Prairies in the Midwestern United States (where we grow most of the corn for Ethanol) do burn (or rather, did), and do have significant long term accumulation of carbon in the soil - it's literally why our soil is so good for farming (for now). Like, where on earth do you get the nonsense idea that they're carbon neutral ecosystems? Tallgrass prairie has crazy deep roots that over time can place a lot of carbon in the ground, not even accounting for the persistent char layers that accumulate after periodic burns. Thousands of years of accumulation adds up, and converting it to farming:
1. Removes that service 2. Has one off costs 3. Has ongoing costs, in that biofuels are not (currently) carbon neutral at all 4. Has other negative impacts.
Assuming that biofuels have a one-off cost and then become carbon neutral just isn't supported by the evidence - it's a simplistic argument that is so at odds with the literature that I have a hard time taking it seriously.