Transplanting boreal soils to a warmer region increases soil heterotrophic respiration as well as its temperature sensitivity

Under a warming climate, the boreal forest could become one of the largest terrestrial net CO2 sources, as increasing disturbances and soil organic matter decomposition rates (heterotrophic respiration, Rh) could offset net primary production. Since soil represents the boreal forest's largest C pool, it is critical of correctly predicting future changes in Rh, as well as its sensitivity to temperature (Q10 of Rh). We simulated a soil warming by transplanting soil cores from boreal balsam fir (Abies balsamea, BF) and black spruce (Picea mariana, BS) stands to a more southern Eastern hemlock stand (Tsuga canadensis, EH). We measured Rh and soil properties over 3 years, from June to October. Over three snow-free seasons, soil temperature (first 10 cm, including the FH organic layers) and Rh increased for BF (+3.2 °C, +60% of Rh) and BS cores (+2.3 °C, +27% of Rh). Microbial C concentration decreased by 54-73% in the FH layers of warmed and control cores relative to initial values, despite unchanged chemically labile C, probably due to excised roots and mycorrhizal hyphae. This suggests a possible underestimation of Rh during the experiment. In BF soils only, the increase in Rh was accompanied by an increase in its sensitivity to temperature. Under a +5 °C soil warming, mean predicted Rh of BF soils would increase by 83% rather than by 56%. Relative to BS soils, such increase in sensitivity could be partly due to a higher fraction of chemically labile C (+52%) in the FH layers and a higher mean warming effect. It suggests that for BF forest soils, predicting decomposition rates for a warmer climate based on current temperature sensitivities could be inadequate. However, longer-term studies are needed to see if this increase in Q10 of Rh for BF soils would be maintained for longer periods.