Worldwide, humans generate emissions of the greenhouse gas carbon dioxide (CO2) that burdens the atmosphere with 11.5 gigatons of carbon yearly. Approximately 5 gigatons of these emissions collect in the atmosphere, 3.5 gigatons are absorbed by land ecosystem and the remaining 3 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 explored new paths in both measurement techniques and scientific knowledge of terrestrial ecosystems. OXYFLUX performed the methodological, experimental and modelling work needed to develop O2 as a new tracer for carbon and nitrogen cycle processes at ecosystem level. Oxyflux aimed 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 was organised in four work packages (WP) addressing different aspects of O2 fluxes in terrestrial ecosystems. WP 1 aimed 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 measured the net O2 and CO2 exchange above forest and agricultural ecosystems using micrometeorological approaches such as eddy covariance. In WP 3, we studied oxidative ratios of organic material across ecosystems globally in order to characterise the long-term O2:CO2 ratio of different ecosystems. WP 4 focussed on modelling and synthesis of atmospheric O2 fluxes and budgets using a multi-layer canopy model and a global land surface model.
A key outcome of Oxyflux was the successful merger of high precision O2 measurement techniques from the atmospheric scientific community with chamber and micrometeorological measurement technique used in the ecological scientific community. At this level of complexity and precisions, our system was unique worldwide. It revealed the complex interplay of O2:CO2 exchange ratios at various spatial and temporal scales and showed the potential to use O2 as a new tracer for the carbon cycle in terrestrial ecosystems.
