Abstract
Advanced facilities are needed to evaluate the response of complex ecosystems to projected unique climate conditions not observable in the context of current natural variation or through the use of climate gradients. A next-generation, experimental system for simulating future belowground temperature increases was conceived, simulated, constructed and tested in a temperate deciduous forest in Oak Ridge, Tennessee, USA. The new system uses low-wattage, 3-m deep, circumferentially-installed heaters surrounding a defined soil volume to both add the necessary energy to support a set-point soil temperature differential within the treatment area and to add exterior energy inputs equal to that which might be lost from lateral heat conduction. This approach, which is designed to work in conjunction with aboveground heated chambers, requires only two control positions, (1) aboveground air temperatures at 1 m and (2) belowground temperatures at 0.8 m. The approach is capable of achieving in situ target temperature differentials in the tested range of +4.0 ± 0.5 °C for soils to a measured depth of -2 m located within the aboveground boundary for air heating. These differentials were sustained throughout 2009, and both diurnal and seasonal cycles at all soil depths were retained using this simple heating approach. Measured mean energy inputs required to sustain the target heating level of +4 °C over the 7.1 m2 target area were substantial: 21.1 kW h d-1 m-2 for aboveground heating but 16 times lower for belowground heaters at 1.3 kW h d-1 m-2. Observations of soil CO2 efflux from the surface of the target soil volumes showed CO2 losses throughout 2009 that were elevated above the temperature response curve for control CO2 losses at levels greater than have been reported in previous soil warming studies. Stimulation of biological activity of previously undisturbed deep-soil carbon stocks is the expected source. Long-term research programs may be able to apply similar experimental systems to address uncertainties in process-level responses of microbial, plant, and animal communities in whole, intact ecosystems using this new heating method that capture expected future warming and temperature dynamics throughout the soil profile.
