Carbonyl sulfide: comparing a mechanistic representation of the vegetation uptake in a land surface model and the leaf relative uptake approach
<p>Land surface modellers need measurable proxies to constrain the quantity of carbon dioxide (CO<span class="inline-formula"><sub>2</sub></span>) assimilated by continental plants through photosynthesis, known as gross primary production (GPP). Carbonyl sulfi...
| Main Authors: | , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-05-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/18/2917/2021/bg-18-2917-2021.pdf |
| Summary: | <p>Land surface modellers need measurable proxies to
constrain the quantity of carbon dioxide (CO<span class="inline-formula"><sub>2</sub></span>) assimilated by
continental plants through photosynthesis, known as gross primary production
(GPP). Carbonyl sulfide (COS), which is taken up by leaves through their
stomates and then hydrolysed by photosynthetic enzymes, is a candidate GPP
proxy. A former study with the ORCHIDEE land surface model used a fixed
ratio of COS uptake to CO<span class="inline-formula"><sub>2</sub></span> uptake normalised to respective ambient
concentrations for each vegetation type (leaf relative uptake, LRU) to
compute vegetation COS fluxes from GPP. The LRU approach is known to have
limited accuracy since the LRU ratio changes with variables such as
photosynthetically active radiation (PAR): while CO<span class="inline-formula"><sub>2</sub></span> uptake slows under
low light, COS uptake is not light limited. However, the LRU approach has
been popular for COS–GPP proxy studies because of its ease of application
and apparent low contribution to uncertainty for regional-scale
applications. In this study we refined the COS–GPP relationship and
implemented in ORCHIDEE a mechanistic model that describes COS uptake by
continental vegetation. We compared the simulated COS fluxes against
measured hourly COS fluxes at two sites and studied the model behaviour and
links with environmental drivers. We performed simulations at a global scale,
and we estimated the global COS uptake by vegetation to be <span class="inline-formula">−</span>756 Gg S yr<span class="inline-formula"><sup>−1</sup></span>,
in the middle range of former studies (<span class="inline-formula">−</span>490 to <span class="inline-formula">−</span>1335 Gg S yr<span class="inline-formula"><sup>−1</sup></span>). Based
on monthly mean fluxes simulated by the mechanistic approach in ORCHIDEE, we
derived new LRU values for the different vegetation types, ranging between
0.92 and 1.72, close to recently published averages for observed values of
1.21 for C<span class="inline-formula"><sub>4</sub></span> and 1.68 for C<span class="inline-formula"><sub>3</sub></span> plants. We transported the COS using the monthly
vegetation COS fluxes derived from both the mechanistic and the LRU
approaches, and we evaluated the simulated COS concentrations at NOAA sites.
Although<span id="page2918"/> the mechanistic approach was more appropriate when comparing to
high-temporal-resolution COS flux measurements, both approaches gave similar
results when transporting with monthly COS fluxes and evaluating COS
concentrations at stations. In our study, uncertainties between these two
approaches are of secondary importance compared to the uncertainties in the
COS global budget, which are currently a limiting factor to the potential of
COS concentrations to constrain GPP simulated by land surface models on the
global scale.</p> |
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| ISSN: | 1726-4170 1726-4189 |