Brüggemann, N.

Gutknecht, J.

Meier, R.

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KIT – a cooperation betweenKarlsruhe Research Centerand University of KarlsruheMoisture and Temperature Dependence of δ13C and δ18O of CO2 from Heterotrophic Soil RespirationN. Brüggemann, J. Gutknecht, R. MeierCenter of Stable Isotopes, Forschungszentrum Karlsruhe, Institute of Meteorology and Climate Research (IMK-IFU), Kreuzeckbahnstrasse 19, D-82467 Garmisch-PartenkirchenII. ExperimentThe dependencies of δ13C and δ18O of CO2from heterotrophic soil respiration on soil temperature and soil moisture were investigated. Samples of the organic layer (O) and humified A horizon (Ah) of a Norway spruce (Picea abies (L.) Karst) forest (Höglwald close to Augsburg, Germany) were incubated at different temperature and moisture levels in gas-tight incubation tubes (exetainers), followed by isotope-ratio mass spectrometer (IRMS) analysis of the CO2formed during heterotrophic soil respiration. The increase of CO2 mixing ratios during the incubations, monitored by IRMS at different sampling times, allowed for calculation of the δ13C and δ18O of the respiratory source of the CO2 by the Keeling plot methodology.I. 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. Heterotrophic respiration contributes approximately half of the total respiratory backflow of CO2 from terrestrial ecosystems to the atmosphere, but too little is known about environmental controls of C and O isotopic signatures of CO2 released from soils to be able to reliably quantify the contribution of different CO2 flux components (photosynthesis, above- and belowground autotrophic as well as heterotrophic respiration) to net ecosystem CO2 exchange.III. MethodSamples of the O layer below the needle litter and of the Ah horizon were taken on the 10th of May 2007 and 30th of August 2007, taken back to the lab on the same day, and kept refrigerated at 5°C until incubation and analysis. Preparation of refrigerated soil samples started two days prior to analytical determination of the δ13C and δ18O of soil-respired CO2. First, samples were sieved at 4 mm in order to homogenate the sample material and to exclude fine roots. Subsequently, the sieved samples of O and Ah horizon were air-dried for one night. Then, some g of soil material, depending on the temperature and humidity treatment, were placed into 12 mL exetainers (Labco Ltd., UK). For each temperature and moisture level and each horizon 18 replicates were analyzed. The specific amount of water required to achieve the target soil moisture level was added to each exetainer. To avoid drying of samples, the exetainers were sealed with parafilm. Samples were then incubated for one night at the respective temperature, which was controlled with a custom-made sample tray connected to a water bath. The parafilm was removed and the samples were aerated for another 1–2 hours before sealing the exetainers with the appropriate septum screw caps prior to IRMS analysis with a gas bench coupled to a continuous-flow mass spectrometer (DeltaPlusXP, ThermoFisher, Bremen, Germany).O AhConnection to water bathRoutine temperature check with PT100Gas benchSpecially designed autosampler-water bath boxIV. ResultsValues for δ13C are given in ‰ vs. PDB, values for δ18O in ‰ vs. SMOW; MWHC = maximum water holding capacity.V. SummaryThe δ13C of the CO2 respired from root-free organic layer and Ah horizon did not show any significant relation to soil water content. In contrast, δ13C had a negative significant linear relationship with temperature. Data fluctuated around an average δ13C of -25.6‰ for the Ah horizon and -26.4‰ for the organic layer, reflecting the fact that the organic layer was more depleted in 13C than the Ah horizon. Likewise, the CO2 from the organic layer was more depleted in 18O than the CO2 from the Ah horizon. In contrast to the δ13C results, no significant relationship of δ18O to temperature could be observed. However, a highly significant decrease in δ18O of respiratory CO2 from both soil layers with increasing soil moisture was found, decreasing from an average of +14‰ vs. SMOW at 25% MWHC to +4‰ at 100% MWHC.Fig. 1. Increase in headspace CO2mixing ratios above soil from the O layer and from the Ah horizon, respectively, during a 20 h measurement period. Fig. 2. Keeling plots for the determination of δ13C of soil-respired CO2; treatment at 10°C and 50% MWHC (R2= 0.966 and 0.998 for Ah and O, respectively).Fig. 3. δ13C (top panels) and δ18O (bottom panels) at different temperature (left panels) and soil moisture levels (right panels), expressed as % of MWHC, for soil samples of the organic layer and Ah horizon. Significant linear relationships between δ13C and temperature as well as between δ18O and water holding capacity were found.Fig. 4. δ18O values for each temperature treatment (differently coloured symbols) at different soil moisture levels for Ah (top) and O horizon (bottom).Fig. 5. Relationship between δ13C of soil-respired CO2 of Ahhorizon (top panels) and organic layer (bottom panels) with δ13C of soil carbon (left panels) and with soil C/N ratios (right panels). Red lines display best linear fit (blue encircled data point omitted); including all data results in a linear fit indicated by the blue line.Contact: Dr. Nicolas Brüggemann (nicolas.brueggemann@imk.fzk.de)Tel.: +49-(0)8821-183226, Fax: +49-(0)8821-183294

Moisture and temperature dependence of δ¹³C and δ¹⁸O of CO₂ from heterotrophic soil respiration

https://publikationen.bibliothek.kit.edu/220073245/3815323

Moisture and temperature dependence of δ¹³C and δ¹⁸O of CO₂ from heterotrophic soil respiration

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