C4 plants contain a slightly higher percentage of carbon-13 than C3 plants. Is this because of carbon obtained from the soil or the atmosphere?

I have read that plants using different chemical pathways, such as millet and wheat, incorporate distinctive proportions of stable carbon isotopes from the SOIL.

Do C4 plants need more carbon-13, or does that process just happen to result in accumulation of more carbon-13?

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    $\begingroup$ Always good to cite things you've read. $\endgroup$ – Bryan Krause Sep 19 '20 at 0:30

Only a small fraction of plant carbon is soil-derived: e.g. from Majlesi et al 2019:

although the majority of plant C was obtained from atmosphere by photosynthesis, a significant portion (up to 3–5%) of C in plant roots was derived from old soil"

(in an experiment with Scots pine and reed canary grass).

As laid out by this review of C4 photosynthesis, the primary reason for differential isotopic make-up of C3 and C4 plants is that the biochemical pathways they use fractionate isotopes differently — not because the plants' requirements are different.

Because of the very different affinities of Rubisco and PEP carboxylase for CO$_2$ , C$_3$ and C$_4$ plants differ in the carbon isotopes that they assimilate. While most atmospheric CO$_2$is made up of ${}^{12}$C, a small fraction includes ${}^{13}$C. Rubisco has a fairly high $K_m$ and hence low affinity for CO$_2$, and preferentially fixes ${}^{12}$C. The ratio of ${}^{13}$C to ${}^{12}$C ($\delta^{13}$C) in a C$_3$ plant is thus much lower than the ratio in the atmosphere; the carbon in structural and storage carbohydrates contains much less ${}^{13}$C. PEP carboxylase has a lower $K_m$ for CO$_2$ than Rubisco and hence a higher affinity; as a consequence it discriminates much less against ${}^{13}$C than Rubisco does, so that the $\delta^{13}$C is appreciably closer to atmospheric.

Majlesi, Soroush, Jukka Juutilainen, Anne Kasurinen, Promise Mpamah, Tatiana Trubnikova, Markku Oinonen, Pertti Martikainen, and Christina Biasi. “Uptake of Soil-Derived Carbon into Plants: Implications for Disposal of Nuclear Waste.” Environmental Science & Technology 53, no. 8 (April 16, 2019): 4198–4205. https://doi.org/10.1021/acs.est.8b06089.


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