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Along with predicted climate change, increased risks for summer drought are projected for Central Europe. However, large knowledge gaps exist in terms of how drought events influence the short-term ecosystem carbon cycle. Here, we present results from 13CO2 pulse labeling experiments at an intensively managed lowland grassland in Switzerland. We investigated the effect of extreme summer drought on the short-term coupling of freshly assimilated photosynthates in shoots to roots as well as to soil CO2 efflux. Summer drought was simulated using rainout shelters during two field seasons (2010 and 2011). Soil CO2 efflux and its isotopic composition were measured with custom-built chambers coupled to a quantum cascade laser spectrometer (QCLAS-ISO, Aerodyne Research Inc., MA, USA). During the 90 min pulse labeling experiments, we added 99.9 atom % 13CO2 to the grass sward. In addition to the isotopic analysis of soil CO2 efflux, this label was traced over 31 days into bulk shoots, roots and soil. Drought reduced the incorporation of recently fixed carbon into the shoots, but increased the relative allocation of fresh assimilates below ground compared to the control grasslands. Contrary to our hypothesis, we did not find a change of allocation speed in response to drought. Although drought clearly reduced soil CO2 efflux rates, about 75% of total tracer uptake in control plots was lost via soil CO2 efflux during 19 days after pulse labeling, compared to only about 60% under drought conditions. Thus, the short-term coupling of above- and below-ground processes was reduced in response to summer drought. The occurrence of a natural spring drought in 2011 lead to comparable albeit weaker drought responses increasing the confidence in the generalizability of our findings.

We present the continuous data record of atmospheric CO2 isotopes measured by laser absorption spectroscopy for an almost four year period at the High Altitude Research Station Jungfraujoch (3580 m a.s.l.), Switzerland. The mean annual cycles derived from data of December 2008 to September 2012 exhibit peak-to-peak amplitudes of 11.0 μmol mol−1 for CO2, 0.60‰ for δ13C and 0.81‰ for δ18O. The high temporal resolution of the measurements also allow to capture variations on hourly and diurnal time scales. For CO2 the mean diurnal peak-to-peak amplitude is about 1 μmol mol−1 in spring, autumn and winter and about 2 μmol mol−1 in summer. The mean diurnal variability in the isotope ratios is largest during the summer months too, with an amplitude of about 0.1‰ both in the δ13C and δ18O, and a smaller or no discernible diurnal cycle during the other seasons. The day-to-day variability, however, is much larger and depends on the origin of the air masses arriving at Jungfraujoch. Backward Lagrangian particle dispersion model simulations revealed a close link between air composition and prevailing transport regimes and could be used to explain part of the observed variability in terms of transport history and influence region. A footprint clustering showed significantly different wintertime CO2, δ13C and δ18O values depending on the origin and surface residence times of the air masses. Based on the experiences gained from our measurements, several major updates on the instrument and the calibration procedures were performed in order to further improve the data quality. We describe the new measurement and calibration setup in detail and demonstrate the enhanced performance of the analyser. A precision of about 0.02‰ for both isotope ratios has been obtained for an averaging time of 10 min.

In this study we present a combined soil/canopy chamber system that allows the investigation of carbon flow through the atmosphere-plant-soil system via a 13CO2 canopy labelling approach – especially when using short vegetation such as tree saplings. The developed chamber system clearly separates soil and canopy compartment in order to (a) prevent physical diffusion of 13C tracer into the soil chamber during a 13CO2 canopy pulse labelling (b) study stable isotope processes in soil and canopy individually and independently. In combination with novel laser spectrometry, measuring CO2 (Aerodyne Research Inc.) and H2O (Los Gatos Research Inc.) isotopologue mixing ratios at a rate of 1 Hz, we were able to trace the label transport from leaves to roots in small beech saplings (Fagus sylvatica L.) without interference due to contamination of the soil matrix and/or canopy re-labelling via tracer returning from soil respiration. A very tight coupling between above- (photosynthesis) and belowground (soil respiration) processes was found, where newly assimilated carbon fixed from the 13CO2 atmosphere re-appeared in soil respiration 2 h after it has been photosynthetically fixed. We were able to demonstrate that leaf metabolism acts on substrate for soil respiration on a diurnal timescale, with input of fresh photosynthates during daytime and starch re-mobilisation during nighttime. Long-term fluctuations in the δ13C of soil respiration, as observed under reduced water availability, could not be described by any biological or instrumental mechanism, as they did occur in an atypical ca. 15 hourly rhythm – potential mechanisms driving these fluctuations are hypothesized.