration. In the next steps of this project, we will determine whether climate change and/or land use intensity has an effect on soil organic carbon fractions with different residence times. Our preliminary results show that land use intensity has an overriding effect on the temperature sensitivity of microbial processes compared to long-term climate change.