Temperature sensitivity of soil respiration in a low-latitude forest ecosystem varies by season and habitat but is unaffected by experimental warming

Experimental warming of forest ecosystems typically stimulates soil respiration (CO₂ efflux), but most warming experiments have been conducted in northern latitudes (>?40°N) with relatively young soils. We quantified the influence of experimental warming on soil respiration (R_T) in two adjacent forest habitats—a mature, closed canopy forest and a gap where trees were manually removed— on highly-weathered Ultisols of the southeastern U.S. (33°N). Using temperature variation, both natural and induced by experimental warming, we also quantified the temperature sensitivity of R_T, defined as the activation energy, E_A in the Arrhenius equation. Experimental warming (either + 3 °C or + 5 °C above ambient) did not significantly increase soil respiration rate or cumulative CO₂ loss over the 3 years of the experiment, and did not influence the temperature sensitivity of soil respiration, once the influence of natural temperature variation was taken into consideration. Despite the absence of an experimental warming effect, we observed that E_A varied on monthly time scales, and varied differently in each habitat. Soil moisture and habitat also influenced R_T, but the effects were not consistent, and varied by month. Our results suggest that although R_T does depend on temperature, the sensitivity of R_T to temperature variation is influenced primarily by factors like microclimate and plant phenology that can change on relatively short (<?monthly) time scales. Thus, using the temperature sensitivity of R_T to predict future CO₂ losses due to warming is only reasonable if monthly variation in E_A is incorporated into models for lower-latitude subtropical ecosystems with highly weathered soils, such as those in this study. Finally, our results suggest that higher temperatures may not enhance R_T in highly-weathered, C-poor soils to the extent that has been reported in prior studies of high-latitude soils, which may constrain ecosystem-atmosphere carbon exchanges and feedbacks to the climate system.