Autotrophic component of soil respiration is repressed by drought more than the heterotrophic one in dry grasslands

Summer droughts projected to increase in central Europe due to climate changes strongly influence the carbon cycle of ecosystems. Persistent respiration activities during drought periods are responsible for a significant carbon loss, which may turn the ecosystem from a sink into a source of carbon. There are still gaps in our knowledge regarding the characteristic changes taking place in the respiration of the different components of the ecosystem in response to drought events. In the present study, we combined a physical separation of soil respiration components with continuous measurements of soil CO2 efflux and its isotopic (C-13) signals at a dry grassland site in Hungary. The physical separation of soil respiration components was performed by means of inox meshes and tubes inserted into the soil. The root-excluded and root- and mycorrhiza-excluded treatments served to measure the isotopic signals of the rhizospheric, mycorrhizal fungi and heterotrophic components, respectively. In the dry grassland investigated in the study the three components of the soil CO2 efflux decreased at different rates under drought conditions. During drought the contribution made by the heterotrophic components was the highest (54 +/- 8 %; mean +/- SE). Rhizospheric component was the most sensitive to soil drying with its relative contribution to the total soil respiration dropping from 66 +/- 7 (non-stressed) to 35 +/- 17%(mean +/- SE) under drought conditions. According to our results the heterotrophic component of soil respiration is the major contributor to the respiration activities during drought events in the dry grassland ecosystem studied.