Abstract:Soil temperature affects virtually all other ecosystem processes and will be the variable most directly altered with global warming. We initiated a multi-factor global change experiment to explore the effects of infrared heat loading (HT) and water table level (WL) treatment on soil temperature in bog and fen peatland mesocosms. We found that the temperature of the experimental mesocosms varied highly by year, month, peatland type, soil depth, heat loading and water table manipulation. The highest effect of heat loading on the temperature at 25 cm depth was found in June for the bog mesocosms (3.34 – 4.27 °C) but in May for the fen mesocosms (2.32 – 4.33 °C) over the two-year study period. The effects of water table levels in the bog mesocosms were only found between August and January, with the wet mesocosms warmer than the dry mesocosms by 0.48-2.03 °C over the two-year study period. In contrast, wetter fen mesocosms were generally cooler by 0.16-3.87 °C. Seasonal changes of temperatures elevated by the heat loading also varied by depth and ecosystem type, with temperature differences at 5 and 10 cm depth showing smaller seasonal fluctuations than those at 25 and 40 cm in the bog mesocosms. However, increased heat loading did not always lead to warmer soil temperatures, especially in the fen mesocosms, where high latent heat loss during the growing season and sensible heat loss during the freezing season when snow cover was significantly reduced likely resulted in temporary soil cooling. Both heat loading and water table manipulations have also changed the length of the non-frozen season, including the starting and ending dates.