Soil respiration is stimulated by elevated CO 2 and reduced by summer drought: three years of measurements in a multifactor ecosystem manipulation experiment in a temperate heathland (CLIMAITE)

Abstract

This study investigated the impact of predicted future climatic and atmospheric conditions on soil respiration ( R S ) in a Danish Calluna‐Deschampsia‐ heathland. A fully factorial in situ experiment with treatments of elevated atmospheric CO 2 (+130 ppm), raised soil temperature (+0.4 °C) and extended summer drought (5–8% precipitation exclusion) was established in 2005. The average R S , observed in the control over 3 years of measurements (1.7 μmol CO 2 m −2 sec −1 ), increased 38% under elevated CO 2 , irrespective of combination with the drought or temperature treatments. In contrast, extended summer drought decreased R S by 14%, while elevated soil temperature did not affect R S overall. A significant interaction between elevated temperature and drought resulted in further reduction of R S when these treatments were combined. A detailed analysis of short‐term R S dynamics associated with drought periods showed that R S was reduced by ~50% and was strongly correlated with soil moisture during these events. Recovery of R S to pre‐drought levels occurred within 2 weeks of rewetting; however, unexpected drought effects were observed several months after summer drought treatment in 2 of the 3 years, possibly due to reduced plant growth or changes in soil water holding capacity. An empirical model that predicts R S from soil temperature, soil moisture and plant biomass was developed and accounted for 55% of the observed variability in R S . The model predicted annual sums of R S in 2006 and 2007, in the control, were 672 and 719 g C m −2 y −1 , respectively. For the full treatment combination, i.e. the future climate scenario, the model predicted that soil respiratory C losses would increase by ~21% (140–150 g C m −2 y −1 ). Therefore, in the future climate, stimulation of C storage in plant biomass and litter must be in excess of 21% for this ecosystem to not suffer a reduction in net ecosystem exchange.

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