Carbon losses due to soil warming: Do autotrophic and heterotrophic soil respiration respond equally?

Global warming has the potential to increase soil respiration ( R S ), one of the major fluxes in the global carbon (C) cycle. R S consists of an autotrophic ( R A ) and a heterotrophic ( R H ) component. We combined a soil warming experiment with a trenching experiment to assess how R S , R A , and R H are affected. The experiment was conducted in a mature forest dominated by Norway spruce. The site is located in the Austrian Alps on dolomitic bedrock. We warmed the soil of undisturbed and trenched plots by means of heating cables 4 °C above ambient during the snow‐free seasons of 2005 and 2006. Soil warming increased the CO 2 efflux from control plots ( R S ) by ∼45% during 2005 and ∼47% during 2006. The CO 2 efflux from trenched plots ( R H ) increased by ∼39% during 2005 and ∼45% during 2006. Similar responses of R S and R H indicated that the autotrophic and heterotrophic components of R S responded equally to the temperature increase. Thirty‐five to forty percent or 1 t C ha −1 yr −1 of the overall annual increase in R S (2.8 t C ha −1 yr −1 ) was autotrophic. The remaining, heterotrophic part of soil respiration (1.8 t C ha −1 yr −1 ), represented the warming‐induced C loss from the soil. The autotrophic component showed a distinct seasonal pattern. Contribution of R A to R S was highest during summer. Seasonally derived Q 10 values reflected this pattern and were correspondingly high (5.3–9.3). The autotrophic CO 2 efflux increase due to the 4 °C warming implied a Q 10 of 2.9. Hence, seasonally derived Q 10 of R A did not solely reflect the seasonal soil temperature development.