Development of an online O2-CO2 soil profile probe for flux estimations

Reliable and accurate measurements of gas fluxes between soil and atmosphere are fundamental to calculate global greenhouse gas budgets. Chamber methods and the eddy covariance method are the most often used methods to measure soil-atmosphere gas fluxes. The gradient method can provide additional information about the localization of gas production within the soil profile. Using this approach gas fluxes in the soil profile are calculated by multiplication of the vertical concentration gradient of a gas in the soil by the effective gas diffusion coefficient of the soil. Technical progress in the field of small gas sensors has made it possible to integrate online CO₂ sensors into soil gas profile monitoring systems, which greatly facilitates soil CO₂ monitoring. While soil CO₂ fluxes have been widely studied during the last decade, the “forgotten half” of this respiratory flux, the counter wise flux of atmospheric oxygen (O₂) into the soil is rarely investigated, although it is known that O₂ availability is the key for many soil processes.

Our objective was to develop and test a soil gas profile probe for online CO₂ and O₂ measurements with high temporal resolution that allows (1) soil–atmosphere flux estimation and (2) estimation of soil respiration profiles for both CO₂ and O₂ including (3) the apparent respiratory coefficient (CO₂ efflux divided by O₂ influx).

We developed a multilevel O₂-CO₂ profile probe with small build-in online sensors in multiple depths. The design was based on a modified version of a recently developed CO₂ profile probe (Osterholt and Maier, 2020, Osterholt et al 2023). The probe consists of one 3D print segment per depth each containing one small NDIR CO₂ and one O₂ sensor. Extensive laboratory tests with different O₂ sensors were necessary to exclude, or identify and quantify, possible biases in the O₂ measurements due to expected environmental changes during field measurements (such as barometric pressure, soil temperature and relative humidity). The segments can be combined to probes of different length. For the installation of the sampler a hole has to be drilled, into which the sampler is inserted. We present first results from laboratory experiments and a field experiment, focusing on methodological issues and the technical performance of the measuring system.

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

Osterholt, L. and Maier, M.: Development of an in-situ CO_­2 gradient sampler, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7272, https://doi.org/10.5194/egusphere-egu2020-7272,  2020.

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.