Yeah idk. I'm pretty skeptical. A few notes mostly just for myself...
The main citation for the finding takes you to this paper:
https://pmc.ncbi.nlm.nih.gov/articles/PMC4810679/
Which describes their experimental methodology which was basically a chamber based approach to compare what they are calling "ambient" to what they are calling elevated levels of CO2. They are calling "ambient" ~350ppm, and elevated ~550 ppm, and under these conditions, they see a difference in nutrient content, at least among c3 plants. Which is fine.. and probably to be expected considering their evolutionary history..
red line is when c4 plants evolved, c4 plants evolved in an era of extremely low atmospheric CO2, and so they evolved some patches to deal with the CO2~ concentration issue.
But the issue I take is that their, laboratory conditions for "ambient" CO2 versus field CO2... they don't reflect reality in fields where you have decomposing material in the soil, and plants actively growing and respiring. That would typically already be in their elevated range of 400-550, heck even 600 ppm, if its a well irrigated high carbon system. And yes it drops during the day while active photosynthesis is happening but really only down to their "ambient" range (maybe 325-375 ppm). There is just a lot of decomposition and other kinds of respiration happening where crops are actually grown.
So yes, in an experimental 350 C3 plants would do better, but in reality all plants are already growing in much higher CO2 concentrations, because decomposition is happening below the soil surface. And I would also think that if you are increasing your organic material in that soil, which would increase decomposition and then ambient CO2 around the plants in the meter or so where they are actually growing, the benefits from increased soil carbon are going to far outweigh any negatives you would get from a higher background rate of CO2~...
I'd like to see this replicated outside of chambers to really trust it, especially considering that most plants already grow in what they call ambient conditions, and the consequences of a small decrease in soil organic carbon are far more consequential than that of elevated atmospheric CO2 around the plant...
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