A warming climate has the potential to increase both the autotrophic and heterotrophic components of soil respiration. The resultant increase in CO2 flux to the atmosphere could create a positive feedback loop, leading to even greater warming. Experimental soil warming of 4 to 5 degrees C was applied from September 2010 to October 2014 in a sugar maple-dominated northern hardwood forest. Soil warming caused fine-root respiration rates to increase, but partial temperature acclimation did occur, limiting the resultant increase in CO2efflux to 19%, which is far less than would be expected with no acclimation. Bulk soil respiration and soil nitrogen mineralization both increased, indicating decomposition of soil organic matter was enhanced. The objective of this study was to investigate the reversibility of those responses during the first years after cessation of the experimental warming. During the growing season of 2015, when soil warming no longer occurred, fine-root (< 1 mm) respiration, soil respiration and net nitrogen mineralization were measured monthly, to determine if responses occurring under warming persisted or reversed.
Specific root respiration rates were 20% lower in the formerly warmed plots at the beginning of the growing season, consistent with the partial temperature acclimation that had occurred in response to soil warming. Over the growing season this acclimation reversed and by August no differences in fine-root respiration rates existed for roots from formerly warmed and non-warmed plots. Soils that were previously warmed had lower rates of soil respiration (-20%) and lower rates of nitrogen mineralization (-11%) throughout the post-warming study, indicating that labile organic matter had been depleted during the four plus years of warming. These results suggest that temperature acclimation of roots was a plastic, rapidly reversible response to warming. However, depletion of soil organic matter in response to climatic warming is a real possibility that could have longer term impacts on soil C pools and nutrient cycling.