Periodic Reporting for period 4 - OXYFLUX (Oxygen flux measurements as a new tracer for the carbon and nitrogen cycles in terrestrialecosystems)
Reporting period: 2021-04-01 to 2023-03-31
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.
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.
We successfully developed a novel O2 measurement system consisting of custom-made soil, branch and stem chambers and a high precision O2 and CO2 analyser unit. This allowed us to make – for the first time worldwide - high precision (< 1 ppm for O2), continuous measurements of O2:CO2 exchange ratios (ER) simultaneously for different ecosystem components.
The chamber measurements at trees at the Leinefelde forest site showed – in contrast to previous assumptions - large temporal and spatial variability in ER indicating either additional CO2 sources, effects of nitrogen assimilation or post-respiratory processes. Our results show that the common practice of measuring CO2 emissions at the stem surface can potentially result in significant bias of reported “stem respiration”.
We established a new flux tower in an agricultural ecosystem for combined O2, CO2, and N2O flux measurements. For O2 fluxes, we developed a flux-gradient approach with high precision O2 and CO2 measurements. The mean molar O2:CO2 flux ratio was -1.18+-0.1 but also with substantial seasonal changes due to crop development. A cooperation with an industry partner let to the development of a new tuneable laser direct absorption spectrometer for O2, which allows fast measurements of O2 mole factions thus enabling micrometeorological measurements.
Oxidative ratios (OR) of organic material as a proxy for ER were studied in three activities: a) Variability within one forest ecosystem, b) Influence of land-use change on OR in Indonesia, and c) Variability across sites distributed globally. In all three activities we found substantial spatial variability and strong differences across species and across ecosystem components. With more than 2000 samples of 100 different plant species we were able to fill some important data gaps in the globally available OR dataset.
For understanding O2 dynamics within a forest ecosystem, we used and further developed the multi-layer canopy model, CANVEG for simulating O2 fluxes. We then used the model to show that flux-gradient measurements of O2 can be used to partition net CO2 fluxes into gross primary productivity and ecosystem respiration. A new photosynthesis model based on a mechanistic representation of nitrogen assimilation was implemented and an analytical solution for the combined photosynthesis and stomata conduction equation was derived.
A major impact of Oxyflux was on building a new scientific community in an emerging research field by organising an international workshop and sessions at conferences and well as initiating joint field experiments and collaborations with partners from Europe, Israel, and the US.
The chamber measurements at trees at the Leinefelde forest site showed – in contrast to previous assumptions - large temporal and spatial variability in ER indicating either additional CO2 sources, effects of nitrogen assimilation or post-respiratory processes. Our results show that the common practice of measuring CO2 emissions at the stem surface can potentially result in significant bias of reported “stem respiration”.
We established a new flux tower in an agricultural ecosystem for combined O2, CO2, and N2O flux measurements. For O2 fluxes, we developed a flux-gradient approach with high precision O2 and CO2 measurements. The mean molar O2:CO2 flux ratio was -1.18+-0.1 but also with substantial seasonal changes due to crop development. A cooperation with an industry partner let to the development of a new tuneable laser direct absorption spectrometer for O2, which allows fast measurements of O2 mole factions thus enabling micrometeorological measurements.
Oxidative ratios (OR) of organic material as a proxy for ER were studied in three activities: a) Variability within one forest ecosystem, b) Influence of land-use change on OR in Indonesia, and c) Variability across sites distributed globally. In all three activities we found substantial spatial variability and strong differences across species and across ecosystem components. With more than 2000 samples of 100 different plant species we were able to fill some important data gaps in the globally available OR dataset.
For understanding O2 dynamics within a forest ecosystem, we used and further developed the multi-layer canopy model, CANVEG for simulating O2 fluxes. We then used the model to show that flux-gradient measurements of O2 can be used to partition net CO2 fluxes into gross primary productivity and ecosystem respiration. A new photosynthesis model based on a mechanistic representation of nitrogen assimilation was implemented and an analytical solution for the combined photosynthesis and stomata conduction equation was derived.
A major impact of Oxyflux was on building a new scientific community in an emerging research field by organising an international workshop and sessions at conferences and well as initiating joint field experiments and collaborations with partners from Europe, Israel, and the US.
Until the OXYFLUX project, the potential of high-precision atmospheric O2 measurements at the ecosystem level had not been fully exploited, largely owing to the significant technical challenges faced in measuring O2 to an accuracy and precision of a few ppm or less against a background mole fraction of 21%. A major methodological novelty in Oxyflux was the design and construction of an automated gas exchange measurement system consisting of custom-made open steady-state chambers for soils, tree trunks (stems) and tree branches (each with four replicates) as well as micrometeorological profiles allowing for continuous O2 flux measurements of different ecosystem components.
In summary, we were able to develop interdisciplinary solutions to challenges arising when high precision atmospheric O2 measurements are merged with terrestrial ecosystem applications and we facilitated the first high-precision long-term continuous measurements of O2 exchange simultaneously for various ecosystem components.
In summary, we were able to develop interdisciplinary solutions to challenges arising when high precision atmospheric O2 measurements are merged with terrestrial ecosystem applications and we facilitated the first high-precision long-term continuous measurements of O2 exchange simultaneously for various ecosystem components.