Background/Question/Methods It has been shown that field warming experiments have lead to a stimulation of soil respiration that diminishes over time.  The primary hypothesis to explain this response is that availability of labile soil organic matter declines as the result of temperature stimulated decomposition rates, leading to a decline in the CO₂ efflux.  However, an alternative hypothesis is that microorganisms acclimate to warming, so that their metabolism is less temperature sensitive thus leading to a reduced CO₂ efflux as well. 
Extracellular enzymes are produced by bacteria and fungi to break down complex organic matter into simple compounds that can be taken up by the cell.  We hypothesized that soil microorganisms may acclimate to temperature by altering the production of extracellular enzymes.  To test this hypothesis, we used soils from a warming experiment conducted at the USDA-ARS High Plains Grasslands Research Station, Cheyenne, WY.  The plots were warmed with infrared heaters leading to a 2°C increase in air temperature and a 5°C increase in soil temperature in the first five cm.  Soils were collected three months after the initiation of the warming, roots removed and stored at -80°C until enzyme analysis.  Enzyme assays for β-glucosidase, cellobiohydrolase, xylosidase, N-acetyl glucosamidase, and leucine amino peptidase were performed at 15 and 25°C using fluorescent substrates.
Results/Conclusions Large changes in soil enzyme activities and their temperature sensitivities were observed.  The activities of enzymes involved in both C (β-glucosidase, cellobiohydrolase ,and  xylosidase) and N (N-acetyl glucosamidase and leucine amino peptidase) cycling were increased by warming when assayed at 15°C.  However, when assayed at 25°C the increases due to field warming were smaller, suggesting a reduction in enzyme temperature sensitivity.  This is the first evidence of a specific mechanism for microbial acclimation to climate warming.