HR: 08:00h
AN: B11E-01    [Abstracts]
TI: Observing ¹³C Tracer Kinetics in CO₂ Respired by a Temperate Grassland Ecosystem: Different Measurement Methods Give Different Answers
AU: * Gamnitzer, U
EM: ulrike.gamnitzer@wzw.tum.de
AF: Lehrstuhl für Grünlandlehre, Technische Universität München, Am Hochanger 1, Freising, 85350, Germany
AU: Schäufele, R
EM: schaeufele@wzw.tum.de
AF: Lehrstuhl für Grünlandlehre, Technische Universität München, Am Hochanger 1, Freising, 85350, Germany
AU: Schnyder, H
EM: schnyder@wzw.tum.de
AF: Lehrstuhl für Grünlandlehre, Technische Universität München, Am Hochanger 1, Freising, 85350, Germany
AB: To study carbon fluxes in grassland ecosystems, a ¹³C tracer technique with open-top-chambers was set up for field use and applied to a temperate grassland. The open-top-chambers were optimised with respect to homogeneity of CO₂ distribution, exclusion of wind incursion and prevention of biases in soil CO₂ efflux due to pressure effects. This included dispersion of air entering the chamber and installing a buffer volume at the top exit, as well as varying the opening diameter at the top and the air flow through the chamber. During tracer experiments in September 2006 and Mai 2007, labelling was accomplished by flushing the chambers during daytime with air containing CO₂ at ambient concentration but with ¹³C of the CO₂ differing from ambient by up to 40‰. ¹³C in the chambers showed little variation during bright daytime, but was depleted by about 1‰ in the early morning and late evening (when assimilation rates and thus tracer uptake were low) compared to bright daytime. Along with day-to-day variation in the same order than during bright daytime, this provided constant labelling conditions in the chambers. The tracer was observed in total ecosystem respired CO₂ during nighttime by measuring CO₂ concentration and isotopic composition online in the field with two methods: closed static chamber mode and open dynamic chamber mode. The two methods gave significant, reproducible differences in observed ¹³C of respired CO₂. These translated in differences in the fraction of labelled carbon in respired CO₂, with a larger fraction of tracer observed in the closed static mode: After two weeks of labelling, in the closed static mode about 70-80% of respired CO₂ were labelled, compared to 40-50% in the open dynamic mode. Additional measurements of total ecosystem respiration in a cuvette system in the laboratory (thus avoiding the influence of varying environmental conditions) agreed with the field measurements in the open dynamic mode, but not with those in the closed static mode. Despite the difference between modes, the tracer kinetics of respired CO₂ observed in each of the modes supported a two-source model: Respiration was fed from one source which was not affected by the tracer and another source which was turned over by current photosynthesis within several days.
DE: 0428 Carbon cycling (4806)
DE: 0439 Ecosystems, structure and dynamics (4815)
DE: 0452 Instruments and techniques
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting