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Oxygen flux measurements as a new tracer for the carbon and nitrogen cycles in terrestrial ecosystems

Periodic Reporting for period 3 - OXYFLUX (Oxygen flux measurements as a new tracer for the carbon and nitrogen cycles in terrestrialecosystems)

Reporting period: 2019-10-01 to 2021-03-31

Worldwide, humans generate emissions of the greenhouse gas carbon dioxide (CO2) that burdens the atmosphere with 10 gigatons of carbon yearly. Approximately 4.4 gigatons of these emissions collect in the atmosphere, 3 gigatons are absorbed by land ecosystem and the remaining 2.6 gigatons end up in the oceans. Atmospheric oxygen (O2) measurements have proven to be one of the most powerful tools to study the carbon cycle at global scale and to quantify the CO2 sink of terrestrial ecosystems and oceans. At ecosystem level, O2 is closely related to CO2 through photosynthesis and respiration, and is influenced by sources of nitrogen during plant uptake. O2 thus carries valuable information about ecosystem processes that cannot be learned from CO2 alone. However, the potential of O2 measurements at ecosystem level has not been exploited. The major hindrance has been the technical challenges faced to measure atmospheric O2 at ppm level against a background concentration of 21%.

Motivated by the enormous insights gained from O2 measurements at global level, OXYFLUX explores new paths in both measurement techniques and scientific knowledge of terrestrial ecosystems. OXYFLUX performs the methodological, experimental and modelling work needed to develop O2 as a new tracer for carbon and nitrogen cycle processes at ecosystem level. Oxyflux aims at providing the mechanistic understanding for a unique approach to (a) partition CO2 fluxes in e.g. forest ecosystems, (b) improve understanding of the carbon and nitrogen cycle in arable land, and (c) identify the sensitivity of O2 fluxes in terrestrial ecosystems to environmental change and thus helping to constrain global scale CO2 sink partitioning.

Oxyflux is organised in four work packages (WP) addressing different aspects of O2 fluxes in terrestrial ecosystems. WP 1 aims at quantifying O2 fluxes of ecosystem components (soils, trunks and branches) by designing and building custom-made gas exchange chambers for soils, trunks and branches. WP 2 measures the net O2 and CO2 exchange above forest and agricultural ecosystems using micrometeorological approaches such as eddy covariance. In WP 3, we study oxidative ratios of organic material across ecosystems globally in order to characterise the long-term O2:CO2 ratio of different ecosystems. WP 4 focusses on modelling and synthesis of atmospheric O2 fluxes and budgets using a multi-layer canopy model and a global land surface model.
We built a mobile lab deployable to field sites with a high precision O2 and CO2 analyzer and the required calibration unit. The instrument performance is excellent reaching the standard of high precision O2 measurements at tall tower stations. Further we designed and built a unique automated O2 and CO2 gas exchange measuring complex at the forest site Leinefelde in Central Germany consisting of eight soil chambers, 4 branch chambers and 4 trunk cambers. The chambers are open steady state chambers and are made of material that is not interacting with O2. All chambers are connected with a custom-made switching unit and then with the high precision O2 and CO2 analyser. The automated operation of all chambers is controlled via a central computer and with self-written control software.

For O2 eddy covariance flux measurements above the forest canopy, we use a prototype tunable laser direct absorption spectrometer (O2-TLDAS, Aerodyne Research Inc.). This instrument was developed by the manufacturer based on our request and is worldwide unique. It has a measurement frequency (2-5 Hz), which is suitable for EC measurements above a forests. Another advantage of the O2-TLDAS is that this laser-based analyzer simultaneously measures O2, CO2 and H2O concentrations in the measurement cell. The instruments has been carefully tested in the lab and at the field site for precision and accuracy and is now ready for continuous operation at the forest site.

Additionally, we implemented O2 processes in the multi-layer ecosystem model CANVEG and use the model for testing hypotheses regarding CO2 flux partitioning and O2 exchange of leaf photosynthesis. Furthermore, we use organic material to evaluate the oxidative ratios of land ecosystems. For that, we collected analysed leaves, wood, and soil from our forest site in Germany across a full growing season. For assess spatial variability in contrasting biomes, we collaborate with international partners from about 30 sites worldwide, where samples are currently collected for us and shipped to Germany.
This is the first time that O2 fluxes at an terrestrial ecosystem are studied in such a comprehensive way combining chamber based measurements, eddy covariance, modelling and biochemistry. This is only possible by bringing together expertise on high precision O2 measurements from the atmospheric sciences with micrometeorological approaches from the ecosystem sciences and gas exchange and biochemistry measurements from ecophysiology.

The mobile lab for high precision O2 measurements and the self-made automated gas exchange chambers measuring continuously in a forest site are worldwide unique. Similarly, the laser-based spectrometer for high frequency O2 measurements is the first commercially available prototype suitable for O2 eddy covariance measurements.

After intensive instrument development, Oxyflux now aims at providing unique datasets of O2 fluxes in terrestrial systems and contributing exciting sciences on the terrestrial carbon cycle.
self-made automated branch gas exchange chambers for O2 and CO2
High precision O2 and CO2 analyzer
Mobile lab for high precision O2 and CO2 measurements
flux tower for O2 eddy covariance measurements