A modeling approach to investigate drivers, variability and uncertainties in O<sub>2</sub> fluxes and O<sub>2</sub> : CO<sub>2</sub> exchange ratios in a temperate forest
<p>The O<span class="inline-formula"><sub>2</sub></span> : CO<span class="inline-formula"><sub>2</sub></span> exchange ratio (ER) between terrestrial ecosystems and the atmosphere is a key parameter for partitioning global oce...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-10-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/20/4087/2023/bg-20-4087-2023.pdf |
| Summary: | <p>The O<span class="inline-formula"><sub>2</sub></span> : CO<span class="inline-formula"><sub>2</sub></span> 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<span class="inline-formula"><sub>2</sub></span> consumed per mole of
CO<span class="inline-formula"><sub>2</sub></span> produced. Due to the technical challenge in measuring directly the
ER of entire terrestrial ecosystems (ER<span class="inline-formula"><sub>eco</sub></span>), little is known about
variations in ER at hourly and seasonal scales, as well as how different
components contribute to ER<span class="inline-formula"><sub>eco</sub></span>. In this modeling study, we explored the
variability in and drivers of ER<span class="inline-formula"><sub>eco</sub></span> and evaluated the hypothetical
uncertainty in determining ecosystem O<span class="inline-formula"><sub>2</sub></span> fluxes based on current
instrument precision. We adapted the one-dimensional, multilayer
atmosphere–biosphere gas exchange model “CANVEG” to simulate hourly
ER<span class="inline-formula"><sub>eco</sub></span> from modeled O<span class="inline-formula"><sub>2</sub></span> and CO<span class="inline-formula"><sub>2</sub></span> fluxes in a temperate beech
forest in Germany.</p>
<p>We found that the modeled annual mean ER<span class="inline-formula"><sub>eco</sub></span> ranged from 1.06 to 1.12 mol mol<span class="inline-formula"><sup>−1</sup></span> within the 5-year study period. Hourly ER<span class="inline-formula"><sub>eco</sub></span> showed
strong variations over diel and seasonal cycles and within the vertical
canopy profile. The determination of ER from O<span class="inline-formula"><sub>2</sub></span> and CO<span class="inline-formula"><sub>2</sub></span> mole fractions
in air above and within the canopy (ER<span class="inline-formula"><sub>conc</sub></span>) varied between 1.115 and
1.15 mol mol<span class="inline-formula"><sup>−1</sup></span>. CANVEG simulations also indicated that ecosystem
O<span class="inline-formula"><sub>2</sub></span> fluxes could be derived with the flux-gradient method using measured
vertical gradients in scalar properties, as well as fluxes of CO<span class="inline-formula"><sub>2</sub></span>,
sensible heat and latent energy derived from eddy covariance measurements.
Owing to measurement uncertainties, however, the uncertainty in estimated
O<span class="inline-formula"><sub>2</sub></span> fluxes derived with the flux-gradient approach could be as high as
15 <span class="inline-formula">µ</span>mol m<span class="inline-formula"><sup>−2</sup></span> s<span class="inline-formula"><sup>−1</sup></span>, which represented the 90 % quantile of the
uncertainty in hourly data with a high-accuracy instrument. We also
demonstrated that O<span class="inline-formula"><sub>2</sub></span> fluxes can be used to partition net CO<span class="inline-formula"><sub>2</sub></span>
exchange fluxes into their component fluxes of photosynthesis and
respiration if ER<span class="inline-formula"><sub>eco</sub></span> is known. The uncertainty of the partitioned
gross assimilation ranged from 1.43 to 4.88 <span class="inline-formula">µ</span>mol m<span class="inline-formula"><sup>−2</sup></span> s<span class="inline-formula"><sup>−1</sup></span>
assuming a measurement uncertainty of 0.1 or 2.5 <span class="inline-formula">µ</span>mol m<span class="inline-formula"><sup>−2</sup></span> s<span class="inline-formula"><sup>−1</sup></span>
for net ecosystem CO<span class="inline-formula"><sub>2</sub></span> exchange and from 0.1 to 15 <span class="inline-formula">µ</span>mol m<span class="inline-formula"><sup>−2</sup></span> s<span class="inline-formula"><sup>−1</sup></span> for net ecosystem O<span class="inline-formula"><sub>2</sub></span> exchange, respectively. Our analysis
suggests that O<span class="inline-formula"><sub>2</sub></span> measurements at ecosystem scale have the potential to
partition net CO<span class="inline-formula"><sub>2</sub></span> fluxes into their component fluxes, but further
improvement in instrument precision is needed.</p> |
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| ISSN: | 1726-4170 1726-4189 |