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 CO2 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 m2 roofs during 25 days in July/August 2008 and 2009. Soil warming permanently increased the CO2 efflux from soil, whereas throughfall exclusion led to a sharp decrease in soil CO2 efflux (45% and 50% reduction during roof installation in 2008 and 2009, respectively). In 2008, CO2 efflux did not recover after natural rewetting and remained lowered until autumn. In 2009, CO2 efflux recovered shortly after rewetting, but relapsed again for several weeks. Drought offset the increase in soil CO2 efflux by warming in 2008 (growing season CO2 efflux in t C ha−1: control: 7.1 ± 1.0; warmed: 9.5 ± 1.7; warmed + roof: 7.4 ± 0.3; roof: 5.9 ± 0.4) and in 2009 (control: 7.6 ± 0.8; warmed + roof: 8.3 ± 1.0). 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 CO2 efflux during and after summer drought.

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Abstract.Peatlands store large amounts of organic carbon, but the carbon stock is sensitive to changes in precipitation or water table manipulations. Restoration of drained peatlands by drain blocking and flooding is a common measure to conserve and augment the carbon stock of peatland soils. Here, we report to what extent flooding affected the contribution of heterotrophic and rhizosphere respiration to soil CO 2 efflux in a grass-dominated mountain fen in Germany. Soil CO 2 efflux was measured in three un-manipulated control plots and three flooded plots in two consecutive years. Flooding was achieved by permanent irrigation during the growing seasons. Radiocarbon signatures of CO 2 from different sources including soil CO 2 efflux, incubated peat cores and live grass roots were repeatedly analyzed for partitioning of soil CO 2 efflux. Additionally, heterotrophic respiration and its radiocarbon signature were determined by eliminating rhizosphere respiration in trenched subplots (only control). In the control plots, rhizosphere respiration determined by 14 C signatures contributed between 47 and 61 % during the growing season, but was small (4 ± 8 %) immediately before budding. Trenching revealed a smaller rhizosphere contribution of 33 ± 8 % (2009) and 22 ± 9 % (2010) during growing seasons.Flooding reduced annual soil CO 2 efflux of the fen by 42 % in 2009 and by 30 % in 2010. The reduction was smaller in 2010 mainly through naturally elevated water level in the control plots. A one-week interruption of irrigation caused a strong short-lived increase in soil CO 2 efflux, demonstrating the sensitivity of the fen to water table drawdown near the peat surface. The reduction in soil CO 2 efflux in the flooded plots diminished the relative proportion of rhizosphere respiration from 56 to 46 %, suggesting that rhizosphere respiration was slightly more sensitive to flooding than heterotrophic respiration.

Peatlands store large amounts of organic carbon, but the carbon stock is sensitive to changes in precipitation or water table manipulations. Restoration of drained peatlands by drain blocking and flooding is a common measure to conserve and augment the carbon stock of peatland soils. Here, we report to what extent flooding affected the contribution of heterotrophic and rhizosphere respiration to soil CO₂ efflux in a grass-dominated mountain fen, Germany. Soil CO₂ efflux was measured in three un-manipulated control plots and three flooded plots in two consecutive years. Flooding was achieved by permanent irrigation during the growing seasons. Radiocarbon signatures of CO₂ from different sources including soil CO₂ efflux, incubated peat cores and live grass roots were repeatedly analyzed for partitioning of soil CO₂ efflux. Additionally, heterotrophic respiration and its radiocarbon signature were determined by eliminating rhizosphere respiration in trenched subplots (only control). In the control plots, rhizosphere respiration determined by ¹⁴C signatures contributed between 47 and 61% during the growing season, but was small (4%) immediately before budding. Trenching revealed a smaller rhizosphere contribution of 33% (2009) and 22% (2010) during growing seasons.

Flooding reduced annual soil CO₂ efflux of the fen by 42% in 2009 and by 30% in 2010. The reduction was smaller in 2010 mainly through naturally elevated water level in the control plots. A 1-week interruption of irrigation caused a strong short-lived increase in soil CO₂ efflux, demonstrating the sensitivity of the fen to water table drawdown near the peat surface. The reduction in soil CO₂ efflux in the flooded plots diminished the relative proportion of rhizosphere respiration from 56 to 46%, suggesting that rhizosphere respiration was slightly more sensitive to flooding than heterotrophic respiration. We conclude that the moderate decrease in rhizosphere respiration following flooding arises from a gradual change in vegetation in this fen ecosystem