Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ<sup>17</sup>O of atmospheric CO<sub>2</sub>
<p>Understanding the processes that affect the triple oxygen isotope composition of atmospheric <span class="inline-formula">CO<sub>2</sub></span> during gas exchange can help constrain the interaction and fluxes between the atmosphere and the biosphere. We co...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-07-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/17/3903/2020/bg-17-3903-2020.pdf |
| Summary: | <p>Understanding the processes that affect the triple oxygen isotope
composition of atmospheric <span class="inline-formula">CO<sub>2</sub></span> during gas exchange can help constrain
the interaction and fluxes between the atmosphere and the biosphere. We
conducted leaf cuvette experiments under controlled conditions using three
plant species. The experiments were conducted at two different light
intensities and using <span class="inline-formula">CO<sub>2</sub></span> with different <span class="inline-formula">Δ<sup>17</sup>O</span>. We directly
quantify the effect of photosynthesis on <span class="inline-formula">Δ<sup>17</sup>O</span> of atmospheric <span class="inline-formula">CO<sub>2</sub></span> for the first time. Our results demonstrate the established
theory for <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> is applicable to <span class="inline-formula">Δ<sup>17</sup>O(CO<sub>2</sub>)</span> at
leaf level, and we confirm that the following two key factors determine the effect of
photosynthetic gas exchange on the <span class="inline-formula">Δ<sup>17</sup>O</span> of atmospheric
<span class="inline-formula">CO<sub>2</sub></span>. The relative difference between <span class="inline-formula">Δ<sup>17</sup>O</span> of the <span class="inline-formula">CO<sub>2</sub></span>
entering the leaf and the <span class="inline-formula">CO<sub>2</sub></span> in equilibrium with leaf water and the
back-diffusion flux of <span class="inline-formula">CO<sub>2</sub></span> from the leaf to the atmosphere, which can
be quantified by the <span class="inline-formula"><i>c</i><sub>m</sub>∕<i>c</i><sub>a</sub></span> ratio, where <span class="inline-formula"><i>c</i><sub>a</sub></span> is the <span class="inline-formula">CO<sub>2</sub></span> mole
fraction in the surrounding air and <span class="inline-formula"><i>c</i><sub>m</sub></span> is the one at the site of oxygen
isotope exchange between <span class="inline-formula">CO<sub>2</sub></span> and <span class="inline-formula">H<sub>2</sub>O</span>. At low <span class="inline-formula"><i>c</i><sub>m</sub>∕<i>c</i><sub>a</sub></span> ratios
the discrimination is governed mainly by diffusion into the leaf, and at
high <span class="inline-formula"><i>c</i><sub>m</sub>∕<i>c</i><sub>a</sub></span> ratios it is governed by back-diffusion of <span class="inline-formula">CO<sub>2</sub></span> that has equilibrated
with the leaf water. Plants with a higher <span class="inline-formula"><i>c</i><sub>m</sub>∕<i>c</i><sub>a</sub></span> ratio modify the <span class="inline-formula">Δ<sup>17</sup>O</span> of atmospheric <span class="inline-formula">CO<sub>2</sub></span> more strongly than plants with a lower <span class="inline-formula"><i>c</i><sub>m</sub>∕<i>c</i><sub>a</sub></span> ratio. Based on the leaf cuvette experiments, the global value for discrimination against <span class="inline-formula">Δ<sup>17</sup>O</span> of atmospheric <span class="inline-formula">CO<sub>2</sub></span> during photosynthetic gas exchange is estimated to be <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M33" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">0.57</mn><mo>±</mo><mn mathvariant="normal">0.14</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="ca37baaffa40a84cf7796ab0b502fe57"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-17-3903-2020-ie00001.svg" width="64pt" height="10pt" src="bg-17-3903-2020-ie00001.png"/></svg:svg></span></span> ‰ using <span class="inline-formula"><i>c</i><sub>m</sub>∕<i>c</i><sub>a</sub></span> values of 0.3 and 0.7 for C<span class="inline-formula"><sub>4</sub></span> and C<span class="inline-formula"><sub>3</sub></span> plants, respectively. The main uncertainties in this global estimate arise from variation in <span class="inline-formula"><i>c</i><sub>m</sub>∕<i>c</i><sub>a</sub></span> ratios among plants and growth conditions.</p> |
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| ISSN: | 1726-4170 1726-4189 |