Some praries burn regularly and some don't. Most (as I said above, most land types) do not have significant ongoing accumulation of carbon in the soil. Obviously there are some biomes that do. Citing the fact that prairies burn as proof that they aren't long-term carbon neutral just seems to me like a low-quality argument if you are trying to challenge what I have suggested here, that land use changes are a one-off carbon cost which is paid off by biomass being used in fuels which replace fossil carbon.
You're also replying to someone who completely ignored the difference between the amount of carbon biomass and the change over time of that amount.
If I am completely mistaken and land types which are plausibly converted from unmanaged to biofuel crops are in fact accumulating significant stored carbon on an ongoing basis then tell me so and I stand corrected.