Background/Question/Methods   Soil respiration is the largest flux of carbon dioxide to the atmosphere, releasing more carbon than fossil fuel combustion. Since temperature affects soil respiration, on a global scale, even a small warming-induced increase in carbon dioxide emission from soils could act as a positive feedback to climate change. Currently, climate-carbon models represent the relationship between soil respiration and temperature using an exponential function with a fixed respiratory quotient (Q10) of 2. However, the temperature sensitivity (apparent Q10) of soil respiration varies with spatial and temporal changes in soil moisture, substrate availability and plant phenology. Thus, a simple exponential Q10 function can over- or underestimate soil respiration, leading to errors in projected carbon budgets under future climate scenarios. We examined temperature sensitivity of soil respiration (roots+ microbes) and microbial respiration alone to climatic changes in an old-field community in Waltham, MA. The study was conducted at The Boston-Area Climate Experiment, which exposes the vegetation to four levels of warming (ambient, +1.3, +2.7, and +4oC) and three levels of precipitation (drought, ambient, wet).

Results/Conclusions   The results showed that the apparent Q10 of soil respiration was strongly influenced by seasonal variation in soil moisture. Temperature was a good predictor of apparent Q10 during spring and fall seasons when moisture was not limiting. The lowest apparent Q10 values were observed in drought treatments during the summer. Moisture stress reduces the diffusion of soluble carbon substrates and extracellular enzymes, limiting microbial activity and hence soil respiration. Warming coupled with moisture stress decreased the apparent Q10 of soil respiration. Based on these results, we argue that models of soil respiration should incorporate both temperature and moisture as controlling variables.