A modeling approach to investigate drivers, variability and uncertainties in O2 fluxes and O2 : CO2 exchange ratios in a temperate forest.

The O 2 : CO 2 exchange ratio (ER) between terrestrial ecosystems and the atmosphere is a key parameter for partitioning global ocean and land carbon fluxes. The long-term terrestrial ER is considered to be close to 1.10 mol of O 2 consumed per mole of CO 2 produced. Due to the technical challenge in measuring directly the ER of entire terrestrial ecosystems (ER eco), little is known about variations in ER at hourly and seasonal scales, as well as how different components contribute to ER eco. In this modeling study, we explored the variability in and drivers of ER eco and evaluated the hypothetical uncertainty in determining ecosystem O 2 fluxes based on current instrument precision. We adapted the one-dimensional, multilayer atmosphere–biosphere gas exchange model "CANVEG" to simulate hourly ER eco from modeled O 2 and CO 2 fluxes in a temperate beech forest in Germany. We found that the modeled annual mean ER eco ranged from 1.06 to 1.12 mol mol -1 within the 5-year study period. Hourly ER eco showed strong variations over diel and seasonal cycles and within the vertical canopy profile. The determination of ER from O 2 and CO 2 mole fractions in air above and within the canopy (ER conc) varied between 1.115 and 1.15 mol mol -1. CANVEG simulations also indicated that ecosystem O 2 fluxes could be derived with the flux-gradient method using measured vertical gradients in scalar properties, as well as fluxes of CO 2 , sensible heat and latent energy derived from eddy covariance measurements. Owing to measurement uncertainties, however, the uncertainty in estimated O 2 fluxes derived with the flux-gradient approach could be as high as 15 µ mol m -2 s -1 , which represented the 90 % quantile of the uncertainty in hourly data with a high-accuracy instrument. We also demonstrated that O 2 fluxes can be used to partition net CO 2 exchange fluxes into their component fluxes of photosynthesis and respiration if ER eco is known. The uncertainty of the partitioned gross assimilation ranged from 1.43 to 4.88 µ mol m -2 s -1 assuming a measurement uncertainty of 0.1 or 2.5 µ mol m -2 s -1 for net ecosystem CO 2 exchange and from 0.1 to 15 µ mol m -2 s -1 for net ecosystem O 2 exchange, respectively. Our analysis suggests that O 2 measurements at ecosystem scale have the potential to partition net CO 2 fluxes into their component fluxes, but further improvement in instrument precision is needed. [ABSTRACT FROM AUTHOR]