Continuous measurements of O2:CO2 flux exchange ratios above a cropland in central Germany

The O₂:CO₂ exchange ratio of land-atmosphere fluxes (ER) can be used to identify sources and sinks of CO₂ in land ecosystems. During photosynthesis, the O₂:CO₂ ER at the leaf level is approximately -1 mol mol^-1, reflecting the uptake of one mole of CO₂ associated with the release of one mole of O₂. However, the ER at the level of entire ecosystems is largely unknown.

Here we present a unique dataset of two years of continuous O₂ and CO₂ flux measurements at the agricultural FLUXNET site Reinshof (51°29'24.0"N, 9°55'55.2"E, DE-Rns) near Göttingen, Germany, in 2023 and 2024. Fluxes were calculated using flux-gradient approaches with air sampled from three inlets situated at 0.5, 1.0 and 3.0 m above ground. Dry mole fractions of O₂ and CO₂ were measured using a modified Oxzilla II differential oxygen analyzer (Sable Systems, USA) and a Li-820 CO₂ infrared gas analyser (LiCor Biosciences, USA), respectively.

The results show that O₂ and CO₂ mole fractions and net O₂ and CO₂ fluxes were strongly anticorrelated. The O₂:CO₂ flux ER showed a distinct annual cycle, with values around -1.5 mol mol^-1 under bare soil conditions and -1.1 mol mol^-1 during the main growing season when sugar beet (2023) and winter wheat (2024) was grown, respectively. An influence from anthropogenic emissions was observed during the winter with stable atmospheric stratification, when winds originated from the city centre of Göttingen or the nearby road. The longer vegetation period of sugar beet in 2023 was well reflected by extended O₂ release and CO₂ uptake, as well as ERs at around -1.1 mol mol^-1.

In conclusion, the O₂:CO₂ ER of a cropland showed considerable seasonal variability, which offers the opportunity to use O₂ flux measurements as a tracer of the carbon cycle.