Soil H218O labelling reveals the effect of drought on C18OO fluxes to the atmosphere

Above- and belowground processes in plants are tightly coupled via carbon and water fluxes through the soil-plant-atmosphere system. The oxygen isotopic composition of atmospheric CO2 and water vapour (H2Ov) provides a valuable tool for investigating the transport and cycling of carbon and water within this system. However, detailed studies on the coupling between ecosystem components and environmental drivers are sparse. Therefore, we conducted a (H2O)-O-18-labelling experiment to investigate the effect of drought on the speed of the link between below-and aboveground processes and its subsequent effect on (COO)-O-18 released by leaves and soils. A custom-made chamber system, separating shoot from soil compartments, allowed separate measurements of shoot-and soil-related processes under controlled conditions. Gas exchange of oxygen stable isotopes in CO2 and H2Ov served as the main tool of investigation and was monitored in real time on Fagus sylvatica saplings using laser spectroscopy. (H2O)-O-18-labelling showed that drought caused a slower transport of water molecules from soil to shoot, which was indicated by its direct derivation from independently measured concentrations and O-18/O-16 ratios of CO2 and H2Ov, respectively. Furthermore, drought reduced the O-18 equilibrium between H2O and CO2 at the shoot level, resulting in less-enriched (COO)-O-18 fluxes from leaf to atmosphere compared with control plants. Compared with the shoot, O-18 equilibrium was not instantaneous in the soil and no drought effect was apparent.