What part, roughly, of the carbon in a plant comes from the soil? As opposed to the atmosphere?
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&quot;

(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 &amp; Technology 53, no. 8 (April 16, 2019): 4198–4205. https://doi.org/10.1021/acs.est.8b06089.