I accept your point about tallgrass prairies but you're back to the double-counting. The argument that 3 - biofuels are not carbon neutral, is established based on 1 and 2.

Also, it might be true that prairies do long term accumulation of carbon, but not true that this represents more carbon than that which is removed from the environment by biofuel crop growing and harvesting. That was my original claim and I don't think you've shown quantitative evidence that it should be dismissed.