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

Abstract

<p>Ecosystem assimilation and respiration result in anti-correlated fluxes of oxygen (O<sub>2</sub>) and carbon dioxide (CO<sub>2</sub>). While the ecosystem O<sub>2</sub>:CO<sub>2</sub> 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).</p> <p>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 O<sub>2</sub>:CO<sub>2</sub> exchange ratios in a managed European beech forest in central Germany.</p> <p>System 1, consisting of a relatively slow response differential fuel cell O<sub>2</sub> analyzer (Oxzilla FC-2, Sable Systems Inc., USA) together with a non-dispersive infrared CO<sub>2</sub> analyzer (LI-840, LI-COR Biosciences, USA), was used to simultaneously measure O<sub>2</sub> and CO<sub>2 </sub>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.</p> <p>Preliminary analysis of August to December 2018 data show that chamber-based flux estimates for O<sub>2</sub> and CO<sub>2</sub> were anti-correlated at all times, and that the O<sub>2</sub>:CO<sub>2</sub> molar exchange ratios (defined as ‑Δ[O<sub>2</sub>]/Δ[CO<sub>2</sub>]) 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).</p> <p>In system 2, CO<sub>2</sub>, O<sub>2</sub> and water vapor (H<sub>2</sub>O) 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 O<sub>2</sub>:CO<sub>2</sub> exchange ratios in air sampled at 1.5 times the canopy height, i.e. a typical eddy covariance set-up.</p> <p>Analysis of the high-frequency data revealed instrumental noise levels of ≈±12 ppm O<sub>2</sub>. Fourier transformation of high-frequency data obtained during well-mixed boundary layer conditions indicate that turbulent fluctuations of the O<sub>2</sub> signal were insufficiently resolved when compared to the CO<sub>2</sub> 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 O<sub>2</sub> mole fraction (R<sup>2</sup> = 0.6 slope = 0.7, MAE = 1.6 ppm) and in estimated O<sub>2</sub>:CO<sub>2</sub> exchange ratios of 1.01 and 0.97 for system 1 and 2, respectively.</p> <p>Our presentation will expand on the applicability of both O<sub>2</sub> and CO<sub>2 </sub>measurement systems with regard to micrometeorological flux techniques. Specifically, we elucidate on the potential of using O<sub>2 </sub>flux measurements as a constraint for estimating ecosystem-scale gross primary production.</p>