Brüggemann, N.

Gutknecht, J.

Kothieringer, K.

Steigner, D.

English

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KIT – a cooperation betweenKarlsruhe Research Centerand University of KarlsruheContact: Dr. Nicolas Brüggemann (nicolas.brueggemann@imk.fzk.de)Tel.: +49-(0)8821-183226, Fax: +49-(0)8821-183294Short-term Fluctuations of δ13C and δ18O of Carbon Dioxide in the Gas Exchange of Norway Spruce Measured with a Tunable Diode Laser Absorption SpectrometerN. Brüggemann, J. Gutknecht, K. Kothieringer, D. SteignerCenter of Stable Isotopes, Forschungszentrum Karlsruhe, Institute of Meteorology and Climate Research (IMK-IFU), Kreuzeckbahnstrasse 19, D-82467 Garmisch-PartenkirchenI. BackgroundUnderstanding the role of terrestrial ecosystems in governing the isotopic composition of atmospheric CO2 requires knowledge of environmental controls of δ13C and δ18O of CO2 in biosphere-atmosphere CO2 exchange. Leaf–atmosphere CO2 exchange plays an important role in determining δ13C and δ18O of ecosystem CO2, as leaves are a CO2 sink due to photosynthetic assimilation with associated discrimination against the heavier C and O isotopes, but also a CO2 source due to dark and photorespiration as well as the backflow of non-assimilated CO2 out of the leaf after exchanging O isotopes with leaf water, which is usually significantly enriched in 18O compared to soil water due to isotopic fractionation during transpiration. The oxygen isotopic composition of CO2 is the pivotal factor for disentangling the CO2 component fluxes in ecosystem-atmosphere CO2 exchange. However, the disentangling will only succeed if spatial and temporal variability, especially short-term fluctuations, of δ18O of CO2 are known and understood. Tunable diode laser absorption spectroscopy (TDLAS) has recently led to great advances in the field of stable isotope research at the plant and ecosystem level, as it allows for measuring stable isotope ratios in trace gases and water with an unprecedented time resolution. Here we show data from laboratory experiments with Norway spruce saplings in closed chambers and a TDL instrument on short-term fluctuations of δ13C and δ18O of CO2 exchanged between needles, soil compartment and cuvette air. The data presented show on the one hand the feasibility of applying the TDLAS technique to leaf-level gas exchange measurements, on the other hand the relevance of short-term fluctuations of δ18O of CO2 in plant-atmosphere exchange for assessing the contribution of different sources and sinks to whole ecosystem CO2 fluxes.dewpoint drierTGAEquilbrationtankLochblendeCO2Clean airgeneratordew point meteroverflowLochblendeBinosLochblendeLochblendelightpyranometerlightEquilbrationtankLochblendeLochblendeLochblendeLochblendeII. ExperimentA. Plant material. 4-year-old spruce saplings were dug out in a Norway spruce forest (Höglwald close to Augsburg, Germany) in April 2007 with the roots in the original soil, leaving the roots as intact possible. The plants were potted in plastic pots and transferred to the IMK-IFU. During the summer, the plants were kept outside and watered regularly. B. Experimental set-up. The experiments were conducted in September 2007 in an air-conditioned laboratory (~21°C). For each experiment, two spruce saplings were put in gas-tight perspex cuvettes, consisting of a bottom part enclosing the potted roots and a top part enclosing the shoot. Each cuvette was flushed with 2 L min-1 of purified air with a dewpoint (dp) of 1°C and with 380 ppm CO2 with a δ13C of -32‰ vs. PDB. Soil temperature was measured with a PT100 sensor; needle temperature was measured with three fine-wire thermocouples attached to the needles. A global radiation pyranometerwas placed between both shoot cuvettes. Light (Osram Halogen 60 W) was automatically turned on every day at 5:00 LT and turned of at 21:00 LT. The outlet of each cuvette was connected to the TDL instrument (TGA100A, Campbell Scientific, USA) and an IRGA for H2O measurements.C. Measurements. After a pre-run for 3–4 days to adapt plants to the new environment, a measurement sequence was started. On 5 consecutive days, environmental parameters were individually changed for a time period of 8 hours, and the response of shoots and roots was measured. During 9:00 and 17:00 LT the plants were exposed to (1) low air humidity (dp ≈ -30°C), (2) low light  intensity, (3) changing light intensity, (4) low CO2 concentration (280 ppm), (5) high CO2 concentration (700 ppm). The measurements were followed by a post-run of 3 to 4 days, in which the original conditions were maintained.III. Results12CO2flux[µmol g-1h-1 ]13CO2 & 18CO2 flux[µmol g-1h-1]Δδ13C [‰] Δδ18O [‰]H2O flux[mmol g-1h-1 ]temperature[°C]GRGR[w m-2]12CO2flux[µmol g-1h-1 ]13CO2 & 18CO2 flux[µmol g-1h-1]temperature[°C]Δδ13C [‰] Δδ18O [‰]ABCDEFGHDate 2007Fig. 1. Scheme of the experimental set-up30.8.07  2.9.07  5.9.07  8.9.07  11.9.07  δ18 O of net CO2 flux [‰ vs. SMOW]-2000200400600800δ13 C of net CO2 flux [‰ vs. PDB]-60-40-200 spruce 1 spruce 2 31.8.07  3.9.07  6.9.07  9.9.07  12.9.07  δ18 O of net CO2 flux [‰ vs. SMOW]-40-2002040δ13 C of net CO2 flux [‰ vs. PDB]-60-40-200Shoots Roots & soilFig. 3. Time courses of CO2isotopic and H2O fluxes, of environmental variables and of Δδ13C and Δδ18O of CO2 (= difference between cuvette inletand outlet) in experiment 1 forshoots (A-E) and roots/soil (F-H).Fig. 4. Calculated δ13C and δ18O values for the net CO2 flux into/out of shoots and roots/soil of the two Norway spruce saplings of experiment 1. In the roots/soil graphs only data forspruce 1 are shown as the soil cuvette became leaky after some days.Fig. 2. Spruce sapling in the twin cuvette for separate measurements of above- and belowground compartment.22-24 Sep8-11 SepPost-run21 Sep7 SepHigh CO220 Sep6 SepLow CO219 Sep5 SepVariable light18 Sep4 SepLow light17 Sep3 SepLow H2O13-16 Sep30 Aug-2 SepPre-runExp. 2Exp. 1TreatmentTab. 1. Time table for the different treatments in the two experiments.IV. SummaryDuring steady-state photosynthesis, which was reached one hour after exposing the spruce saplings to full light, isotopic 13C fractionation during CO2 uptake was in the range of 20‰, whereas 18O fractionation was in the range of 45‰. At night, 13C fractionation disappeared. In contrast, rapid changes from light to darkness caused a huge increase in δ18O in CO2released by the above-ground plant material of up to 700‰within a few minutes. In the following eight hours of darkness the δ18O values decreased quickly at first, then more gradually to approx. 50-100‰ enrichment. The only significant treatment effect on both δ13C and δ18O was observed during low light intensity, when C and O isotopic fractionation was enhanced (4 Sep in Fig. 4). Elevated CO2 treatment had an increasing effect on 13C discrimination only (7 Sep in Fig. 4). The roots/soil compartment did not show any significant treatment effect nor a visible diurnal cycle in isotopic fluxes.

Short-term fluctuations of δ¹³C and δ¹⁸O of carbon dioxide in the gas exchange of Norway spruce measured with a tunable diode laser absorption spectrometer

https://publikationen.bibliothek.kit.edu/220073244/3815322

Short-term fluctuations of δ¹³C and δ¹⁸O of carbon dioxide in the gas exchange of Norway spruce measured with a tunable diode laser absorption spectrometer

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