The diel imprint of leaf metabolism on the δ13C signal of soil respiration under control and drought conditions

Summary •Recent 13CO2 canopy pulse chase labeling studies revealed that photosynthesis influences the carbon isotopic composition of soil respired CO2 (δ13CSR) even on a diel timescale. However, the driving mechanisms underlying these short-term responses remain unclear, in particular under drought conditions. •The gas exchange of CO2 isotopes of canopy and soil was monitored in drought/nondrought-stressed beech (Fagus sylvatica) saplings after 13CO2 canopy pulse labeling. A combined canopy/soil chamber system with gas-tight separated soil and canopy compartments was coupled to a laser spectrometer measuring mixing ratios and isotopic composition of CO2 in air at high temporal resolution. The measured δ13CSR signal was then explained and substantiated by a mechanistic carbon allocation model. •Leaf metabolism had a strong imprint on diel cycles in control plants, as a result of an alternating substrate supply switching between sugar and transient starch. By contrast, diel cycles in drought-stressed plants were determined by the relative contributions of autotrophic and heterotrophic respiration throughout the day. Drought reduced the speed of the link between photosynthesis and soil respiration by a factor of c. 2.5, depending on the photosynthetic rate. •Drought slows the coupling between photosynthesis and soil respiration and alters the underlying mechanism causing diel variations of δ13CSR.