Fig. 1.

Setup of the ¹³C pulse labeling (Left) and the tracing of the fate of ¹³C in respiratory fluxes and plant and soil microbial pools (Right). A transparent plastic sheet was erected from scaffolds, enclosing the whole-tree crown to form a labeling chamber. ¹³CO₂ was added to the chamber from gas bottles via a mass flow controller. The ¹³CO₂ concentration was monitored with LGR CCIA 46d isotope laser spectrometers. The air temperature was adjusted to maintain ambient temperature with an air-conditioning system involving a closed coolant system so that no air exchange between inside and outside the chamber occurred. One branch, stem, and soil chamber per tree were connected via a manifold with automatically controlled solenoid valves to the isotope laser spectrometer. Within a radius of 8 m (along three transects per tree), soil collars were installed to determine the spatiotemporal pattern of soil-respired ¹³CO₂ that was measured in the gas samples. Samples of different plant organs and the soil microbial biomass were taken to determine the ¹³C enrichment at different time points after labeling.