Will warming enhance soil organic matter decomposition? Evidence from an in situ warming experiment and process-based soil laboratory incubations

Background/Question/Methods

One of the largest potential positive feedbacks to climate warming is CO₂ arising from the enhanced decomposition of soil organic matter (SOM) in warmer soils. Soil microbial acclimation to warming temperatures, depletion of a responsive pool of labile carbon, and soil drying, however, all have the potential to decrease the soil respiration response to warming, yet their effects are not well understood. The goal of this study was to combine measurements of the in situ soil respiration response to ecosystem warming in an open-air warming experiment at two southern boreal forest sites in Minnesota with laboratory studies directed at understanding the mechanisms underlying those effects. The in situ experiment uses infrared heat lamps and soil heating cables to heat 7-m² plots continuously during the growing season by 1.8 or 3.6°C. We measured soil CO₂ flux every two weeks during the snow free season from 2009 to 2011. Laboratory soil incubations used repeated measurements of the temperature response of microbial respiration with southern boreal forest soils incubated at three different moisture contents and at two different temperatures to determine the relative importance of microbial acclimation, carbon pool, and soil moisture effects in controlling the release of CO₂ from warmer soils. 

Results/Conclusions

In three years of in situ experimental warming, soil respiration increased in the two heated treatments relative to control plots. Soil (microbial + root) respiration increased by 6 and 20% in the +1.8 and +3.6°C treatments, respectively. Microbial respiration, measured in root exclusion collars, showed a similar warming enhancement of 4% and 14% in the +1.8 and +3.6°C treatments. These results suggest that enhanced SOM decomposition could be a significant source of CO₂ to the atmosphere in a warmer world. Supporting this, laboratory incubations of southern boreal forest soil showed that when water was not limiting, the temperature sensitivity (i.e Q₁₀) of soil microbial respiration increased as the incubations progressed (i.e. as respiration became dominated by the respiration of more recalcitrant SOM). In contrast, the same laboratory study found that the main effects of simulated soil drying and warming both decreased the temperature sensitivity of soil microbial respiration, meaning that warming-induced soil drying and soil microbial thermal acclimation could decrease the potential warming enhancement of soil organic matter decomposition. Together, these results suggest that warming has the potential to release large amounts of carbon stored in recalcitrant SOM, but may be offset by soil drying and soil microbial acclimation.