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

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Tapping into the potential of oxygen to study the terrestrial carbon cycle

Researchers designed a new way to gain high-accuracy oxygen measurements at an ecosystem level.

Climate Change and Environment icon Climate Change and Environment

Studying the cycle of carbon around our planet can reveal the health of ecosystems, something particularly relevant to the major twin crises of climate change and declining biodiversity. Oxygen (O2) is strongly linked to carbon dioxide (CO2) through plant photosynthesis and plant respiration, making it an untapped source of information about ecosystems and the global carbon cycle. Yet gathering high-precision O2 measurements at the terrestrial ecosystem level is technically very challenging compared to CO2, in part because there is far more O2 in the atmosphere. You need to make a similarly precise measurement but at a far higher ratio. “This is similar to using a body scale to check if you have shaved your beard or not in the morning,” explains Alexander Knohl, professor of Bioclimatology at the University of Göttingen and OXYFLUX project coordinator. In the OXYFLUX project, which was funded by the European Research Council, researchers successfully developed a novel O2 measurement system, complete with custom-made chambers to measure the gas exchange from tree branches, trunks and the soil surface, and a high-precision O2 and CO2 analyser unit. “This allowed us to make – for the first time worldwide – high-precision continuous measurements of O2 to CO2 exchange ratios, simultaneously for different components of an ecosystem over an entire growing season,” says Knohl.

Developing an innovative O2 analyser

The new O2 analyser unit is a custom-made system, comprising a temperature-controlled mobile trailer containing the analyser, a calibration unit, a gas handling unit, and air drying systems. “We designed and built our own gas exchange chambers for measuring the exchange of CO2 and O2 from the soil surface, and from the trunks and branches of beech trees,” adds Knohl. The multi-chamber system was installed at the Leinefelde forest site in Germany, a pure beech stand around 140 years old. The system was fully automated and controlled by custom-made software, taking quasi-continuous measurements of CO2 and O2 exchange across an entire growing season. The team also set up a new experimental site at an agricultural farm close to Göttingen, to measure O2, CO2 and nitrous oxide (N2O) fluctuations.

Imbalanced fluxes uncovered

The major innovation of OXYFLUX was to merge methods from two scientific communities: the high-precision O2 measurements from atmospheric sciences, and gas exchange measurements from ecosystem sciences. “This was due to the close collaboration with researcher Andrew Manning and his team from the University of East Anglia, United Kingdom, myself and a really great team of co-workers!” notes Knohl. The team found substantial imbalances in O2 and CO2 fluxes from different ecosystem components as well as in entire ecosystems, implying for example that the common practice of measuring only CO2 can potentially result in significant biases of reported ‘trunk respiration’ from trees.

Merging scientific communities

The project also inspired an emerging terrestrial O2 flux community, through an international workshop in Göttingen which brought together scientists from Germany, Israel, the Netherlands, the United Kingdom and the United States. “OXYFLUX was just the start for an exciting new research agenda that my group will pursue over the next decade,” remarks Knohl, who notes they have successfully demonstrated the feasibility of high-precision O2 flux measurements in ecosystem studies. “We hope to inspire more terrestrial ecologists and atmospheric scientists to join together in studies of O2, to better understand our planet’s fragile ecosystems,” he adds.

Keywords

OXYFLUX, ecosystem, carbon cycle, terrestrial, tree, trunk, branches, atmosphere, oxygen, measurement

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