It is though. Or if you like, it's transforming carbon from "in the ground" to "not in the ground", which is fundamentally the only thing that matters, since you're adding to a closed system.

The point I was making is that cash crop farming is not a self-contained system even if the earth as a whole mostly is. (Much) more carbon goes into its production than comes out in the crop. Most of that takes the form of operating machinery, but there's also the production of fertilizers, and also the incremental loss of carbon in topsoil through tillage and erosion.

I am singling out maize (and, well, soy) because the scale of it is absolutely crazy; also on the whole most of the product isn't for direct human consumption (animal feed and ethanol). (Also BTW I work on software for running machinery which operates in those fields.)

Anyways, this is intrinsic to farming, which is an extractive process. The question is how to more smartly manage it in the long run.