Climate change may considerably impact the carbon (C) dynamics and C stocks of forest soils. To assess the combined effects of warming and reduced precipitation on soil CO 2 efflux, we conducted a two-way factorial manipulation experiment (4°C soil warming+throughfall exclusion) in a temperate spruce forest from 2008 until 2010. Soil was warmed by heating cables throughout the growing seasons. Soil drought was simulated by throughfall exclusions with three 100 m 2 roofs during 25 days in July/August 2008 and 2009. Soil warming permanently increased the CO 2 efflux from soil, whereas throughfall exclusion led to a sharp decrease in soil CO 2 efflux (45% and 50% reduction during roof installation in 2008 and 2009, respectively). In 2008, CO 2 efflux did not recover after natural rewetting and remained lowered until autumn. In 2009, CO 2 efflux recovered shortly after rewetting, but relapsed again for several weeks. Drought offset the increase in soil CO 2 efflux by warming in 2008 (growing season CO 2 efflux in t C ha -1: control: 7.11.0; warmed: 9.51.7; warmed+roof: 7.40.3; roof: 5.90.4) and in 2009 (control: 7.60.8; warmed+roof: 8.31.0). Throughfall exclusion mainly affected the organic layer and the top 5 cm of the mineral soil. Radiocarbon data suggest that heterotrophic and autotrophic respiration were affected to the same extent by soil warming and drying. Microbial biomass in the mineral soil (0-5 cm) was not affected by the treatments. Our results suggest that warming causes significant C losses from the soil as long as precipitation patterns remain steady at our site. If summer droughts become more severe in the future, warming induced C losses will likely be offset by reduced soil CO 2 efflux during and after summer drought.
