Great question (and we are reaching the outside edge of my knowledge here). Something like 3-5% of carbon in plants is taken up from the soil by plant roots. I don’t fully understand the mechanism, but the organic carbon percentage is an important competent in the calculation of how much artificial nitrogen a crop is going to need, so I guess it’s probably some biochemical process for making the nitrogen available.

The organic carbon percentage is closely watched by farmers and is something of an indication of soil health. ie if your crop rotation is reducing the OC% over time then you probably need to reconsider it. It’s one of the reasons burning crop stubbles is a much rarer practice now.