EGU2020-18822, updated on 09 Dec 2022
EGU General Assembly 2020
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Measuring oxygen fluxes in a European beech forest - results from the OXYFLUX project

Alexander Knohl1, Jan Muhr1, M. Julian Deventer1, Emanuel Blei1, Jelka Braden-Behrens1, Edgar Tunsch1, Mattia Bonazza1, Penelope A. Pickers2, David Nelson3, Mark Zahniser3, and Andrew C. Manning2
Alexander Knohl et al.
  • 1University of Goettingen, Bioclimatology, 37077 Göttingen, Germany (
  • 2Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, United Kingdom
  • 3Aerodyne Research, Inc., Billerica, United States

Ecosystem assimilation and respiration result in anti-correlated fluxes of oxygen (O2) and carbon dioxide (CO2). While the ecosystem O2:CO2 molar exchange ratio is usually assumed constant at ≈1.1 on longer timescales, variations for individual ecosystem compartments or shorter timescales have been reported in the past. We hypothesize that these exchange ratio variations can reveal information about underlying biotic and abiotic processes in plants or soil that cannot be inferred from traditional net ecosystem exchange measurements. To date, oxygen measurements have not been widely implemented in ecosystem research due to the technical challenge of detecting very small variations (ppm-level) against an atmospheric background of ≈21% (≈210,000 ppm).

We evaluate the performance and applicability of two commercial oxygen analyzers Integrated into custom-built gas handling and calibration systems, and report first results from measurements of O2:CO2 exchange ratios in a managed European beech forest in central Germany.

System 1, consisting of a relatively slow response differential fuel cell O2 analyzer (Oxzilla FC-2, Sable Systems Inc., USA) together with a non-dispersive infrared CO2 analyzer (LI-840, LI-COR Biosciences, USA), was used to simultaneously measure O2 and CO2 mole fractions in air sampled from soil, stem, and branch chambers. Chambers were operated in an open flow-through steady-state design aimed at equilibrium mole fractions within a few hundred ppm of atmospheric background. Using a multiplexer valve design, we measured chambers sequentially by directing chamber air at a controlled flow rate to the gas analyzing system.

Preliminary analysis of August to December 2018 data show that chamber-based flux estimates for O2 and CO2 were anti-correlated at all times, and that the O2:CO2 molar exchange ratios (defined as ‑Δ[O2]/Δ[CO2]) varied considerably over time and between the different ecosystem compartments (soil, stems, and branches) with a median (interquartile range) of 0.94 (0.75 to 1.09).

In system 2, CO2, O2 and water vapor (H2O) measurements were performed with a fast response (5 Hz) gas analyzer using tunable infrared laser direct absorption spectroscopy (TILDAS, Aerodyne Research Inc., USA). We measured fluctuations in O2:CO2 exchange ratios in air sampled at 1.5 times the canopy height, i.e. a typical eddy covariance set-up.

Analysis of the high-frequency data revealed instrumental noise levels of ≈±12 ppm O2. Fourier transformation of high-frequency data obtained during well-mixed boundary layer conditions indicate that turbulent fluctuations of the O2 signal were insufficiently resolved when compared to the CO2 power spectra. When averaging high-frequency data to 2-min aggregates, instrumental noise was reduced to ≈±1 ppm, similar to the precision of system 1. At this timescale, contemporaneous measurements of above-canopy air revealed agreement between the fuel cell and the laser systems, both in O2 mole fraction (R2 = 0.6 slope = 0.7, MAE = 1.6 ppm) and in estimated O2:CO2 exchange ratios of 1.01 and 0.97 for system 1 and 2, respectively.

Our presentation will expand on the applicability of both O2 and CO2 measurement systems with regard to micrometeorological flux techniques. Specifically, we elucidate on the potential of using O2 flux measurements as a constraint for estimating ecosystem-scale gross primary production.

How to cite: Knohl, A., Muhr, J., Deventer, M. J., Blei, E., Braden-Behrens, J., Tunsch, E., Bonazza, M., Pickers, P. A., Nelson, D., Zahniser, M., and Manning, A. C.: Measuring oxygen fluxes in a European beech forest - results from the OXYFLUX project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18822,, 2020.


Display file