A modeling approach to investigate drivers, variability and uncertainties in O2 fluxes and O2 : CO2 exchange ratios in a temperate forest
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Published:2023-10-06
Issue:19
Volume:20
Page:4087-4107
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Yan YuanORCID, Klosterhalfen AnneORCID, Moyano FernandoORCID, Cuntz MatthiasORCID, Manning Andrew C.ORCID, Knohl AlexanderORCID
Abstract
Abstract. The O2 : CO2 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 O2 consumed per mole of
CO2 produced. Due to the technical challenge in measuring directly the
ER of entire terrestrial ecosystems (EReco), little is known about
variations in ER at hourly and seasonal scales, as well as how different
components contribute to EReco. In this modeling study, we explored the
variability in and drivers of EReco and evaluated the hypothetical
uncertainty in determining ecosystem O2 fluxes based on current
instrument precision. We adapted the one-dimensional, multilayer
atmosphere–biosphere gas exchange model “CANVEG” to simulate hourly
EReco from modeled O2 and CO2 fluxes in a temperate beech
forest in Germany. We found that the modeled annual mean EReco ranged from 1.06 to 1.12 mol mol−1 within the 5-year study period. Hourly EReco showed
strong variations over diel and seasonal cycles and within the vertical
canopy profile. The determination of ER from O2 and CO2 mole fractions
in air above and within the canopy (ERconc) varied between 1.115 and
1.15 mol mol−1. CANVEG simulations also indicated that ecosystem
O2 fluxes could be derived with the flux-gradient method using measured
vertical gradients in scalar properties, as well as fluxes of CO2,
sensible heat and latent energy derived from eddy covariance measurements.
Owing to measurement uncertainties, however, the uncertainty in estimated
O2 fluxes derived with the flux-gradient approach could be as high as
15 µmol m−2 s−1, which represented the 90 % quantile of the
uncertainty in hourly data with a high-accuracy instrument. We also
demonstrated that O2 fluxes can be used to partition net CO2
exchange fluxes into their component fluxes of photosynthesis and
respiration if EReco is known. The uncertainty of the partitioned
gross assimilation ranged from 1.43 to 4.88 µmol m−2 s−1
assuming a measurement uncertainty of 0.1 or 2.5 µmol m−2 s−1
for net ecosystem CO2 exchange and from 0.1 to 15 µmol m−2 s−1 for net ecosystem O2 exchange, respectively. Our analysis
suggests that O2 measurements at ecosystem scale have the potential to
partition net CO2 fluxes into their component fluxes, but further
improvement in instrument precision is needed.
Funder
H2020 European Research Council
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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