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I see many repeated claims that plants absorb $CO_2$ from the air.

$CO_2$ goes into the stomata, while $H_2O$ evaporates and leaves those same stomata. The $CO_2$ dissolves in the water in the plant sap, and then goes to wherever it's needed.

It occurred to me that in loose moist topsoil, there is a tremendous surface area for $CO_2$ to leave air and get into the water. And that water does get absorbed by plant roots. Meanwhile there is plentiful O2 that soil bacteria can use to oxidize organic matter in the soil and convert it to $CO_2$. In my imagination, this looks like a far more efficient way to collect $CO_2$. Instead of wait for the $CO_2$ from decaying organic matter to diffuse out of the soil into the air above and then enter stomata and dissolve in water there, cut out the diffusion step. The rising sap would carry $CO_2$ to wherever it will be used.

It seems to me this would be easy to test. Grow plants in sealed containers with their stems sticking through small holes in an impervious barrier. Introduce $CO_2$ containing $C_{14}$ either into the air above with the leaves, or into the air below with the soil the roots are in. Observe which way gets more $CO_2$ incorporated into the plants.

Have experiments like this been done? My quick search did not find experiments like that. It found only the many-times-repeated claim that plants absorb $CO_2$ through their stomata.

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    $\begingroup$ Carrying CO2 "in the sap" is not that easy of a step, especially compared to simple diffusion: as plants do not significantly decrease the local CO2 level, diffusion is basically a free pump insuring constant CO2 concentration. Furthermore, while CO2 does dissolve into the water, its concentration is rather low naturally. (Natural fluids that transport CO2 in large quantities, such as blood, have enzyme like carbonic anhydridase to create bicarbonate for mass transport; which soil water won't have.). Also, it maybe possible to disprove this theory by just cutting a leave and using the c14. $\endgroup$ – Eliane B. Jul 16 '18 at 16:06
  • $\begingroup$ You see many repeated statements (not claims) that plants absorb CO2 from the air through the stomata of their leaves. For example, you can read an account of this in Lodish et al., Molecualr Cell Biology. This is because the phenomena has been studied by numerous professional scientists for over 50 years. Your question shows a lack of the research required before posting on this list. If you had done that you might have discovered that typical experiments use (rootless) leaves and manipulate the CO2 concentration. I have voted to close. $\endgroup$ – David Jul 16 '18 at 21:22
  • $\begingroup$ Thank you for the link. This link shows that plants absorb CO2 in the mesophyll, from the air space. Water from the xylem evaporates into this space and leaves through stomata; this drives the movement of water from the roots. O2 created by plants also leaves through stomata. It does not show that the CO2 comes only through stomata and not with the water. Using rootless leaves would prevent roots from collecting CO2, though. Still, normal photosynthesis with no root and only normal CO2 air levels would plausibly say that CO2 from air is enough. $\endgroup$ – J Thomas Jul 17 '18 at 7:23
  • $\begingroup$ Eliane B, thank you for responding. I think that since CO2 level in soil air can reach as high as 8% during wheat cultivation, it seems plausible to me that plants might find a way to sequester that resource rather than just let it diffuse into the outer air and then concentrate it again from there. And that probably happens. nature.com/articles/415451a $\endgroup$ – J Thomas Jul 17 '18 at 8:19
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Thanks to David's comment above, I learned enough vocabulary to do a better lit search.

Yes, $CO_2$ supplied to roots is absorbed and distributed by the xylem to places where it is photosynthesized. This is not in doubt.

The remaining question is how much of the $CO_2$ used by plants comes through stomata from the outside air (where it is present in concentration around 0.04%), and how much of it comes to roots from subsurface air (where it is present in concentration that varies widely, up to 8%).

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