B097-0002
Fluxes of Oxygen (O2) in a European Beech Forest

Tuesday, 15 December 2020
Poster
Alexander Knohl1, Jan Muhr1, Malte Julian Deventer1, Emanuel Blei1, Jelka Braden-Behrens1, Mattia Bonazza1, Christian Markwitz1, Edgar Tunsch1, Penelope Pickers2, David D Nelson3, Mark S Zahniser3 and Andrew C Manning2, (1)University of Goettingen, Bioclimatology, Göttingen, Germany, (2)University of East Anglia, Norwich, United Kingdom, (3)Aerodyne Research Inc, Billerica, MA, United States
Abstract:
In terrestrial ecosystems, fluxes of carbon dioxide (CO2) are typically anti-correlated to fluxes of oxygen (O2) via the processes of photosynthesis and respiration. The molar exchange ratio O2:CO2 is widely assumed to be around -1.1 on larger spatial and temporal scales. Some recent work, however, suggests that individual components of ecosystems such as soils or tree trunks may have considerable variation in their O2:CO2 exchange ratios. Up to now, O2:CO2 measurements have rarely been performed due to the technical challenges of measuring small fluctuation in O2 at the ppm level against the large atmospheric background of around 21% (≈210,000 ppm).

Here we report O2:CO2 exchange ratios from chambers and profile measurements at the FLUXNET site Leinefelde, a mature beech forest in Central Germany.

We measured concurrent O2 and CO2 exchange ratios from soil (n=8), stems (n=4), and branches (n=4) in custom-made open flow-through steady-state chambers in a quasi-continuous manner since summer 2019. O2 and CO2 mole fraction were determined with a differential fuel cell O2 analyzer (Oxzilla FC-2, Sable Systems Inc., USA) and a non-dispersive infrared CO2 analyzer (LI-840, LI-COR Biosciences, USA), respectively. Time resolution of the O2 fuel cell analyzer was 2 min with a precision of ≈±1 ppm.

Data from August to December 2019 show that the O2 and CO2 fluxes from the chambers were anti-correlated during day and night time. The O2:CO2 exchange ratios fluctuated considerably temporally and between the different ecosystem components with a median (interquartile range) of 0.94 (0.75 to 1.09).

In a vertical profile from the forest soil to the roughness sublayer above the forest canopy we measured CO2, O2 and water vapor (H2O) mole fractions with a high frequency (2 Hz) tunable laser spectrometer (TILDAS, Aerodyne Research Inc., USA). Instrument precision for O2 mole fraction was 12 ppm at 2 Hz and when averaged ≈±1 ppm at 2 min.

In the vertical profile, measurements of the O2:CO2 ratio show large variability between day and night conditions, but also between adjacent hourly sampling intervals, with a O2:CO2 range spanning from 0.7 to 1.5. Finally, we are analyzing the O2:CO2 ratio profile measurements in an inverse Lagrangian flux framework and present first results of the ecosystem-scale exchange fluxes.

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