How and how accurately can we measure soil O2-CO2 fluxes?

Soil gas fluxes are an important signal for ecosystems and various soil functions, as soils can be both sources and sinks of greenhouse gases (GHG). Because of  this critical role, soil gas research has focused mainly on GHGs, while other important gas species have received much less attention. Soil O₂ concentration is key to many soil processes, such as ammonification, nitrification and denitrification, and root growth. Studying the relationship between of CO₂ and O₂ exchange at the soil-atmosphere interface or within the soil profile would be key to better understanding these processes. Gas fluxes between terrestrial ecosystems or the soil surface and the atmosphere are typically measured using the eddy covariance approach (or related micrometeorological approaches), chamber-based measurements, or the flux gradient method. Knowing the precision of the individual sensors used in the methods, but even more so the overall uncertainty of a measurement method, including all steps from sensor to calculation is essential (e.g. the minimum detectable flux). Yet, it is a requirement that is rarely adequately assessed. While there are a variety of suitable CO₂ sensors and setups commonly used for measuring CO₂ fluxes in soil, the measurement of O₂ fluxes in soil is still in its beginning.  

Our aim was 1) to develop a chamber method for online CO₂ and O₂ measurements and 2) to improve a soil gas profile probe for online CO₂ and O₂ measurements (Maier et al. 2024) to calculate the apparent respiratory coefficient (CO₂ efflux divided by O₂ influx). We used a multilevel O₂-CO₂profile probe with built-in online sensors based on the previously published design of an online CO₂ sampler (Osterholt et al., 2023). For the chamber measurements, we used an automatic LI-COR chamber equipped with the same sensors and an additional AAB CO₂ laser spectrometer. Extensive laboratory tests with a large sand column and controlled injections of CO₂ and O₂ were performed to test the effects of temperature and air pressure on the chamber system and the soil profile sampler. We present the results of these laboratory experiments, focusing on the technical performance of the measurement system and its impact on the uncertainty of the estimates of CO₂ and O₂ fluxes and the respiration coefficient.

Acknowledgements

This research was supported by the German Research Foundation (DFG, MA 5826/4-1 project number: 535470615)

Maier, M.; Osterholt, L.; Levintal: Development of an online O₂-CO₂ soil profile probe for flux estimations, EGU General Assembly 2024, EGU24-6777; https://doi.org/10.5194/egusphere-egu24-6777,  2024.

Osterholt, L.; Kolbe, S.; Maier, M. (2022): A differential CO₂ profile probe approach for field measurements of soil gas transport and soil respiration #. In J. Plant Nutr. Soil Sci. 185 (2), pp. 282–296. DOI: 10.1002/jpln.202100155.