[griffzhowl]

Most of the land in question isn't a mature ecosystem in equilibrium, and if it is there are other arguments against turning that into cornfields. But let's say the worst case is that it's carbon neutral.

The point is that biofuel production isn't carbon neutral. Maybe it could be theoretically, but in practice it takes energy inputs in the production process like in the fertilizers, processing of crop into fuel, and transport/distribution, which makes it net positive for atmospheric carbon.